Focal · The engine will, of course, smoke and vent abnormally during the first few minutes after starting when the sumps have been flooded. Start the engine in low-speed ground idle

A SERVICE PUBLICATION OFLOCKHEED AERONAUTICALSYSTEMS COMPANY

EditorCharles 1. Gale

Art DirectorAnne G. Anderson

Vol. 18, No. 3, July-September 1881

CONTENTS

Focal PointJim Stewart,Subcontracts AdministratorSubcontracts and Machined PartsDepartment

T56 Engine Oil SystemMaintenanceA comprehensive guide to engineoil loss prevention.

Photographic Support: John Rossino

Cover: The first of more than two dozennew C-130 airlifters planned for acquisitionby the U.S. Naval Air Reserve, this C-130Thas been assigned to recently reactivatedVR 54 Squadron. The USNR unit is based atLouisiana’s New Orleans Naval Air Station.

The back panel shows another view of VR54’s brand-new Hercules, this time over theCrescent City’s famed sports facility, theLouisiana Superdome.

Focal

A Special Partnership

Allison-manufactured 501/T56 turbopropengines have been used exclusively to powerHercules aircraft from the YC-130 to the aircraft’spresent configuration, the Advanced C-l 30H.This is neither happenstance nor merely an artifactof previous practice. It is the consequence of aspecial, ongoing partnership between two compa-nies which occupy unique positions of leadershipin the world of aerospace technology.

Allison Gas Turbine Division has workedcloselv with Lockheed over the years to provide a

Jim Stewart power plant whose performance criteria’ meet allof the mission profile requirements for the various

C-l 30 and L-l 00 models that have evolved. The result is a quality productthat offers a constantly improving record of performance, durability, andefficiency.

This issue of Service News features an article which details proventechniques that maintenance personnel can use in troubleshooting the5Ol/T56 engine’s lubrication system. This valuable guidance from Allison isan example of that company’s commitment to working jointly with Lockheedto provide exemplary support for the operation and servicing of its productson Lockheed aircraft.

Allison’s commitment to close support of Lockheed products alsomanifests itself in a very literal way. The engine manufacturer’s SoutheasternZone Office is physically located on the grounds of Lockheed’s Mariettamanufacturing facility. From this vantage point, the Allison team provides 24-hour-a-day direct technical service and product support for Lockheed’sHercules production effort, and for a number of Hercules operators as well.

This kind of close, working partnership makes a difference that counts.It helps transform policy and promises into the kind of solid, hands-on productsupport whose value can be measured. It is just one more way that we atLockheed strive to ensure that each and every Lockheed customer realizes themaximum benefit from the quality and capability that is built into everyHercules airlifter.

Sincerely,

mes H. Stewart,Subcontracts AdministratorSubcontracts and Machined Parts Dept.

H. L. BURNETTE DIRECTOR

FIELD SUPPLYSUPPORT SUPPORT

TECHNICALPUBLICATIONS

RM&S CUSTOMERDESIGN TRAINING

J. D. Adams J. L. Bailey A. G. Hunt H. D. Hall S. S. Clark

A Guide to Oil Loss Prevention

by Leon D. Smith, Manager, Southeastern ZoneAllison Gas Turbine Division

The lubrication system of the Allison 5Ol/T56 en-gine has proven itself effective and reliable in literallymillions of flight hours during more than three decadesof service. Like all mechanical systems, however, prob-lems can and do occur.One lubrication system problemthat the T56 engine maintainer may encounter sooner orlater is unexplained oil loss.

Engine oil loss, excessive consumption, or just plain“dumping” that has no obvious cause can challenge theexpertise and test the patience of the most experiencedpower plant specialist. Furthermore, there are poten-tially so many factors involved that it is difficult to giveadvice that will be both general enough to cover all casesand specific enough to be useful. But there are someprocedures that should be followed and things to look forthat can help. Please note that this discussion is opti-mized for Allison Series III engines like the 501-D22Aand T56-A-15 Older engines, although fundamentallysimilar, may require some modification of the proce-dures described.

