INSTRUCTIONS REGARDING THE

D I S M A N T L I N G , O V E R H A U L I N G ,

REPAIRING, CALIBRATING, TESTING

AND INSTALLING THE

KBB-KOLLSMAN

RATE OF CLIMB

I N D I C A T O RMark IB

KELVIN, BOTTOMLEY AND BAIRD LIMITEDKELVIN WORKS, BASINGSTOKE, HANTS

FOREWORDThe K.B.B. Rate of Climb Indicator

possesses some unique features thesignificance of which must be fullyunderstood if the satisfactory servicingof the instrument is to be adequatelyaccomplished.

In the Rate of Climb Indicator moreprobably than in any other instrumentit is unwise to attempt, and indeedalmost impracticable to lay down hardand fast rules which will cover everypossible contingency. We must, there-fore, be content with enunciating prin-ciples to guide the repairer on the linesto be followed.

In the present instance our desire tosatisfy an insistent demand for informa-tion not readily available elsewhere haslargely been countered by an equallyurgent necessity to conserve paper.

But it was evident from the start thatadditional explanatory notes would beof inestimable value to the repairer andwe set out to provide the maximumwhich the exigencies of the space limita-tions would permit.

As the notes increased it was realisedthat it would be advantageous toseparate the more important of themcompletely from the main body of thebook. The principal ones were there-fore segregated into a section whichhas been headed ' Technical Notes.'

In this form they will now have a wideappeal as they explain the operation ofthe important points of the mechanismwith the aid of diagrams in a mannerwhich leaves no doubt as to the functionof the devices concerned.

All the test apparatus required isdescribed, and illustrations of all com-ponent parts are provided, while othernecessary details are very fully dealtwith.

We commend this volume to theinstrument repairer as a combinedeffort to remove his difficulties. Wehope it will be used as a text bookand a reference book.

If you wish any further guidance weshall always be happy to help you in anyway we can.

January 1945

CONTENTSGENERAL DESCRIPTION Page IPRINCIPLE OF OPERATION 2

RATE OF CLIMB INDICATOR, MARK IB*I DISMANTLING:—

a Removal o f mechanism . . . . . . . 3b Dismantling o f case assembly . . . . . . 4c Dismantling o f mechanism . . . . . . . 5

2 EXAMINATION AND OVERHAUL:—a Handstaff pinion . b Bridge jewel . . . 6c Rocking shaft assembly . . . . . . . 6

3 REPLACEMENT OF HAIRSPRING 74 CALIBRATING SPRINGS 85 EXAMINATION AND REPLACEMENT OF DIAPHRAGM AND METERING UNIT 86 RE-ASSEMBLY:—

a Re-assembly o f mechanism . . . . . . . 9b Straightening bent handstaff . . . . . . 1 0c Adjustment of handstaff and sector . . . . . 11d Cleaning of mechanism e Assembly of calibration bracket 11

7 REPLACEMENT AND ADJUSTMENT OF ZERO SETTING MECHANISM . 128 GENERAL INSPECTION BEFORE CALIBRATION 129 CALIBRATION AND ADJUSTMENT:—

a Preliminary setting u p o f apparatus . . . . . 1 3b Calibration a t 1,000 ft./min. climb and descent . . . 1 3

10 SETTING DIVE OVERLOAD STOP 1411 SETTING CLIMB OVERLOAD STOP 1512 STATIC BALANCE OF MECHANISM:—

a Purpose o f method . . . . . . . . 1 5b Procedure 16

13 CALIBRATION (continued):—a at 2,000 ft./min. descent b at 3,000 ft . /min. descent . 17c at 4,000 ft./min. descent d on climb side . . . 18

14 RE-ASSEMBLY 1815 CHECK FOR LEAK 1916 BELL JAR TEST 1917 TESTS AND TOLERANCES 20

RATE OF CLIMB INDICATOR (EARLIER TYPE) 21

TECHNICAL NOTESZERO SETTING MECHANISM 22USE OF ZERO ADJUSTMENT MECHANISM 22CALIBRATION AND ADJUSTMENT 23DIAPHRAGM DIVE OVERLOAD STOP 23DIAPHRAGM CLIMB OVERLOAD STOP 24DIAPHRAGM AND METERING UNIT 24

CALIBRATION APPARATUSINCLINED MANOMETER CALIBRATION STAND 26LEAK-TESTING APPARATUS 27TABLE FOR DETERMINING ERROR 30

SMALL TOOLS 31SPECIAL TOOLS 33INSTALLATION DETAILS 34LISTS OF SPARES 35

GENERAL DESCRIPTION

The K.B.B. Rate of Climb IndicatorMark IB* is a simple means of measuringthe rate of change of atmospheric pres-sure due to change in altitude of theaircraft in which the instrument is fitted.

A single pointer is mounted on aspindle in the mechanism. This pointermoves round a dial graduated in evendivisions each representing one thou-sand feet per minute and each divisionis subdivided into graduations of 200ft./min. The range is from 0 to 4,000ft./min. climb and descent.

The moulded case is of one-piececonstruction and is rendered airtightexcept for the static nipple. It is fittedwith a stout glass to permit the instru-ment to be used in pressure cabinmachines.

The pointer and some of the lettersand graduations of the dial are coatedwith luminous compound to make themlegible under night flying conditions, orthey may be coated with a fluorescentcompound which glows under ultra-violet floodlighting.

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PRINCIPLE OF OPERATION

The principle of operation of thisinstrument can be followed from thesketch. It consists of a sensitive dia-phragm which operates a pointer througha link and suitable mechanism. Thediaphragm is connected directly to thestatic nipple. This static nipple incor-porates a leak which allows a slowpassage of air from the inside of the caseto the static connection. The case mustbe airtight except for this leak.

When the static pressure is changing,corresponding to a change in altitude,the pressure in the diaphragm changesmore rapidly than the pressure in thecase, and hence the diaphragm is de-flected. This deflection causes a move-ment of the pointer, which indicates

the rate of climb or descent of the air-craft. When the static pressure issteady, the pressures inside the dia-phragm and case are equal, and the de-flection of the pointer is zero.

The leak is specially designed to com-pensate for changes in the temperatureand density of the air, and consists ofan arrangement of small capillaries andorifices. It can be shown both mathe-matically and experimentally that suchan arrangement gives efficient tempera-ture and altitude compensation over awide range of conditions.

The function of the diaphragm, theleak and of some of the other workingparts is described more fully in 'Tech-nical Notes ' on page 22.

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DISMANTLING, OVERHAUL,CALIBRATION AND RE-ASSEMBLY

I Dismantlingla Removal of mechanism

Insert the instrument upright on awooden block, or similar support.Remove the outer snap ring using theblade of a sharp penknife to prise upthe end, taking care not to chip theglass. The glass will then probably comeout quite easily if a suction pad is used.If this is not available, working the glassbackward and forward gently with thefingers will generally release it from thegasket. Any difficulty is likely to beconfined to instances where the rubberof the gasket has perished, and wherethis has happened, some additional per-suasion may be called for. Normally therubber gasket and the retaining ringwill come out together.

The pointer should then come easilyfrom a straight pull with thumb andforefinger. Avoid putting any strain onthe handstaff as this may bend or breakit.

Remove the inner snap ring. Insertthe pointed rod in the hole at the end ofthe ring, and relieve the tension of thering on the case by bending the ring ina little towards the centre of the dial(fig. I). It will then lift out quite easily.

Remove the dial screws using a smallwatchmaker's screwdriver, and invertthe case to allow the dial to fall into thehand. Dial and pointer should now be

sent off for luminosity check, and anynecessary reluminising and rebalancingof pointer should be carried out at once,so that the parts are ready when re-assembly takes place.

Remove the countersunk screw in thefront of the mechanism body plate (fig.2), then lift out the spigot and the 54-

tooth wheel. The ring which holds themechanism can now be snapped out bylevering under one edge with a smallscrewdriver (fig. 3).

Now turn the case over and loosenthe three screws which secure themetering unit to the bottom of the case.Make quite sure that the nipple screwsare not slackened in error. For guidanceon this point we might explain that themetering unit screws alternate with thenipple screws round the nipple, and it

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is always one of the metering unitscrews which occupies the hole in theshortest distance between the nippleand the edge of the case (fig. 4). The

screws have probably been touched withBakelite varnish, but this will flake offas soon as the screw is turned. It maybe necessary to press on the screws tobreak the seal.

Invert and withdraw the case fromthe mechanism. It is very importantthat the utmost care should be taken inthis operation. The diaphragm is ofextremely delicate construction and willnot stand up to any rough handling.The least dent in the material (which isthin foil) will render it useless (fig. 5).

\b Dismantling ofcase assembly

Before proceeding further with themechanism, presuming that the case issound and can be used again, remove thezero adjusting pinion from the case.Press down the retaining ring on theshoulder of the pinion (fig. 6) to reveal

a small cross-pin driven through thepinion and stem. In the newer instru-ments this ring will not be included.

With tweezers push the pin as farthrough as possible, then turn the pinionround and withdraw the pin from theother side of the stem. The stem willthen drop out through the front of thecase leaving the pinion and spring stillinside. These can be lifted out.

The purpose of the spring is to keepthe stem tightly in contact with theleather washer, so that the case will bequite airtight at this point.