Lockheed SERVICE NEWS V18N3

It is also important to bear in mind that the informa-tion contained in this article is not intended to replacecurrently approved technical data or supplant acceptedmaintenance procedures. This presentation is merely acompilation of proven maintenance techniques designedto improve the efficiency of your troubleshooting effort.The concepts are based upon the ideas and experiencesgained by Allison product support engineers in manyyears of close collaboration with U.S. military units andother 501/T56 engine maintainers around the world.

System Description

To be able to troubleshoot an oil loss problemefficiently, one must first be thoroughly familiar with theentire engine oil system and know how it works. Mostimportant, the troubleshooter must know what influenceeach component has on the total system. This includesthe nacelle (QEC kit) components as well as the basicengine parts.

The overall 501/T56 engine lubrication systemconsists of two essentially independent systems (Fig. 1).They are the pressure lubrication system and the dry-sump scavenge recovery system. These two systems,commonly known as the “pressure side” and the “scav-enge side,’ actually have only one thing in common, and

3

that is the oil they pump. They do share a few mechani-cal devices, however, which we will describe later on.

Each of the two systems is made up of two subsys-tems. One is for the reduction gear assembly and theother is for the engine power section. The reductiongear assembly thus has its own pressure system and scav-enge system, as does the power section. Althoughtechnically separate, the four subsystems do interrelateboth systemically and mechanically to one degree oranother.

Two independent pressure pumps, one for thereduction gear assembly and one for the power section,move oil from a common oil supply tank through theirrespective distribution systems and provide lubricationand cooling to the various internal surfaces. Two dry-sump scavenge systems route recovered oil back to theoil tank through a common return system.

On the way back to the supply tank, the oil is filteredand cooled. At this point the oil is a heavily aerated,foamy mixture. When it reaches the tank, the oil isseparated from the air it has entrained in its passagethrough the lubrication system. The oil drains into thetank and the air escapes through the tank pressurizingvalve and out the nacelle drain mast to the atmosphere.

A Troubleshooting Overview

When the problem is oil dumping, the pressure sidesof these systems can be virtually eliminated as possiblesources of the trouble. Remember that the pressuresystems supply oil to the engine. When the symptom isdumping, the problem is not that oil is not being sup-plied, but that it is not being removed from the engineand returned to the tank.

About the only time the pressure side can causedumping is when one or more of the oil jets have en-larged metering holes. This could allow more oil to flowinto the engine than can be pumped out by the scavengerecovery system. This kind of problem does not just“happen,” however. It is normally discovered on thefirst run after an oil jet has been replaced.

Another possibility is that oil is simply leaking fromthe engine before the scavenge system can recover it.The troubleshooter must remember that every oil dump-ing case is different. The component that was replacedto correct the most recently encountered oil problem maynot be, and probably will not be, the culprit the next timearound. Every step in the isolation procedures must beaccomplished in each and every case in order to isolateand identify the faulty component.

The reports given by the flight crew are often crucialin getting an investigation off on the right foot. Forinstance, they may mention that the indication of oil loss

Figure 1.

occurs only at high altitude. Or, that oil dumping occursonly during a nose-up attitude. Here is a list of questionswhose answers can be most helpful in isolating the causeof an oil dumping problem.

1.

2.

What was the oil consumption rate?

Was the oil leak visible to the crew? If so, fromwhere?

3.

4.

5.

Was the oil loss evident only at high altitude?

At what altitude did the oil dumping occur?

After descent, at what altitude did the oil dump-ing stop?

6.

7.

How long into the flight did the dumping begin?

What was the aircraft attitude when the dump-ing began?

A Lockheed SERVICE NEWS V18N3

8. Did the oil dump begin immediately after initialstart up?

9. Does the oil dump while the aircraft is on theground, as well as in flight?

10. Was there a rise in oil temperature before orduring the dumping incident?

11. Did the affected engine require the oil coolerflap to be opened more often than the otherengines to maintain proper oil temperature?

12. Was there a noticeable reduction of oil quantityon the oil quantity indicator before the actualdumping began? If so, how much did the quan-tity drop and how long did it take before thedumping began?

Oil loss, oil consumption, or dumping problems canusually be grouped into three broad areas: static oilleaks, internal leakage, and scavenge system ineffective-

Lockheed SERVICE NEWS V18N3 5

ness. Let us take an in-depth look at each of thesecategories individually.

Static Oil Leaks

Static oil leaks usually show up after an engine hasbeen sitting unused for a period of time.The period canbe anything from a few hours to several days. Note thedistinction in terminology between static leaks andinternal leakage. Static leaks occur in an engine at rest,while internal leakage shows up only during actualengine operation.

The most common cause of static leakage, alsocalled gravitational flooding, is an improperly func-tioning oil retention check valve. This may occur eitherin the power section oil filter assembly or in the reduc-tion gearbox oil pump assembly (Fig. 2).

It should be kept in mind that gravitational floodingcan be a transient condition, caused by a particle offoreign material temporarily lodged in the seat of one of

Figure 2. Improperly functioning oil retention check valves are a common cause of static oil leakage.

the oil retention check valves. Sometimes there will beevidence of oil dripping out of the bottom compressorbleed valves, and sometimes there will be oil in the tailpipe or intake. On the other hand, there may be noexternal indication of oil leakage at all.

When a static leak is suspected, perform the follow-ing confirmation check. Verify that the oil quantity gageof the affected engine shows at least four gallons presentto provide lubrication, and then motor the engine briefly.

NOTE: Do not add oil to the tank if the oil levelis below full, but above four gallons.

If the oil quantity increases significantly during themotoring process, it is likely that an excessive amount ofoil has found its way into the sumps and is being scav-enged back to the supply tank during the motoringprocess. We know this problem can be temporary, so ifthere is no indication that the engine is a repeat offender,it should be run to clear it of foreign material andchecked again. The engine will, of course, smoke andvent abnormally during the first few minutes afterstarting when the sumps have been flooded.

Start the engine in low-speed ground idle. Thenclose the bleed air valve so that the bleed air from whatis probably a well-oiled compressor will not enter theaircraft pneumatic system. Operate the engine at flightidle or higher until the engine is clear of the blue smokefrom the residual oil. Note that it is a good idea tocompressor-wash the engine before returning it toservice to get rid of all the unwanted oil film and avoid

the possibility of fumes entering the air conditioningsystem.

The oil retention check valves are not always theculprits; there are other components in the system thatcan cause persistent static oil leaks. One or more of thefollowing components may contribute to the problem:

l

l

l

l

l

6

Power section oil filter O-ring deteriorated ordamaged.

Power section oil filter check valves not seated.

Power section oil filter bypass valve not seatedproperly within the accessory drive housingfront cover.

Power section oil pump pressure regulatingvalve not seated.

Power section oil pump pressure regulatingvalve O-ring at valve outside diameter deterio-rated, damaged, or missing.

Power section oil pump drive and idler shaftseals between the pressure and scavenge pumpgears damaged, deteriorated, or missing.

Reduction gear assembly oil pump check valvenot seated.

Reduction gear assembly oil filter too long,causing oil pump check valve to remain off seat.


l Reduction gear assembly oil pump drive shaftseal deteriorated, damaged, worn, or missing.

If static oil leaks become a persistent problem witha particular engine, further in-depth troubleshootingmust be accomplished to identify and correct the mal-functioning component. There are effective isolationprocedures in the maintenance manuals which will helpyou identify and locate the specific causes of static oilleaks. Following through on these procedures will solvemost static oil loss problems.

system. When an engine consumes or dumps oil, it willoften do so after accumulating oil in one or more of threemajor areas: (1) the combustion assembly inner cas-ing-the “lighthouse” assembly, (2) the reduction gearassembly, and (3) the accessory drive housing (Fig. 3).

To troubleshoot an oil loss problem effectively, thetroubleshooter must first determine whether or not theengine is actually accumulating oil. The followingprocedure is designed to ascertain if oil is being accu-mulated, and if so, where.

Internal Leakage vs. Scavenge Problems Creating a ‘Known Condition”

If static leaks can be ruled out as a source of oil loss,which of the remaining choices is it? Internal leakage oran ineffective scavenge system?