When the stem falls out, the leatherwasher will probably remain adheringto the bottom of the counterbore. Liftit out with tweezers and fit anew washerwhich has previously been steeped incastor oil.

This replacement of leather washershould be carried out in most instanceswhen an instrument is dismantled foroverhaul. The cost of doing so isnegligible and much trouble due to airleakage has afterwards been traced toneglect of this small point.

If there is insufficient time to dismantlethe stem, a drop of pharmaceuticalcastor oil on the existing leather washermay improve it very considerably.

Now, set aside the case for a newpaper washer to be fitted on the nippleinside the case. This is essential to makean airtight joint when the metering unitis replaced later on. The washer is fixedwith Bakelite varnish which requires 24hours to dry.

At all times when working with themechanism out of the case, care mustbe taken to avoid damage to the hand-staff and to the diaphragm. The most

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satisfactory position for laying themechanism on a bench, therefore, isresting on the pillar at the base of thediaphragm, held upright by the capillarytubing where it bends to join the meter-ing unit (fig. 7). (In this position thelocating pin in the mechanism plate is atthe top.)

\c Dismantling of mechanismAs frequent mention is made of the

various parts of the mechanism bodycasting, it is desirable that these partsshould be easily identified. To assistin this direction an outline sketch of thecasting is given in fig. 8.

The casting, which is of an aluminiumalloy, comprises a plate (on which thedial is mounted) and two pillars forsupporting the mechanism, strengthenedby a connecting web. On one pillar isan arm which takes the bearing for therocking shaft and the hairspring post,while the arm on the web carries thecalibration bracket and springs.

At the back of the plate are two bossesfor the bridge. A locating pin is drivenin to the plate to enable the mechanismto be located accurately in the caseduring assembly. For simplicity the bossfor the zero adjustment mechanism hasbeen omitted in the sketch.

Discretion should be used about theextent of the dismantling carried out.There is no virtue in stripping themechanism right down as a matter ofroutine. Indeed, this frequently entailsan appreciable amount of work whichis quite unnecessary and unproductive,and it tends to disturb adjustments andsettings which are better left alone.

Procedure can be divided under a fewbroad headings and these should be takenas a guide to the limits implied.

Remove the two screws securing thecalibration bracket to the spacers andto the arm on the connecting web ofthe casting (fig. 9) then the screw and

washer holding the stiffening plate tothe pillar of the mechanism body (fig.10). This releases the calibration

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bracket and the stiffening plate.Slacken all calibrating screws (fig. 11).

This is to prevent any risk of re-assemblywith the screws protruding through, anoperation which might damage thecalibrating springs. The screws havebeen coated lightly with Bakelite var-nish to hold them in position, and todissolve this, the bracket should beimmersed in methylated spirits for abouta quarter of an hour.

Meantime, remove the screws secur-ing the bridge to the bridge boss andlift off bridge (fig. 12). This will releasethe handstaff pinion.

2 Examination and overhaul2a Handstaff pinion

Immerse the handstaff in benzine fora few moments to remove grit and dirt.Examine for faulty teeth or bent pivotand for marks showing signs of wear onthe shoulders of the pivot, using a 2"eyeglass. This examination is veryimportant since a very great deal

depends on the satisfactory performanceof the handstaff. If the pivot is bentthis can frequently be rectified afterre-assembling. Any bending would mostprobably be between the jewel and thetop of the mechanism plate. (See para-graph 'Straightening bent handstaff'below.) If the teeth are faulty or signsof wear are present, replace.

2b Bridge jewel

Examine the jewel on the bridge forcracks or chipping round the edge of thehole. If present, tap out old jewel onthe stake, and replace. Deal similarlywith the jewel in the mechanism body.

2c Rocking shaft assembly

Check that the sector is perfectlyflat, and that the teeth are undamaged.If the meshing of the sector and hand-staff pinion has been too tight, damagesometimes occurs to the sector teeth,which are softer than the pinion. Thisdamage is, fortunately, rarely observed.It is occasioned by faulty adjustment onsome previous servicing of the instru-ment by a repairer not thoroughlyfamiliar with the adjustments.

It is difficult to get satisfactory resultsfrom a sector which has been bent andstraightened, and faulty teeth will causeerratic readings. If either is observed,the rocking shaft should be replaced.

Confirm that the handstaff pinion ismidway round the arc of the sector. Ifit is badly placed this will require to becorrected and the necessary action ismentioned in the paragraph 'Adjust-ment of Handstaff and Sector ' below.

Check pivots and jewels. With roughusage the jewels will be more easilydamaged than the pivots which are verystrong. A broken pivot can be extractedwith pliers, and a new one pressed in.When performing this operation,remem-ber that the rocking shaft is of delicateconstruction and use great care.

The jewel in the mechanism body platecan, if damaged, be driven out on thestake and replaced, and the jewel in thearm on the pillar by loosening the secur-ing screw.

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3 Replacement of hairspring

A hairspring is fitted to the rockingshaft to prevent backlash in the pointer,and to steady the mechanism while inoperation. In a very few instances onlyis it necessary to fit a new hairspring,because its travel in controlling theaction of the rocking shaft is but afraction of a single turn, and damage istherefore likely to occur mainly whenhandling—either in dismantling or re-assembly—unless in the case of a crash.

When replacement is called for, com-mence by removing the rocking shaftfrom the mechanism body. The correctprocedure for this is first of all tostraighten the pin holding the link andto withdraw it, thus releasing the rock-ing shaft from the link. This will befound a little difficult owing to the smallsize of the pin, but in view of the fewoccasions when it is likely to be necessary,the extra trouble involved is worthwhile spending.

When removing the link for anyreason at all, always note the hole inwhich the bearing pin was located, andmake quite sure that the pin is replacedin this hole. When fitting a new dia-phragm it is possible that the same holemay not be the correct one—this canonly be confirmed by trial at l,000 ft./min.calibration—but it should be standardpractice to replace the pin in the samehole, since it will perhaps save a greatdeal of time later at this adjustment.

The alternative to removing the bear-ing pin is to loosen the screw holdingthe calibration arm, and thus release thecalibration arm from the rocking shaft.This is very much easier to accomplish,of course, but the great objection tothis procedure is that it immediatelydisturbs the setting of the link and therocking shaft, and restoring this settingis troublesome and requires much carewhen calibrating at 1,000 ft./min. climband descent.

Following the removal of the pin orthe loosening of the calibration arm,withdraw the taper pin securing theend of the hairspring to the hairspringpost and loosen the jewel in the arm onthe mechanism body pillar (fig. 13).The rocking shaft can now be liftedclear of the mechanism.

Remove the faulty spring by prisingthe collet off with tweezers, making amental note of the direction in whichthe spring winds relative to the collet.Remove and examine the disc under-neath the hairspring. If it is undamagedit may be used again, but it should bepolished to remove any marks whichmay have been caused by the faultyhairspring rubbing. It has been foundthat such marks sometimes result inslight friction with the coils of the newhairspring and disturb readings.

Replace the disc. The new hairspringis supplied already colleted. Press it onto the end of the rocking shaft, makingsure that it winds in the correct direc-tion, and adjust until the end of thespring is located just above the end ofthe calibration arm, where the arm ispinned to the link. Check also that theturns do not rub against one anotherand that the spring is free from contactwith the disc.

Now, mount the rocking shaft assem-bly, and swing it outward until thesector is over the locating pin on theplate of the mechanism body, and restingagainst the pillar. Grasp the end of thehairspring carefully with tweezers, andput a half-turn of tension on it by bring-ing it round and slipping it through thehole in the hairspring holder, thensecure it in this position by the taperpin.

Confirm by inspection that the hair-spring is lying quite flat. If not, someslight adjustment of the position of thehairspring post may be necessary. Thisis easily effected by easing the screwsecuring it to the arm on the pillar.

Now swing the rocking shaft backinto its correct operating position. The

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hairspring now has one complete turnof tension.

The correct setting of the hairspringis essential. Any inaccuracy in the settingwill become apparent when an attemptis made to calibrate the mechanism at1,000ft. min. climb and descent. It willbe clear from the construction of themechanism that if the tension is toogreat, its opposition to the diaphragmmotion on ciimb will cause a minus error,and its co-operation with the diaphragmmotion on descent a plus error.

Adjust the endshake of the rockingshaft as necessary by altering the positionof the jewel in the pillar arm. It is diffi-cult to express on paper the limit ofend-shake, but a useful guide is to con-sider about 0002" as a fair amount. Afeeler gauge might be used until sufficientexperience is gained. This value couldbe accepted for most occasions whereendshake is referred to, except wherethe parts themselves are extremelysmall, when the amount will be propor-tionately less.

4 Calibrating springs

These should not be disturbed unlessthey have suffered damage. Any kink orbend in the springs renders them un-suitable for calibration and they wouldthen require to be replaced.

If it is desired to remove them,slacken the screws securing them tothe arm on the connecting web of thecasting (fig. 14).

When refitting, make quite sure thatthey are exactly one above the other.This is most important to enable the

calibrating stem to function equally wellfor climb and descent and to avoid thelink rubbing on one or the other. Thespace available is so small that this adjust-ment is critical. Before the securingscrews are tightened home, the springnearest to the diaphragm should begently led into position using a pair ofpliers or tweezers (fig. 15).

5 Examination and replace-ment of diaphragm andmetering unit

The function of these is discussed atsome length in the Technical Noteswhich will be found on page 11.