When an engine accumulates oil before it dumps, itgenerally points toward some sort of scavenge problem.There are other situations, however, in which the enginewill lose oil but not accumulate it anywhere. If noaccumulation has occurred, we know oil is being scav-enged. In such cases, it is likely that the oil loss iscaused by internal leakage somewhere in the powersection.

To be able to troubleshoot an engine that may ormay not have residual oil throughout the internal cavi-ties, an unknown condition, one must begin the trouble-shooting procedures by creating a known condition.Follow the steps of this procedure in sequence. Notethat it is a good idea to carry out the following procedurewhen the oil is at normal operating temperature.

In practice, it is not always easy to distinguishbetween internal leakage and an ineffective scavenge

Figure 3. Accumulated oil will often collect in one or moreof three major areas.

1.

2.

3.

Remove the power section accessory drive magneticdrain plug. Drain all of the oil from the accessorydrive housing, and then reinstall the plug.

Remove the front (lower) magnetic drain plug fromthe reduction gear assembly. Drain all of the oilfrom the reduction gear assembly and then reinstallthe plug.

Remove and drain both scavenge lines supplying theexternal scavenge pump at the accessory drivehousing. Reinstall the lines after draining them.

REDUCTION GEAR ASSEMBLY

SCAVENGE OIL TUBE

DRAIN PLUG

ACCESSORY DRIVE

LIGHTHOUSE ASSEMBLY


Note that at this point in the troubleshooting proce- lighthouse pressure connection on the diffuser casedure, the amount of oil that drains from these areas just forward of the 11 o’clock bleed air manifoldis not important. A small amount of static oil leak- adapter. The pressure should be read with a gageage at these locations is normal. which indicates inches of mercury (in. Hg).

4. Record, exactly, the amount of oil in the oil tank,using the dipstick.A measured amount of oil is nowin the tank, and the reduction gear assembly, acces-sory drive housing, and lighthouseareas are empty.In other words, a known condition exists.

6. Operate the engine at cruise power for a minimum of30 minutes, or until the oil begins to dump out thedrain mast, whichever occurs first. During the run,record the oil scavenge back pressure readings at oiltemperatures of 60°C 70°C and 9O'C, and peakscavenge back pressure. Record peak lighthousepressure at a stabilized takeoff power setting.5. Attach both a scavenge back pressure gage and a

lighthouse pressure gage to the engine.The scav-enge back pressure gage should be connected to apoint where “pump out” pressure can be read.Onesuch location is the main pressure scavenge oil pumpmagnetic plug. To connect to the pump, modify amagnetic drain plug by drilling and tapping a hole inthe center for a No. 4 hose fitting.

Another possibility is the external scavenge filter“in” point. Connection to the filter can be made byremoving the external filter bypass indicator buttonand inserting the special adapter provided with thePN 23008696 test set, which is available fromAllison. Either of these points will show the backpressure the pump senses during operation.Figure4 shows the locations referred to in this step.

Lighthouse pressure should be determined byattaching the pressure gage to the engine at the

7. Shut the engine down. Measure the quantity of oilremaining in the oil tank. Record the amount the oillevel has dropped from the previous measurement.

Remember that if the oil was not at operating tem-perature when you began the check, it will have ex-panded because of the increase in temperature.Thedifference can be up to 2 quarts, depending on howcold the oil was initially. This must be consideredwhen figuring the level reduction on the dip stick.That is why the oil should be at normal operatingtemperature before carrying out this procedure.

8. Within 5 minutes after engine shutdown, remove themagnetic drain plugs from the accessory drivehousing and the reduction gear assembly.Catch andmeasure the amount of oil that drains from theseareas separately.

Figure 4. Determine “pump out” pressure at the main oil pump magnetic plug or the external oil filter “in” point, and lighthous epressure at the pressure connection on the diffuser case.

8 Lockheed SERVlCE NEWS V18N3

9. Under normal operating conditions, the oil drainedfrom the accessory drive housing should not be morethan 1 quart. The oil drained from the reductiongear assembly should not be more than 2.5 quarts.The amount of oil collected over and above theseamounts can be considered accumulated oil.

Analyzing the Results

Mathematically determine the sum of the oil drainedfrom both the reduction gear assembly and accessorydrive housing. Subtract this total from the original dip-stick reading.Taking into account up to 2 quarts for heatexpansion of the oil, if applicable, the remainder shouldequal what was recorded on the dipstick after shutdown.

If the oil reduction on the dipstick was more than thesum of oil collected from the reduction gear assemblyand accessory drive housing combined, the remaining oilis probably in the lighthouse assembly. This amountshould not be more than 1 pint.

This method is most accurate when very little or nodumping occurred during the run. For this reason it isimperative that the engine be shut down as soon as oildumping begins.

If lighthouse accumulation is suspected, remove theoil scavenge lines from the external scavenge pump.Catch and measure the amount of oil drained from thesetwo lines. Any amount over 1 pint can be considered oilaccumulation. If, by using these procedures, it isdetermined that 110 oil has accumulated in any of thesethree major areas, the most probable cause for the oilloss is internal leakage.

Internal Leakage

There is no cut-and-dried method of pinpointing theactual cause of all internal leakage problems withoutdisassembling the engine. However, there are somethings that can be done to help isolate the problem areas.

There are several locations where internal leakagemost often occurs, all of which are in the power section.Worn or deteriorated O-ring seals, or a worn or damagedcompressor front bearing labyrinth seal in the compres-sor front bearing and extension shaft area can causeinternal leakage (Fig. 5).

The lighthouse area can experience internal leakagefrom either end caused by damaged crush gaskets, wornor damaged labyrinth air or oil seals, or even a crack inone of the combustion chamber inner liners.The turbinerear bearing sump area can suffer leakage because ofineffective crush gaskets, turbine rear scavenge pumpfailure, or a worn or damaged turbine rear bearing laby-rinth seal (Fig. 6).


Three common locations for internal leakage are shown below.

Figure 5. Compressor front bearing labyrinth seal.

Figure 6. Turbine rear bearing labyrinth seal.

Figure 7. Compressor rear bearing labyrinth seal.

One tool that can aid in the process of elimination isthe lighthouse pressure gage. By combining the light-house pressure indications with physical evidence of anoil leak, one can determine which part of the lighthousearea is at fault.

For example, if the lighthouse pressure is over 11 in.Hg, coupled with evidence of oil loss, the most probablecause is an ineffective compressor rear bearing labyrinthair seal (Fig. 7). This is due to the lighthouse beingoverpressurized by compressor discharge air flowingover the worn or damaged labyrinth air seal.

This overpressurization forces the oil in the turbinefront bearing area to flow over its labyrinth seal and outthe exhaust. Remember, each of these labyrinth seals isa dual-stage seal. They are made up of an air seal and anoil seal combined in one seal assembly. It is thereforepossible for only one part of the seal to be a problemwhile the other part is still serviceable.

The reverse can also take place. The turbine frontbearing labyrinth air seal could be damaged or worn,causing an overpressurization of the lighthouse assem-bly. This will force the oil in the compressor rearbearing sump out through its oil seal and out its vents.However, the indication of this problem would be highlighthouse pressure coupled with oil venting from thecompressor rear bearing vents.

What can make troubleshooting this area even moreconfusing is that oil can vent out the turbine exhaust areawhen only the turbine front bearing oil seal is worn ordamaged. This, however, produces a different symp-tom, in that the oil will be lost even though lighthousepressure remains normal. This happens because the oilseal is worn or damaged, and the oil can flow across thisseal and out the exhaust. There is no cause for thelighthouse pressure to build up.

To complicate things still further, the reverse canhappen as well. Oil can dump out of the compressor rearbearing vents because the compressor rear bearinglabyrinth oil seal is worn or damaged. The oil simplyflows across the bad seal and out the vents. There isagain no reason for lighthouse pressure to build. Theexternal symptom seen in this case is oil dumping out ofthe compressor vents, with the lighthouse pressurenormal. Table 1 at the right may make all this a littleeasier to understand.

If there is evidence of oil coming from the threeo’clock and nine o’clock turbine inlet struts, the problemis probably the inner combustion liner crush gaskets. Tocorrect this problem, replace the gaskets and use thefollowing torquing procedures. Tighten the bolts inorder around the circumference until the required torqueis obtained. Allow the bolt tension to stress-relieve for15 or 20 minutes.