Examine the diaphragm closely forany dents, marks or scratches. If anyof these are observed, the complete

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diaphragm and metering unit must bescrapped. The slightest mark preventsthe instrument from calibrating to agood reading. Don't, however, confusethese with the marks inseparable fromthe production of formed blanks on apress tool. Forming marks are generallyradial and uniformly distributed roundthe outer parts of the diaphragm.

To replace the diaphragm and meter-ing unit, remove the pin which securesthe calibrating stem to the centre pieceof the diaphragm, and also the pin whichholds the link to the centre piece.Remove the climb stop by looseningthe screw and lock-nut (fig. 16) thenremove the two screws at the side ofthe diaphragm mounting piece (fig. 17).

Support the diaphragm by holding therim gently between thumb and fore-finger, and when quite loose, slip out bygrasping the capillary tube with theother hand, and lift clear (fig. 18).

Fit new diaphragm assembly in reverseorder. Care must be taken here toensure that the tubing is not touchingthe diaphragm, or it will disturb expan-sion. The tube will bend away easily inthe fingers.

Dismantle the metering unit by re-moving the screw with the special screw-driver for the purpose (fig. 19). Clean

all parts thoroughly, removing any tracesof dust or foreign matter. On noaccount should cleaning fluid be em-ployed in this operation. Re-assemblecarefully.

6 Re-assembly

6a Re-assembly of mechanism

Hold the mechanism level with theeye, diaphragm plate upward, and checkthat the calibrating stem, when heldvertically, head down, hangs free, andthat it will swing evenly to either side(fig. 20). This is most important, and

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the examination must be carried out withcare since the movement is very smallindeed. Fig. 21 gives an enlarged view

of the calibrating stem between thecalibrating springs. If the calibratingstem does not swing freely, remove thepin securing it to the diaphragm centrepiece, insert a fine needle in the hole atthe end of the calibrating stem, and feelfor any binding. If present, broach outwith a fine broach. Re-assemble andre-test.

Test that the link between the dia-phragm and the calibration arm also isquite free. It must not bind. Test byholding the link with fine pointedtweezers at each end in turn and at thesame time make sure that there is justa little endshake to ensure freedom.The same procedure should be carried

out for the link as for the calibratingstem, if any binding is suspected.

Polish the pivots on the handstaffpinion in elder pith. Clean out jewelson bridge and mechanism plate withpegwood sharpened to a fine point. Fitthe handstaff pinion and the bridge.The handstaff should be quite free. Testby holding the instrument upright inone hand and tapping the handstaffgently with the forefinger nail with anup-and-down movement. It should bepossible to hear the pinion drop on thejewel under its own weight (fig. 22).

The amount of endshake should bekept very small—the least possible com-patible with freedom. The endshakecan be adjusted by altering the positionof the jewel in the bridge. This is effectedon the staking tool.

If sticky, remove bridge and againexamine jewels and pinions, and if theseare all in order, replace and check thatthe handstaff is dead upright and notbearing to one side. This can be ascer-tained by holding the instrument leveland looking across the mechanism plate,while turning the mechanism round.

6b Straightening bent handstaff

If the handstaff is bent a little, it canfrequently be straightened. Bending isinvariably at the long pivot, and usuallytakes place where the handstaff leavesthe bearing in the mechanism plate.The special tweezers described else-where are inserted in the cavity in theplate, and the handstaff coaxed in theappropriate direction until it is straight.Ordinary pliers should not be used forthis job, as they may not straighten thehandstaff, but simply bend it back atanother point. The tweezers are easilyobtained and very simply adapted forthe purpose. They do the operation somuch more efficiently that their produc-tion is encouraged (fig. 23).

Here is a practical tip of special valueto instrument repairers who also handleK.B.B., Mark XIVA sensitive altimeters.The same handstaff is employed in bothmechanisms, and if the damaged hand-staff were mounted in a Mark XIVAmechanism, it would be much moreeasily reached with tweezers or pliers,

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while a quick flick of the sector canrotate the handstaff rapidly to observethe effect of the straightening efforts.

6c Adjustment of handstaffand sector

Make sure that the depth between thesector teeth and the handstaff pinion iscorrect. This can be tested by holdingthe mechanism in one hand and gentlydepressing the handstaff with the thumbof the same hand to hold it steady, thengripping the side of the sector with apair of tweezers and moving the sectorbackward and forward crosswise (fig.24). There should be very slight move-ment between the teeth of the sector

and the pinion. The correct depth is ofthe utmost importance for a steadyreading, but the exact amount can onlybe determined after a little practicalexperience.

If a new rocking shaft or diaphragm

has been fitted, or the compensatingplate adjusted, it will be necessary toset the sector. It is essential that thepinion should rest midway round thearc. The setting is done by using a pairof smooth pliers and a special sector key.Grip the rocking shaft gently with thepliers. Place the key over the rivetedend of the sector shoulder and turn thesector to the required position (fig. 25).

Extreme care should be taken not totwist the pliers as strain on the rockingshaft will break the pivots.

6d Cleaning of mechanism

Place the mechanism in a shallow dishcontaining some clean benzine, and washwith a fine camel-hair brush or fine paintbrush. Every precaution must be takenwhile washing to prevent the benzinefrom entering the diaphragm throughthe climb stop or getting into the meter-ing unit.

6e Assembly of calibrationbracket

Assemble in reverse order from dis-mantling, making sure that the twocalibrating springs are depressed to-gether to allow the bracket to passsafely over them. Be sure that no cali-brating screws are protruding. If thereshould be any, slacken them until theirtips are concealed within the arms ofthe calibration bracket.

Care should be taken to see that thedive stop passes between the link andthe calibrating stem. It is imperative

II

that the stop should be set in a centralposition between the link and the cali-brating stem so that there will be norubbing or friction when the diaphragmis expanding or contracting. Secure thebracket in position by the two screws.Replace the stiffening plate.

On examining the calibration bracket,a slot will be observed on an extensionof the arm, for which there are no corre-sponding screws. This can be ignored.It was used in an earlier type of instru-ment for the climb and dive stop, but isnow obsolete, having been replaced bythe devices described later.

7 Replacement and adjustmentof zero setting mechanism

This rarely requires taking apart orreplacing. The main point to watch isthat the 36-tooth wheel is tight on thezero adjusting stem. If it should proveto be loose, draw out the taper pinsecuring the stem to the mechanismbody (fig. 26) and withdraw the wheel

and stem along with the washers. Fita new wheel and stem assembly, andreplace the washers in their correctorder—the thin domed phosphor-bronzewasher first and the brass washer on top.The phosphor-bronze washer is to makethe assembly friction tight, and thebrass washer acts as a spacer.

The adjusting stem must be set insuch a position that the pointer can bemoved in either direction by it, there-fore it is fixed in one of the two midwaypositions. For the sake of uniformity thewider portion of the shoulder is turnedaway from the pillar on the mechanismbody, and the taper pin is pressed in atright angles to the diaphragm. Withthe zero setting mechanism in thisposition, any adjustment of the screwon the case in a clockwise direction willresult in the pointer also being movedclockwise.

There is no functional reason why thewider portion of the shoulder cannot beturned towards the pillar, but it isbetter to keep all instruments identicalin performance in this minor point.

The zero adjusting stem must also beaccurately mounted at the end of thecompensating plate. If rotation of thestem does not cause continuous move-ment of the pointer, the compensatingplate is not bearing against the stemproperly, and it will be necessary toachieve this by bending the compensat-ing plate outward a little more, usingpliers. This operation is not often calledfor, and needs great care and attention.No undue force must be used, keepingin mind that the diaphragm must remainin an upright position.

A mark might be made on the 38-toothwheel and a corresponding one on themechanism plate to make sure that whenfinal assembly of the mechanism is takingplace, the position of the zero settingmechanism has not been disturbed.

8 General inspection beforecalibration

Hold the mechanism in both hands sothat the calibration bracket is horizontaland level with the eye, and ti l t it bothsides to make sure that the calibratingstem is swinging freely and evenly.Should it be swinging unevenly, thiscan be remedied by easing the twoscrews securing the bracket to themechanism and gripping the calibratingspring nearest to the diaphragm with apair of pliers and moving it up and downa little to give the required setting.The head of the calibrating stem must be

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set in the centre of the two springs.This is most important. The movementnow possible for the calibrating stem isvery small.

Points to note specially at this stageare :—

(i) The link must not foul the cali-brating springs,

(ii) There must be perfect freedomin all moving parts.

(iii) The screws on the calibrationbracket must be tight but free tomove with a fine needle for pur-pose of calibration.

(iv) The calibrating springs shouldcorrespond with one anotherwhen pressed together, and thereshould be no rubbing on thebracket whatever.

Re-check these points and satisfy your-self that they are all in order.

9 Calibration and adjustment9a Preliminary setting up of

apparatus

As the inclined manometer calibrationframe is in general use on the factoryassembly lines and in most repair shops,reference is made solely to this appara-tus in the notes which follow. This isthe correct apparatus for the Job, andmakeshift fittings will never be satis-factory.

The instrument is first calibrated as asensitive pressure gauge (see TechnicalNotes), therefore the leak permittingthe passage of air through the orifice issealed by rubber tubing and a clip orsmall brass stopper.