10

Now check the torque again. If there is torquewrench movement, retorque again and wait. Repeat theprocess until no movement of the wrench occurs whenrechecking torque. This procedure ensures a completecrush effect on the gaskets and usually corrects theleakage problem.

If the lighthouse pressure was below 11 in. Hg andno evidence of oil leakage from the compressor ventswas noted, try the following:

1. Check the turbine exhaust area for evidence of oilleakage. If oil is present, investigate the turbine rearbearing area. It should be noted that there can beevidence of oil in this area caused by excessivedown-the-shaft venting. This condition is mostoftencoupled with high lighthouse pressure, but it ispossible for it to occur with normal lighthousepressure as well.

2. Remove and inspect a compressor bleed valve forevidence of oil film. Check for oil in the air inlethousing area. If oil is present in either of theseareas, the most probable cause is leakage from thecompressor extension shaft housing O-ring or thefront compressor labyrinth seal.

Scavenge System Problems

If it is determined that oil has accumulated in one ofthe three major areas, the problem is likely being caused

Table 1.

11 in. H gLighthousePressure

Oil fromCompressorVents

‘ProbableCause

. .

At or above No Compressor rearlabyrinth air seal.

At or above Yes Turbine front laby-rinth air seal.

Below No Turbine front laby-rinth oil seal.

Below Yes Compressor rearlabyrinth oil seal.

At or above No Cracked diffuser,inner combustionliner, crush gas-kets, piston rings,or both compres-sor rear & turbinefront labyrinth airseals.Potential problem,needs repair.


Figure 8. Power section scavenge problems can often be traced to ineffective pumps. leaking seals, and restricted scavenge oil tubes.

by an ineffective scavenge system. The followingprocedures can help isolate the cause. Note that theseactions should be taken only if accumulation was evidentin the area under discussion. Each of the three majorareas will be addressed separately. Figures 8 and 10 willbe helpful in tracing the oil flow in the power section andreduction gear assembly scavenge systems.

cannot keep up with the demand. In the early stages, thisusually causes oil loss only at altitude. If not corrected,however, the pump will wear to the point of also beingineffective on the ground.

Lighthouse Accumulation

There is just one reason why oil accumulation occursin the lighthouse assembly. That is that oil is beingpumped into the lighthouse faster than it is being pumpedout. Either there is too much oil being pumped in, whichis usually caused by enlarged jets, or the oil is not beingremoved as fast as it should be. As noted earlier, thelatter situation is much more common than the former.

It should be kept in mind that even a normallyoperating scavenge pump loses effectiveness at highaltitude. At sea level, the oil is being forced into thepump by 14.7 psi atmospheric pressure (on a standardday). This provides a sort of preload effect. At altitude,this pressure is reduced, causing more air and less oil tobe pumped. The higher the altitude, the greater the ratioof air to oil becomes. At cruise altitude, a perfectly goodscavenge pump actually pumps more air than oil.

There are several reasons why the oil may not begetting pumped out at a sufficient rate. The most obvi-ous is that the external scavenge pump is not pumpingeffectively. One possible cause is that the drive shaft issheared and the pump does not pump at all.Normallywhen this happens, the lighthouse area fills with oil in amatter of minutes and oil dumps out the compressor rearbearing seal vents, the turbine exhaust, or anywhere elseit can get out.

All too often, an external scavenge pump is replacedbecause it is thought to be worn, when in fact it only hasa leaking O-ring. The inner O-ring on the pump cover,if leaking, can cause oneelement ofthepump to cavitate.This causes the pump to lose its efficiency (see Fig. 11on page 15).

Still another reason why oil may not be pumped outfast enough is that the oil is not reaching the pump to bepumped. Remember that this is a suction-feed pumpingsystem. Any restriction in the passages to the pump willcause oil accumulation to begin.