Assemble the mechanism into the testcase. Fix the leak to the back of the caseby the three screws, and secure themechanism in position by the inner snapring. Fit a test dial in position by twodial screws and mount a properlybalanced test pointer. Fix the case inthe buzzer frame. Put a drop of Bakelitevarnish on the climb screw and nut toprevent any leak from the diaphragm.Connect up to the calibration frame.Make sure that the vent is open to the

atmosphere and set the pointer exactlyon zero. It is imperative that the pointershould be accurately placed. If not quiteon the mark it must be removed andre-set.

At any time when it is necessary tocheck that the pointer is on zero, makequite sure that the vent is open and thatthe diaphragm is subjected internallyand externally to atmospheric pressureonly. This point is easily overlookedand would introduce false readings.

9b Calibration at 1,000 ft./min.climb and descent

(i) Items affecting pointerdisplacement.

Up to 1,000 feet climb and descent, thecalibrating stem does not come intocontact with the calibrating springs,therefore neither the springs nor theiradjusting screws come into play.

This means that the method necessaryfor calibrating at this point differs fromthe method used at the higher speeds,where the springs form an integral partof the calibrating mechanism.

Up to this point the pointer displace-ment is affected by :—

(i) The setting of the zero adjust-ment.

(ii) The effective length of the linkbetween the diaphragm and therocking shaft. (There are severalalternative holes in the link.)

(iii) The effective radius of the cali-bration arm, and

(iv) The pull from the tension on thehairspring.

(ii) Procedure.

Check against the inclined column atthe two points, then open the vent andreduce reading to zero. If the errorshave been infinitesimal, proceed to thenext calibration point without disturb-ing the setting. If the errors are outsidethe permitted limit, the procedure tobe followed will depend upon the natureof the errors which are observed.Probable errors can be grouped under

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six headings, and may be tabulated asfollows :—

ERRORCLIMB DESCENT

Experience has shown that (c) is themost general error and (d) least to beexpected.

(a) can be corrected by increasing theradius of action of the calibration arm,that is, by moving it a little outward.

(b) can be corrected by decreasingthe radius of action of the calibrationarm, that is, by moving it a little inward.

In the case of (c), (d), (e) and (f), ifthe combined error does not exceed100 feet, the logical solution is to adjustthe calibration arm a little to allow theerrors to be more evenly distributed —for example, plus 10 minus 60 could bemade to read plus 35 minus 35.

Errors on climb and descent to a com-bined total of 100 ft./min. can be adjustedby altering the position of the calibrationarm on the rocking shaft. Errors up to200 ft./min. suggest either that the linkis in the wrong position — which wouldonly occur if a new diaphragm assemblyhad been fitted, or if the link had beenwithdrawn during dismantling and re-assembled in the wrong position-incorrect set up on the hairspring, orincorrect setting of the zero adjustment.An excessive error has also been ob-served where the calibration arm hasbeen removed at some time and verycarelessly replaced.

During these adjustments, make quitesure first of all that no screws are touch-ing the diaphragm.

After each adjustment open the ventand re-set the pointer on zero, thenre-check at the two points, carrying outany further adjustments which may benecessary to reduce the errors to thelowest possible value. Before each test,the pointer must be re-set.

Once the most suitable position ofthe calibration arm has been determined,it must not be disturbed while calibra-ting at higher points.

Altering the position of the calibrationarm will, of course, affect the readings atthe higher test points also, but to alesser degree, and any alteration can betaken care of in calibration at thesepoints.

It may not always be possible to re-move the errors completely at 1,000ft./min., for example where there is aplus error on climb, and no error ondescent. Provided the errors are rela-tively small they may be left meantime,as it will be observed that the furtheradjustments necessary for correction at2,000 ft./min. will have the effect ofcorrecting these minor discrepancies.Before proceeding further, however, setthe overload stops.

10 Setting dive overload stop

With the vent open, check that thepointer is exactly on zero, and if notre-set it, then close the vent and increasethe pressure on the diaphragm until thepointer is brought right round thedescent scale and approaches the 4,000ft./min. climb graduation. Maintain thepressure at this point, slacken the screwsecuring the dive stop to the bracketand depress the stop against the centrepiece of the diaphragm (fig. 27). As

soon as the stop touches the diaphragmcentre piece, the pointer will be ob-served to move back.

Continue the depression until thepointer reaches the position previouslydecided upon equivalent to about4,800 ft./min., then tighten the screw.Make quite sure that the dive stop does

14

not foul the link side of the calibratingstem, and again check that no calibratingscrews are touching the diaphragm.This latter is often the case during thisoperation. Any screws touching shouldbe screwed into the bracket until theyare quite free from the diaphragm.

Now release the pressure controlclip, and bring the pointer back to zero.Replace the clip once more, increasepressure, and check that the pointerwill not go beyond the 4,800 mark onoverload. Maintain the overload positionfor about half a minute, This gives anopportunity to confirm that there is nota slight leak in the diaphragm. If one ispresent, the pointer will graduallyreturn towards zero in this test, inwhich case a replacement diaphragmwill be needed. Should the diaphragmbe dented, this will be observed duringthe test by an oil-can effect. If any dentsare present in the diaphragm, a newdiaphragm must be fitted. On over-loading again, some of the screws maystill be observed touching the dia-phragm. These, as before, must be slack-ened. Open vent and allow the pointerto return to zero. If it does not returnexactly, it will be necessary to re-set itand repeat this operation.

11 Setting of climb overloadstop

If the position of this stop has beendisturbed it will require to be re-set.Reduce the pressure on the diaphragmuntil the pointer is brought round theclimb scale and approaches the descentmarkings. Maintain the pressure at thisvalue, and by means of the special boxspanner (to prevent the nut turning atthe same time) and a fine screwdriver,turn in the screw until the pointerreturns to the same position fixed forthe descent overload, that is, equivalentto about 4,800 ft./min. (fig. 28). Do thiscarefully and gently. Give the screwand nut a touch of Bakelite varnish tofix them at this point, and to preventair leakage.

If the pointer will not reach the 4,800ft./min. mark, of course, the screw willrequire to be eased a little.

Open the clip and allow the pointerto return to zero. Now overload onclimb again by reducing the pressure,and confirm that the pointer does nottravel beyond the 4,800 ft./min. marksetting. Re-adjust if necessary.

12 Static balance ofmechanism

12a Purpose of method

A substantial contribution to theefficiency of the instrument is made bythe static balance of the mechanism, andgreat care has been taken over this inthe design of the component parts. Butsince there are numerous adjustableparts in the mechanism, the exactposition of which cannot be determineduntil the instrument has been calibrated,additional provision is made so that themechanism can be brought into accuratebalance once the final adjustments tothese parts have been made.

This operation is usually carried outhere since the procedure in calibratingat higher ranges has little effect on thebalance.

The adjustment is effected by varyingthe position of a small brass counter-weight mounted on a phosphor-bronze

15

pin on the rocking shaft. The counter-weight is moved along the pin towardsor away from the rocking shaft to alterthe radius of action.

Incomplete balance is shown by thepointer going off zero position when theinstrument is turned round. The balanceis checked by turning the instrumentinto four positions and observing whetherany movement of the pointer takesplace. If this is noticed, suitable correc-tion must be made.

12b Procedure

Disconnect the mechanism from thecalibration stand and remove it from thebuzzer frame. Hold the mechanism ateye level, test dial to the front, andpointer horizontal, and check that thepointer is exactly on zero. If not,remove and re-set. Now turn themechanism round 180 degrees, and ob-serve the position of the pointer. Tapgently while noting this.

If the pointer has moved off zero bytoo great an amount, it will be necessaryto adjust the position of the counter-weight. First touch the counterweightlightly with the finger tip to ascertainin which direction the correction is tobe made, then support the pin andcounterweight with the finger, loosenthe screw, move the counterweight alittle in the appropriate direction andretighten the screw. The screw shouldbe kept at the top of the counterweight.

In this position it is more accessible andhas less effect on the balancing action ofthe counterweight. Re-check to see ifthis has reduced the movement and, ifnecessary, make further adjustments.

Now hold the mechanism with thepointer at zero pointing vertically up-ward. (fig. 29). Tap lightly and observe ifthe pointer remains on zero, then rotate180 degrees until the pointer is pointingdownward, tap again and observe if anymovement off zero takes place. Thecorrection in this instance takes theform of bending the phosphor-bronzepin on which the counterweight ismounted. Once more it will be neces-sary to determine the direction of bend-ing by exerting slight pressure on thecounterweight with the fingertip. Bend-ing should take place in a plane at rightangles to the axis of the rocking shaft.Sketch shows clearly the direction in

which this should be made. No attemptshould be made to bend the pin eitherto left or to right as this will only disturbother adjustments. After bending, re-check and, if necessary, carry out anyfurther adjustments.

Having completed these two opera-tions individually, the instrument shouldbe further checked as follows :

Hold the mechanism at eye level withthe pointer on zero and horizontal.Turn the mechanism 90 degrees, tap,and check position of pointer. Turn afurther 90 degrees, check again ; afurther 90 degrees checking, and finallyback into the original position. If thevariation at any point is too great, theabove individual tests must be repeated.Usually this is not necessary.

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13 Calibration and adjust-ment (contd.)

13a Calibration at 2,000 ft. min.descent

Re-connect to the inclined manometertube, and set the buzzer. Check theprevious calibration at 1,000 ft./min.climb and descent, and if any errorshould be present, repeat the previousoperation. If it were properly carriedout, this should not be necessary.

Now calibrate at 2,000 ft./min. Let ustake descent first, but there is no advan-tage in taking one side before the other.

Increase the pressure and bring theliquid in the column to the appropriatevalue for the metering unit of the instru-ment under test. Maintain at this value.

If the pointer reads 2,000 exactly,proceed to the next test point withoutmaking any adjustments. It is unlikely,however, that this will be the case sincethe positions of the calibrating screwshave been altered during dismantling.

If the pointer indicates a plus readingat 2,000 ft./min., gently screw in thesecond screw from the back of the upperportion of the calibration bracket. Thescrew will then depress the uppercalibrating spring towards the head ofthe calibrating stem, which should fittruly into the hole in the spring. Thespring will be depressed until it reachesthe head of the calibrating stem, whenthe position of the pointer will beaffected. Depression should continueuntil the pointer is moved back exactlyto 2,000 ft./min. Several turns of thescrew may be necessary to achieve this,and great care must be taken as a slipof the screwdriver might damage thediaphragm.

An important point must be madeclear here, and that is that the pressureon the calibrating spring occasioned bya screw being tightened exerts a corre-sponding pressure on the diaphragm,and since the diaphragm is connected tothe inclined column, the level of theliquid in the column will be affected.When any change in the position of ascrew is being made, therefore, the levelof the liquid must be watched andadjusted all the time to maintain it atthe correct value.

With a good diaphragm properly setat 1,000 ft./min., no minus readingshould ever be noticed at 2,000 ft./min.It will be evident from the constructionof the instrument, that to shorten theeffective length of the calibrating springswill make the pointer go back, so thatno amount of working with the screwswill alter a reading to make the pointerplus at 2,000 ft./min.

After obtaining a satisfactory calibra-tion at 2,000 ft./min., re-check at 1,000ft./min. descent. (It is the rule aftereach calibration to return to the previousone and check if it has altered.) If thereading at 1,000 ft./min. descent shouldhave become a minus one as a result ofremoving a plus error at 2,000 ft./min.by adjustment of the second screw,return to 2,000 ft./min., release thesecond screw and re-calibrate at 2,000using the third screw (fig. 30). (The

first screw is seldom used because of atendency to disturb the calibration at1,000 ft./min.) The pointer should bebrought right on by depressing thisscrew. Now re-check at 1,000 ft./min.,descent and it should be observed thatthe minus error at this point has dis-appeared.

There will now be satisfactory calibra-tion at 1,000 and 2,000 ft./min., descent.

13b Calibration at 3,000ft /min.descent

Now proceed to 3,000 ft./min. descent(still leaving the climb side to be dealt

17

with later). If the reading is correct,proceed to the next test point withoutadjustment.

If there should be a plus error, screwin the fifth screw from the fixed end ofthe calibration bracket until it touchesthe spring, and by compressing it bringthe pointer right on the 3,000 ft./min.mark. Now return and check at 2,000.If the 2,000 ft./min. reading should havegone minus, return to 3,000, release thefifth screw and calibrate at 3,000 usingthe sixth screw instead. Return and re-check at 2,000 ft./min.

If the reading at 3,000 showed a minuserror, it is necessary to return to 2,000and recalibrate at that point, takingadvantage of the tolerance permitted,to leave the reading a little plus.

13c Calibration at 4,000 ft. /min.descent

Bring the level of the liquid to theequivalent of 4,000 ft./min. If thepointer is correct, once more no furtheradjustments will be called for. If itshows a plus error, remove this byadjusting the position of the ninthscrew. This can be done with impunitybecause there is no higher test point tobe considered. The seventh, eighth andninth screws (and all the screws on theclimb side) are operated by a tommybar owing to their inaccessibility to ascrewdriver.

When the reading at 4,000 has beenbrought right on, return and re-checkat 3,000. If it is found that the calibra-tion at 4,000 has caused the pointer tominus at 3,000, return to 4,000 and re-adjust the ninth screw until the read-ing at 4,000 is just a little bit plus, butstill, of course, within the permittedtolerance. This will have the effect ofimproving the reading at 3,000.

If the reading at 4,000 should be minus,it is necessary to return to 3,000 andre-calibrate, leaving the reading at thispoint a little more plus. The tolerancelimits of the specification should alwaysbe spread evenly so that there is amplescope for adjustment.

Don't, however, make a practice ofleaving readings at various test points alittle plus to avoid a possible minuserror at the next test point. All read-

ings should as far as practicable be leftwith the minimum obtainable error.

13d Calibration on climb side

If calibration has been satisfactorilycarried out on the descent side, we maynow proceed to effect similar calibrationon the climb side. Procedure is iden-tical with the exception that the screwson the lower side of the calibrationbracket are employed and these are alloperated by a tommy bar owing to theirinaccessibility to a screwdriver (fig. 31).

The notes which are given above underthe different headings are equally applic-able and should be used. No additionalinstructions are required.

In calibrating on climb, the base ofthe head of the calibrating stem comesinto contact with the lower spring, andthe result is similar to the action duringdescent.

14 Re-assembly

Remove the stopper fitted over themetering unit and give the calibratingscrews a touch of Bakelite varnish sothat they will not slacken under vibra-tion. Make sure that a new paper washerhas been fitted to the nipple inside thecase, otherwise the joint between thenipple and the metering unit will not beairtight. This should be done earlier asBakelite varnish takes 24 hours to dry.Confirm also that a new leather washerhas been fitted, or that the old one hasbeen properly treated with castor oil.

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Check that the gauze filter in the nippleis clear of foreign matter.

Fit the mechanism into the case, withthe locating pin in the mechanism plateengaging the slot in the case. Secure themetering unit to the back of the case bythe three screws. Touch them withBakelite varnish to ensure no leakage ofair past them into the case.

Fit the 54-tooth wheel between thezero stem wheel and the pinion in thecase. Take care not to disturb the settingas shown by the mark on the 38-toothwheel while doing this. Replace thespigot, making sure it is seated welldown into the wheel. Secure in positionwith the countersunk screw.

Fit the snap ring to secure the mechan-ism to the case. Place the two ends inthe slots on the mechanism plate andpress the spring down all round into itsgroove with a screwdriver.

Fit luminised dial and fix by the twodial screws. Mount the luminised pointerright on zero. If the pointer has beenre-luminised it must have been correctlyre-balanced also. Fit the inner snap ringassembly, making sure that the locatingpin in the ring is properly fitted into theslot in the case.

Fit retaining ring and a new gasket.Give the gasket a rub with castor oilbefore inserting. This helps to preservethe rubber and assists in making a goodairtight joint. The gasket should beabout an inch longer than the circum-ference of the retaining ring. Pack inwith the point of tweezers (fig. 32). If

it should bulge upward a little in places,this will disappear when the glass ispressed on. This is the next operation.

If the old glass is being used again, rinseit in benzine and remove all traces ofprevious use, and polish carefully onboth sides to remove finger marks.

After the glass, the outer snap ring isfitted. This must be a good tight fit.Bring the two ends together in thecorrect groove at the bottom of the dial,and then run the thumbs round theedge of the case coaxing the snap ringdown into position. If the snap ring istoo loose there will be a leakage of airround the edge of the glass.

The two ends of the snap ring shouldlie together when the ring is in position.If there is a gap, a tighter ring must befitted.

15 Check for leak

Connect the assembled instrument tothe inclined manometer and blowthrough the vent until the liquid isforced up the vertical column to aposition equivalent to 280 mms. ofwater. By means of a clip, grip the tubeand maintain this pressure for abouthalf a minute. If any leak is present, thelevel of the liquid will fall. If the altera-tion in level is within the limit laid downby the specification, then it may beassumed that the case is sufficiently air-tight, and the instrument can be passedfor a bell jar test.

Where a leak is indicated, confirmfirst of all that the clip and all mano-meter fittings are quite tight, and Ifexamination of these confirms that theleakage is in the instrument, open theclip to allow the liquid to be restored tonormal level, and disconnect the instru-ment. Re-check the external screws fortightness, see that the outer snap ringis tight round the glass, and examine thecase for cracks. If this examination stillfails to locate the source of the leakage,the instrument can be reconnected tothe manometer and the pressure re-stored, and a few drops of water or lightoil applied through a fountain-pen fillerto the points mentioned, when bubblesshould indicate the presence of the leak.

16 Bell jar test

This test is an independent check onthe accuracy of the calibration of the

19

instrument and is made by placing theindicator completely cased up andchecked for leak in a frame along withthe sub-standard sensitive altimeter,coupling to a buzzer, connecting to avacuum pump and covering with a belljar.

To check on climb, the buzzer isoperated and air is generally extractedfrom under the bell jar. The controlvalve is adjusted until the pointer ofthe instrument under test indicates1000 ft./min. climb exactly. The handsof the sensitive altimeter should nowbe rotating smoothly.

When the long hand of the altimeterpasses the zero position on the dial,the stop watch is operated and whenthe hand returns to zero after onecomplete revolution (= !,000 ft.) thewatch is stopped. The time in seconds

recorded is compared with the figurein the first column of the table onpage 30, and any error in the instru-ment under test is read off at once.

This is repeated for the other testpoints with appropriate adjustments ofthe control valve and of the number ofrevolutions of the altimeter hand.

As a rule test specifications lay downthe limits of pressure under which thistest has to be made.