Another reason for external scavenge pump ineffec-tiveness is that the pump is worn to the point where it

There are two causes of restricted scavenge oiltubes. One is carbon. When oil gets overheated, it tends


LINE O-RINGS

Figure 9. Good seals, unobstructed lines, and undamaged S-tubes are critical to proper scavenge oil flow.

to break down chemically, leaving a carbon residue.The residue can collect in the oil tubes and passages andcreate restrictions to flow. There is a screen in thebottom of the compressor rear bearing sump to preventlarge pieces of carbon from entering the oil tube, but thisdoes not stop carbon from forming within the tubesthemselves. Figure 9 shows the details of several of thecomponents discussed on this page.

The other cause for restricted oil tubes is air. Air isnot really a restriction, but it can act to inhibit flow.Leaking scavenge line packings at the diffuser portconnection allows lighthouse pressure to enter the oiltube. This breaks the suction to the turbine front bearingsump. There are two O-rings installed on the returnscavenge line to help prevent this from happening.

Another place air can enter the system is at thediffuser S-tubes. These tubes are located inside thediffuser 5 o’clock and 7 o’clock struts. They serve as oilpassageways to transfer the oil from the lighthouse areathrough the compressor air stream to the pump. If one ofthese tubes becomes cracked, compressor discharge aircan enter the tube. The air pressure acts to break thesuction and thus inhibits the free flow of oil to the pump.

There is one rather peculiar possibility that shouldbe considered when cracked S-tubes are suspected. Justwhat happens depends on the severity of the crack. Thekey symptom is that the oil quantity will drop by approx-imately 2 to 3 gallons when the engine is running abovelow-speed ground idle. The lost oil will then be mysteri-ously recovered when the engine is “down-shifted” tolow-speed ground idle.

12

The key here is the fact that the oil quantity returnsto normal at low-speed ground idle. To explain further:in low-speed ground idle, the compressor discharge airis sometimes not hot enough, nor at high enough pres-sure, to expand a hairline crack and force its way into theS-tube, where it can affect the suction of the pump. Thepump therefore functions correctly and the oil is proper-ly scavenged back to the supply tank.

When the engine is upshifted or taken to power, thecompressor discharge temperature increases, as does theair pressure. This tends to open the crack and allow thenow much higher compressor discharge pressure into thesystem, breaking the suction. The oil that is beingpumped into the lighthouse area will now no longer beremoved as rapidly as it should be, and oil accumulatesin the lighthouse assembly as a result.

This causes the oil quantity indicator to show areduction in oil quantity. When the engine is againdown-shifted to low-speed ground idle, the crack willclose and the oil that has accumulated in the lighthousewill be scavenged back to the tank. The oil quantityindicator will now show that the oil has returned.

So when an engine tends to “hide” approximately 2to 3 gallons of oil and then give it back at low-speedground idle, it is likely that an S-tube in the diffuser iscracked. One way to confirm this problem is to removethe lower bleed air duct from the diffuser and inspect theoil tubes for visible oil seepage.Normally, this area isbone-dry. If these tubes are cracked, there will be signsof an oil film residue or even just wetness around the S-tube area.


If a condition of oil accumulation in the lighthouseis neglected long enough, the lighthouse becomescompletely filled with oil. The labyrinth seals will thenusually allow the oil to dump, or the oil will pass throughthe shaft venting system into the rear turbine exhaustarea. So, if oil is found in the turbine exhaust area, andmore than 1 pint of oil was drained from the lighthouseoil tubes, the problem is likely to be inadequate light-house scavenge and not turbine rear bearing seals.

Remember that the reduction gear assembly has twoscavenge pumps (Fig. 10). One pump, the main scav-enge pump, scavenges oil from the sump behind themain diaphragm. The other, the nose scavenge pump,scavenges oil from forward of the main diaphragm.There are two pumps because it requires two pumps toscavenge all of the oil effectively at altitude.

Note that it is not a good idea to allow large amountsof oil to accumulate in the lighthouse. When oil isproperly scavenged from this area, it takes heat with it.This is how the compressor rear bearing and turbinefront bearing remain cool. If the oil remains, so does theheat. This sets up a conditionwhich promotes prematurebearing failure and carbon build-up in the scavengelines. Keep in mind that these things can happen evenwith the scavenge back pressure readings well withinnormal limits.