17 Tests and tolerancesThe complete tests to be carried out,

and the tolerance limits for calibration,balance, leak etc. have not, in general,been quoted in the above notes. Thisinformation should be abstracted fromthe specification to which the instru-ments are being repaired.

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ILLUSTRATIONS OF PARTS REQUIRED TOBRING UP TO DATE OLD TYPE INSTRUMENTS

Earlier type rate of climbindicator

In June 1942 a minor modification wasmade in the construction of the instru-ments, to permit them to be used onpressure-cabin aircraft.

It is possible that some of these earlierinstruments are still in service. Thechange was made round about serialnumber 16750/41.

Should any instruments be receivedfor repair which bear earlier serialnumbers it should be noted that the

case will differ from the present type(unless a change has been made duringan intervening repair). Supplies of theearlier type of case are no longer avail-able for replacements.

Fortunately the remedy is simple andindeed desirable. A new-type case shouldbe used along with the special pressure-cabin-type glass (which the case is milledto take) and the adjusting pinion shouldalso be replaced.

The instrument will then be convertedto the latest pattern. The case, glassand pinion are shown above.

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TECHNICAL NOTES

Zero setting mechanism

After some little time in service, thediaphragm and other working partssettle down, and tend to produce aslight position error of the pointer atzero. To avoid the necessity for remov-ing the glass and resetting the pointer,a zero setting mechanism is provided.While this is designed so that the move-ment of the pointer effected by it islimited in character, yet the risk is alsopresent of it being used to correct errorswhich may arise in adjustment or calibra-tion, and this risk must be guardedagainst.

The mechanism consists of a pinionlocated in one corner of the case,operated by a small screwdriver. Thisarrangement is to avoid disturbance ofthe setting in flight. The pinion isgeared to a 54-tooth idler wheel whichis held in position by a spigot andcountersunk screw. The teeth of thiswheel in turn engage a 36-tooth wheelassembled on the zero adjusting stem.

The stem passes through the mech-anism body and is held in position by ataper pin (fig. 33). Two washers keep it

friction tight. The stem is placed eccen-trically on a shoulder. This is moreclearly seen in fig. 34. It will be evident

that any movement of the stem willcause a corresponding movement of thecompensating plate and thus vary theposition of the diaphragm slightly, This,by action through the link and the rock-ing shaft, will alter the position of thepointer.

Use of zero adjustmentmechanism

When using the zero adjustmentmechanism to correct a slight error atzero, the pointer should not simply bebrought to zero and left at that. Acheck should be made to determinewhether any previous adjustment hasbeen carried out. It is obvious that anaccumulation of adjustments suggestssome fault in the calibration which willnot be corrected by bringing the pointeron zero.

Therefore, first of all take a smallscrewdriver and turn the zero settingmechanism to its point of minimumeffect. This will reveal the total errorwhich it is necessary to correct. If itis small, remove it by adjusting theposition of the stem. If, however, it isvery large, then test the calibration atthe various points before returning theinstrument to service.

Failure to understand the limits andthe correct use of the zero settingmechanism causes many instruments tobe reported unserviceable.

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Calibration and adjustment

Satisfactory calibration can only beachieved by a certain amount of trialand error. This is due to the varyingcharacteristics of the diaphragm whichcannot be predetermined.

It is not practicable to produce alarge number of diaphragms of thecorrect flexibility without externalinfluence being necessary to harness themovement, hence the diaphragm isdesigned and constructed so that it hasa freedom in excess of functional re-quirements. This enables calibration tobe effected by restraining the excessmovement. This restraint, however,does not come into operation until the2,000 ft./min. point is reached.

It will be seen, therefore, that if amechanism cannot calibrate satisfac-torily at 2,000 ft./min. because of largeminus readings, without the restrictinginfluences, a faulty diaphragm is indi-cated. In repair work this possibilitydoes not arise to the same extent, asthe selection of the diaphragm fororiginal assembly has been carefullycarried out, but a fault occasionallydevelops in use, and produces similareffects.

Movement of the diaphragm is con-trolled from 2,000 ft./min. by the actionof two calibrating springs located be-tween the arms of the calibration bracket.The movement of the springs in turn isrestricted by the selection of the positionof the screws on the bracket. The effec-tive length of the spring is determinedby the point of contact of the screws,and calibration consists of finding themost suitable position for these screwswhich will permit the diaphragm toexpand and contract in such a way thatthe pointer registers within the termsof the specification being used.

In practice, only about three screwson each arm are required, but nine areprovided to give an ample reserve ofalternative positions. The screws areformed into three groups. The first,second and third screws from themounting end of the bracket are usedfor calibrating at 2,000 ft./min., thefourth, fifth and sixth for 3,000 ft./min.,and the seventh, eighth and ninth for4,000 ft./min. Not more than one screw

in each group can be employed. Calibra-tion involves determining which screwin each group gives the best results,because in addition to their primaryeffect on the reading at the point undertest, they have a secondary effect on thereadings at previous test points. Thisis made clear in the notes on Procedure.

It has been found that the best cali-bration is obtained when the top screwsin each group can be used, that is, thethird, sixth and ninth screws. This, ofcourse, is not always practicable.

The upper calibrating spring comesinto play during descent, and the lowerone during climb.

Calibration is one part of repair workwhere logical study of the actions andreactions can repay a hundredfold.There is nothing supremely difficultabout it. There are no secrets which arenot unfolded. The problems likely to beencountered are described as fully aspossible. But it is usually during cali-bration that previous omissions becomeevident, and due attention to the cau-tions give in the earlier part of this hand-book will avoid the disappointmentsand delays which would arise at thisstage if they had been neglected.

It is impossible in these brief notes tocover every possible contingency, butthe principles are laid down and theinterpretation and application of themmust be left to the individual. A certainamount of experience will be necessarybefore complete confidence can beexpected.

Diaphragm dive overload stop

A dive overload stop is fitted toprevent damage to the diaphragmshould it be subjected during descent tosudden air-pressure differences much inexcess of the maximum it is designed toregister. It consists of a little brass stopfixed to the underside of the calibrationbracket so placed that when the dia-phragm expands, the centre piece touchesit and further expansion is prevented(fig. 35). It is secured by a screw fittedthrough an elongated slot which per-mits a measure of adjustment of positionto be effected. The position of the stop,therefore, determines the limit of ex-pansion of the diaphragm. To prevent

23

undue strain on the diaphragm, theoverload point is taken as being dia-metrically opposite to the zero positionon the dial, that is, in a position approxi-mately equivalent to 4,800 ft./min., andthe stop has to be set so that the pointerwill not exceed this under any conditionof use.

Diaphragm climb overload stopThe climb overload stop consists of a

screw which passes through the dia-phragm mounting piece into the dia-phragm, and is held in position by a smallhexagon nut (fig. 36). Its function is to

prevent the diaphragm from collapsingcompletely at overload.

Diaphragm and metering unitFor servicing purposes these are con-

sidered as a single unit, but both have aseparate function and it is desirable tounderstand these.

(a) The diaphragmThe pointer, through the rocking

shaft and the link, measures the expan-sion and contraction of the diaphragm.Very small pressure changes have to beregistered, therefore the diaphragm hasto be of an extremely sensitive nature.It is constructed of a very fine brass foilmade to a stringent specification. Theformed blanks are selected and speciallygraded with great care before assembly.

The diaphragm is connected by ashort length of small bore tubing to themetering unit. The loop observed inthe tubing is simply to permit slightaccommodation in fitting. It will beevident that the diaphragm movementis affected by :—

(a) The air pressure inside the dia-phragm (received direct from thenipple through the tubing) incombination with

(6) the air pressure outside the dia-phragm (received through themetering unit), and

(c) the mechanical control exercised toregulate the movement and permitcalibration to specified limits.(This last point is dealt with morefully in the section on Calibration.)

A fine gauze filter is located in thenipple to prevent as far as possible theingress of dust, sand, or other foreignmatter in the diaphragm or meter ing unit.

(b) The metering unitThe design of a suitable metering unit

(fig. 37) is based on several physical

24

considerations. It should be remem-bered that the reading of the instrumentdepends on the pressure difference be-tween the inside and outside of thediaphragm, that is, on the pressuredifference across the metering unit.The metering unit is therefore requiredto give a definite pressure difference forany given rate of climb under differentconditions of temperature and pressure.

In general a metering unit requirescompensation to effect this result. Com-pensation for changes in pressure at agiven temperature is spoken of asaltitude compensation, and compensa-tion for changes in temperature isspoken of as temperature compensation.

The compensation is often mechanicalin nature, but in the K.B.B. instrumentit is obtained without any moving partsby the combination of two basic types ofleaks, (a) an orifice leak, and (b) acapillary tube leak. We will now con-sider the performance of these separ-ately, dealing first with altitude com-pensation and then with temperaturecompensation.

In the case of a capillary leak, it canbe shown that the pressure differenceacross the leak for a constant rate ofclimb increases with decreasing pressure,

at a constant temperature. The instru-ment will therefore show a positiveerror for decreasing pressures. In thecase of an orifice leak, the pressuredifference for a given rate of climbdecreases with decreasing pressure, andthe error is therefore of an opposite signto the error caused by a capillary leakalone. A combination of the two types canbe chosen to give a satisfactory pressurecompensation at a given temperature.

With regard to the temperature com-pensation, the pressure difference acrossthe capilliry leak depends on the vis-cosity of the air, which is proportionalto the absolute temperature, and there-fore decreases with decreasing tempera-ture. In the case of an orifice leak thepressure difference depends on thedensity of the air, and will thereforeincrease with decrease of temperature.It is seen that this effect is opposite tothe effect of temperature on the capil-lary leak, so that compensation can beobtained by a combination of the twotypes of leaks.

In the K.B.B. instrument, the sizes ofthe capillary and orifice leak are chosenso that the readings of the instrumentwill remain correct over a wide rangeof temperature and altitude conditions.

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CALIBRATION APPARATUSThe successful calibration of the rate

of climb indicator requires the provisionof a calibration stand and a leak-testingapparatus, together with a bell jar,frame, stop watch and sensitivealtimeter.

THE FRAME need only be a simplewooden erection capable of holding oneor more rate of climb indicators, and inaddition a sensitive altimeter. It shouldbe provided with a buzzer or othermeans of producing slight vibration toovercome the fictional resistance of themechanisms.

THE BELL JAR. must be sufficientlylarge to cover the frame comfortably.

ANY STOPWATCH of good qualitywill be satisfactory. The duration of eachtest is normally comparatively short sothat only a. large error would affect thereadings.

THE SENSITIVE ALTIMETER shouldbe specially calibrated within plus orminus 10 ft., tested at every 1,000 ft.from minus 1,000 ft. to plus 10,000 ft.As the Instrument will not be used fortest purposes above this limit, accuracyat higher points can safely be sacrificedto produce special accuracy at the lowerpoints.

The remaining items of apparatus aresomewhat special in character and it isnecessary to deal with them in greaterdetail.

Inclined manometer calibra-tion stand

A calibration stand of the type illus-trated (fig. 38) will be found very suit-

able. The apparatus comprises inclinedmanometer, rubber tubing, T connec-tions, bellows, clip, wooden frame.Accurate levelling is essential and ade-quate provision must be made for thisby fitting two spirit levels and adjust-able feet during construction.

Inclination of the tube of the mano-meter allows greater movement ofliquid for the same difference of pressurethan would be possible in a standardvertical manometer, thus permitting amore open scale.

The angle of inclination may be about12 degrees. The magnification of thescale varies with the cosecant of theangle with the horizontal, and in the caseof 12 degrees would be nearly five times(actually 4 8I) giving approximately aninch on the scale for every Five milli-meters of water. It will be evident froma table of cosecants that one degreedifference of angle will make a substan-tial alteration to the scale, so this angleshould be measured carefully.

The scale is graduated in millimetres,taking advantage of the magnificationfactor provided by the inclination of thetube, and should be set so that thedivisions are at right angles to the glasstubing. The reading is taken where themeniscus of the liquid touches the line.It is useful to draw a horizontal line inred ink on the scale directly behind thetube. This is not evident in the portionwhere there is no liquid, but it is greatlymagnified by the liquid and facilitatesreadings by giving the liquid the appear-ance of red fluid.

Owing to the very light pressuresemployed, no special precautions needbe taken against leakage. The cisternshould be conveniently placed on oneside of the frame so that the level of theliquid can be adjusted to bring it abouthalf-way along the inclined portion ofthe tube.

The capacity of the bellows requiredcan only be determined accurately byexperiment since it will depend uponthe nature of the apparatus used. It hasbeen found, however, that two olddiaphragm assemblies from Mark XIVAaltimeters mounted in series usually

26

give sufficient variation to operate thecolumn under normal conditions.

In practice a light oil (such as Duck-ham's S U 12) is employed in preferenceto water, but the scale may still be cali-brated in mms. of water by allowingfor the difference in specific gravities.The level of the liquid in the canisteris adjusted until the level in the tubeunder atmospheric pressure touches thezero line.

The use of the calibration stand isfully dealt with in the section ' Calibra-tion and Adjustment.'

Leak-testing apparatusIt has been mentioned in the Tech-

nical Notes that the variable factor incalibration is the performance of thediaphragm, and that it is not possible topredict this.

On the other hand, the characteristicsof the metering unit do not vary incalibration. They can therefore bedetermined in advance and the meteringunit can then be sealed off and thediaphragm calibrated alone, due allow-ance being made for the effect whichthe unit will have on the mechanism.

This method offers many advantagesand is, in fact, the procedure adopted.The outstanding benefit is that themechanism can then be calibrated with-out the necessity for being fitted intoan airtight case.

A leak-testing apparatus is requiredfor this work, but fortunately it is simpleand inexpensive to produce. The appara-tus consists of a standard leak, and ameans of comparing with it leaks ofunknown values.

The rate of climb indicator requiresa metering unit of a given characteristicand the purpose of the apparatus istherefore to grade the metering unitsaccording to the effective size of theorifice and capillary.

Three grades are employed, and theleaks are sorted as follows :—

(1) Those smaller than standard ;(2) those equal to standard ;(3) those larger than standard.The tolerances of these are, of course,

strictly limited and any which are out-side the limits are rejected. For eachgroup a different set of scale readings is

necessary on the calibration frame. Oncethe group has been ascertained, thencalibration can proceed, provided thescale readings for the calibration framehave already been worked out.

For the leak-testing apparatus, astandard leak is required, and this mustconform to exceptionally close limits.The makers of the indicators can providesuch a standard leak against a specialorder for a comparatively small cost. Asecond one would be required if it wereintended to test Mark IA in addition toMark IB indicators, since the design ofthe former is quite different from thatof the latter.

The apparatus consists of two U-tubesin series as shown in fig. 39. If the

airflow through both leaks is identical,then the readings in both tubes will bethe same.

If, however, there is any difference,the pressure will tend to build up in theone and fall off in the other, which willbe immediately evident by a differencein the U-tube readings. The U-tubescould, therefore, be graduated in sucha way that this difference could be readoff in any convenient scale of units.

This is the result which is achieved bythe apparatus, although to permit easiercomparison the tubes are fitted close toone another, and other connections aretaken to more convenient and less con-spicuous positions behind the panel.The source of pressure is a pump and abell jar reservoir. This is required forproviding a constant pressure for theperiod of the test, and any other means

27

may not be so successful. The tubesmust be so constructed that a verticaldifference of at least 60 millimetres canbe measured on the scale.

Setting up

Connect up the apparatus as shown inthe sketch (fig. 39). It can be built in amanner similar to the calibration frame.It will probably be more advantageousto have the inclined portion of the tubeat a less acute angle to the horizontalthan in the case of the calibration frame,because the pressure difference to bemeasured is greater.

Prepare a scale for the inclined tube,marking it in divisions equivalent tomillimetres, taking into account theangle of inclination of the tube. The zeroshould be in the middle and the rangeshould extend to about plus and minus30 millimetres. Fit this scale convenientto the inclined tubes. Fill the cisternswith a light oil such as Duckham's SUI2until the liquid reaches the zero mark.If the apparatus is open to atmosphericpressure the level in both columns willbe the same. Connect the leak undertest.

Procedure

Bring the level of the column con-nected to the standard leak to 30 mms.pressure, and take the correspondingreading in the column connected to theleak under test. Now apply vacuum andbring the reading to the 30 mm. markat the other end. Again take the appro-priate reading.

Tolerance limits

The leak is then sorted out accordingto the results obtained, in the followingmanner :—

(1) Leaks that are within —½ to—2½ mms. (larger than standard) ;

(2) leaks that are within plus orminus ½ mm. (equal to standard) ;

(3) leaks that are within plus ½ toplus 2½ mms. (smaller than stan-dard).

For acceptance the leaks must notonly be within plus or minus 2½ mms.but the difference between the readingsobtained at the two test points mustalso not exceed 1 mm.

To determine the readings forcalibration-frame scale

Select half a dozen good mechanismswhich have leaks corresponding to oneof the three groups already mentioned,as determined by the leak-testing appar-atus—say the standard leak.

Connect these in turn to the calibra-tion frame and calibrate them in accor-dance with the standard practice de-scribed in pages 13-18 using the following' average ' set of readings :—

1,000 ft./min. ... 5 mms. water.2,000 ' 103,000 204,000 30

After this provisional calibration,case up the mechanisms, paying specialattention to the question of case leak.Care must be taken that the risk of errordue to case leak is reduced to theabsolute minimum.

Now place one of the instrumentsunder a bell jar with a sub-standardsensitive altimeter and check as follows :

Adjust the pressure until the pointerregisters 1,000 ft./min., and control theflow so that the pointer is kept exactlyat this point. The long pointer of thesensitive altimeter should be rotatingsteadily, indicating a constant rate ofclimb.

When the pointer passes the zeroposition, set the stop watch in motionand when the pointer completes onerevolution (equal to 1,000 ft.) stop thewatch.

The error in the instrument (if any)can be found at once by reference to thetable on page 30. For example, if thestop watch indicates 61 seconds for1,000 ft. it is clear that the instrumentis recording high and from the tableswe find there is a plus error of 16 ft./min. This means that the actual rate ofclimb of the instrument when it records1,000 ft./min. is 984 ft./min.

Now check the readings at 2,000,3,000 and 4,000 ft./min. in the same

28

fashion. Repeat for the remaining fiveinstruments.

Connect up the first instrument to thecalibration stand and adjust the pressureuntil the pointer indicates the truespeed obtained at 1,000 ft./min. undertest in the bell jar by the stop-watchmethod, and note the correspondingreading on the manometer. Proceed to2,000 ft./min. and to the other testpoints and record the corresponding

manometer readings. Repeat With theremaining five instruments.

There will now be six sets of readingsfor the inclined column scale. Theseshould be averaged and the resultantset of pressures accepted as representingthe scale readings for an instrumentwith a standard leak.

The same procedure will permitsimilar scale readings to be obtained forleaks of the larger and smaller sizes.

29

TABLE FOR USE WITH STOP WATCH FOR DETERMININGERROR IN RATE OF CLIMB INDICATORS (See Page 20)

SMALL TOOLSAbove is illustrated a typical kit of

small tools containing items founduseful for the repair of K.B.B. Rate ofClimb Indicators. The majority arestandard articles readily available. In afew instances a little modification isrequired.

The special tools can be preparedwithout difficulty in any small workshop,and their production is encouraged inview of the valuable purpose which theyserve. In these instances additionalguiding notes have been appended.

The tools shown comprise :—1. Special key for metering unit secur-

ing screw. Used when dismantlingor re-assembling the unit (seefig. 19 and note below).

2. Hand lever for holding case pinionagainst tension of spring when re-moving cross pin holding zeroadjusting stem during dismantlingof case assembly (see fig. 6 and notebelow.)

3. General purposes screwdriver.4. Light hammer.

Long-nosed pliers.

Top cutting nippers for trimmingnew outer snap rings to size, etc.

Penknife for removing outer snapring, etc.

T wrenches for holding broaches orsmall taps. For general work, onewrench is sufficient.

Pin tongs for holding small broaches.

Clip for pinching rubber tubing ofcalibration apparatus.

Smooth-cut pillar file about 6" long.

Pointed rod for removing innersnap ring (see fig. 1).

Range of small broaches for use. with pin tongs.

Sector key for moving sector inrelation to rocking shaft (see fig.25 and note below).

Closed 6BA spanner for tighteningor slackening nut on stiffeningplate (see fig. 10).

5 .

6.

7.

8.9.

10.

11.

12.

13.

14.15.16.17.

18.

31

Special-purpose screwdriver andbox spanner for adjusting andlocking climb stop (see figs. 16,28 and note below).

Tommy bar for movement of cali-bration bracket screws (see figs. 11and 31).

Special pivot-straightening tweezers(see fig. 23 and note on page 33).

General-purposes tweezers.Caliper tweezers for holding pinionswhile cleaning with brush, etc. (seenote below).

Watchmaker's screwdriver.Elder pith for cleaning pivots.Pegwood for cleaning bearing holesand jewels. Orange stick if avail-able makes the most satisfactorypegwood.

Small hand vice.Double ended pin tongs with twocollets for holding broaches. These

may be used as an alternative tothe tongs shown at item 10. It isnot essential to have both.

30. Combined pivot file and burnisher.

31. Half-round file.

32. Rubber suction pad for removingobstinate glasses, fitted- with asmall metal handle to allow greaterpurchase.

33. Camel-hairbrush.

34. Watchmaker's four-row bristlebrush.

35. Glass brush.

In addition to the items shown, awatchmaker's eyeglass (2½" or 3") isindispensable and a standard staking toolwith set of punches is also required. Asupply of cleaning fluid in a suitable con-tainer must always be at hand. For testpurposes an old dial is required alongwith a properly balanced pointer.

32

19.20.

2 1 .

22.

23.24.

25.26 .27 .

28.29 .

SPECIAL TOOLSThe following notes amplify the in-

formation which may be obtained fromthe photograph on page 31. It has notbeen considered necessary to givedimensional sketches because of thesubstantial tolerances permissible inconstruction. No difficulty will beexperienced in making up any of thetools in question.

Key for metering-unit-securingscrew

Made from ¼" dia. steel rod with twokeyways to fit into the slot in the headof the screw which is 1/32" wide, cutaway in the centre to clear the capillary.• A simpler tool can be made from ascrewdriver ground down and cut toavoid the capillary as before.

Hand lever for case pinion

May be constructed from an old screw-driver by heating the blade, bending atright angles, then sharpening.

Sector key

Cut to fit over the tail of the sectorand made to accommodate the shoulderon the rocking shaft. The dimensionsare not critical but the key must be agood fit to get adequate leverage withoutdistorting either sector or rocking shaft.

Pivot-straightening tweezers

Any-small size tweezers can be con-verted for this purpose. The noseshould be filed down to give a flatsurface, then the width of the bladesreduced to allow them to enter thecavity in the mechanism plate. Thecavity is just over 0 · I " in diameter. Itwould probably give additional grip onthe pivot if the inside of the tweezerblades were slightly grooved.

Climb-stop screwdriver andspanner

This combination comprises a boxspanner to fit a 10BA nut and a long-bladed screwdriver, fine enough to enterthe spanner and engage the slot on thehead of the screw, which is approximately0·11" in diameter.

The length of spanner and screwdrivercan be varied to suit individual require-ments. A milled head on the spanneraffords additional purchase.

Caliper tweezers

It is better to select blunt-nosedtweezers. They are prepared by drillinga small hole in both blades so that thepivots of a pinion will rest comfortably.

33

INSTALLATIONTo install Rate of Climb Indicators

Mark IA, Mark IB and Mark IB*, cut ahole in the instrument board and drillthree holes, as shown in the left-handsketch.

Mount the instrument in the boardfrom the rear and secure in place withthree screws and locknuts.

Connect the nipple on the back of thecase to the static side of the pitot tubethrough a T connection.

RATE OF CLIMB INDICATOR.

34

LISTS OF SPARESIn the following pages will be found

lists of the principal spares which it isrecommended should be carried for theadequate servicing of Rate of ClimbIndicators Mark IB*. These parts areusually in stock and can be had from themakers practically on demand.

In the event of desiring spares whichare illustrated but not included in the

lists, it is sufficient to quote the partnumber when ordering, but it is alsohelpful if the serial number of theinstrument can be given.

Adherence to these lists is encour-aged, as special fixtures are frequentlyrequired to assemble certain of theparts, and these tools are not generallyavailable outside the factory.

35

Recommended list of spare parts forformer and also present types ofrate of climb indicators Mark IB*

Rocking shaft assembly . . . . . SA 309

Diaphragm and metering unit assembly . . SA 1432

Calibration bracket assembly . . . . SA 1426

Mechanism plate jewel assembly . . . SA 1924

Hand assembly (not luminised)† . . . SA29II

Dial (not luminised)† . . . . . P 1368

Glass sealing gasket . . . . . P 6023

Gear spigot . . . . . . P 3855

Outer snap ring . . . . . . P 4196

Spigot mounting zero setting wheel . . P 1541

Gasket between metering unit and nipple . . P 6025

Small jewel mounted . . . . . P 3178

Handstaff Pinion . . . . . . P 1621

Washer P 5650

Calibrating spring . . . . P 4670

Large jewel mounted . . . . P 3177

Handstaff jewel setting assembly . . . SA 1922

Link P 2526

Retaining ring . . . . . . P 4358

† When a hand assembly is obtained unluminised, care should be taken to see thatit is property balanced after it has been luminised, otherwise errors will be intro-duced in the performance of the indicator. When ordering for stock the handassembly and dial should be obtained unluminised by quoting the part numbersshown in the list. If for immediate use, however, they may be obtained luminisedby requisitioning as follows:

Hand assembly, luminised . . . SA 2912

Dial, luminised SA 3449

A dial can be supplied with serial number engraved complete if the appropriatenumber is given when ordering.

SCREWSCounterweight securing screw

Calibration arm securing screw

Calibration bracket adjustment screws

Do. Do.

Do. Do.

Dial screw . . . . .

Zero setting spigot mounting screw .

Screw for securing metering unit to case

Bridge mounting screw

36

. P

. P

. P

. P

. P

. P

. P

. P

. P

1025

9981009

1010

1011

1001

1014

1015

1017

36

SCREWS (Contd.)Jewel clamp screw .

Diaphragm plate mounting screw

Calibrating spring mounting screw

Calibration bracket mounting screw .

Dive overload stop securing screw

Stiffening bracket mounting screw

Do. Do.

Diaphragm mounting screw

. P

. P

. P

. P

. P

. P

. P

. P

1018

1019

1021

1022

1023

862

984

999

The following parts are applicable only to the former type of indicator:—

C a s e a s s e m b l y . . . . . . S A 2 6 I 2

G l a s s P 5 9 5 6

It is recommended to use instead the new toughened glassP5988 which is interchangeable, and which will withstand ahigher pressure difference than P 5956.

The following parts are applicable only to the present type of indicator:—

37

Case assembly . . . . . . SA 2635

Glass . . . . . . P 5982

NOTE.—Bearing pins P 4018 and P 4019 are not included in the above lists. Thesecan be made up as required from Nichrome wire 0·014" and 0·008" diameterrespectively. We have also omitted such items as nuts, washers, etc., of standardBA sizes.

RATE OF CLIMB INDICATOR MARK IB*

38

RATE OF CLIMB INDICATOR MARK IB*

39

VIEW FROM REAR - CASE SECTIONED40

RATE OF CLIMB INDICATOR MARK IB*(OLD TYPE)

RATE OF CLIMB INDICATOR MARK IB*(MODERN TYPE)

VIEW FROM REAR CASE SECTIONED41