If both pumps fail, or either one fails at altitude, theoil will accumulate in both sumps and begin to fill thereduction gear assembly. When the oil level begins toincrease, the oil will usually flow out through the torque-meter and into the compressor front bearing area.

Reduction Gear Assembly Accumulation

Keep in mind that the compressor front bearing areais scavenged through the accessory drive housing by thepower section main pressure scavenge pump. Extra oilflowing into the area will overload this pump, causingaccumulation to begin in the accessory drive housing aswell. Eventually the oil blows by the compressor frontbearing labyrinth seal into the compressor air flow or outthe breather vent line to the drain mast, or both.

Just as in the case of the lighthouse assembly, thereis only one reason why oil accumulates in the reduction

To the troubleshooter, the key to this condition is thelarge amounts of oil that will be collected from both the

gear assembly. That is that oil is being pumped into the reduction gear assembly and theaccessory drive housingreduction gear assembly at a significantly faster rate than magnetic drain plug areas.Also, if one pump has failed,it is being pumped out. The reason that the oil is not the oil quantity indicator will show a reduction of oilbeing removed rapidly enough is usually an inefficient quantity at cruising altitude, but remain relatively stablescavenge pump. at lower altitudes.

Figure 70. Effective reduction gearbox scavenging at altitude depends on efficient operation of both nose and main scavenge pum ps.

NOSE SCAVENGE PUMP

MAGNETIC PLUG

SCAVENGE PUMP

MAGNETIC PLUG -

Lockheed SERVICE NEWS Vl8N3

This occurs because one pump cannot keep up with to handle the added load,Troubleshooters should againthe oil flow at altitude and oil accumulatesThe single keep in mind that once again, all these things can happengood pump may be able to prevent further accumulation with the lighthouse pressure and scavenge back pressureat lower altitudes, but it does not have enough capacity still within prescribed limits.to completely recover the oil that has been accumulatedat altitude. The oil simply stops accumulating. There is one other condition that causes reduction

gear assembly scavenge pumps to be ineffective, and thatOne symptom which is unique to reduction gearbox is a restriction in the common return system.Remember

main scavenge pump failure is that the engine dumps oil that the reduction gear assembly and power section shareor indicates a reduction of oil quantity only when the the same return system from the external scavenge filteraircraft is in a nose-up attitude or during rapid accelera- all the way to the oil tank. Any restriction to the freetion. It is under these conditions that oil normally scav- flow of oil in this system will create a back pressureenged by the nose scavenge pump flows to the rear of the problem for the pumps, which results in a decrease inreduction gear assembly. their effectiveness.

If the main pump is functioning properly, this extra Two indications commonly reveal the presence ofoil can be pumped back to the tank at low altitudes, such this situation. One is that the problem will normally beas during takeoff. However, if the main pump has shown on the scavenge back pressure gage as more thanfailed, the oil will accumulate and eventualIy dump into 30 psi back pressure. The other is that both the reduc-the compressor front bearing area.So, if the engine tion gear assembly and the accessory drive housing willdumps oil only in a nose-up attitude, especially during usually accumulate oil simultaneously. Isolation of thetakeoff, the reduction gear assembly main scavenge causes of oil return system components failure is coveredpump should be suspected. under “Excessive Scavenge Back Pressure” on page 15.

On the other hand, if the engine dumps oil shortly Accessory Drive Housing Accumulationafter a long descent, the probable cause is failure of thenose scavenge pump. In a nose-down attitude, the oil Probably the most common area for 501/T56 oilthat is usually pumped by the main scavenge pump flows accumulation is in the accessory drive housing (Pig. 11).to the front of the reduction gear assembly. The nose There are two pumps located in the accessory drivescavenge pump will usually pump the additional oil back housing, but one, the external scavenge pump, onlyto the supply tank at lower altitudes. However, if the services the lighthouse area and does not affect thenose pump has failed, oil will accumulate in the front of scavenging of the accessory drive housing itself.the reduction gear assembly.

The reason for oil accumulation here is the same asAfter a time during long descents, the oil quantity in the other cases we have discussed: more oil is being

indicator will indicate a decrease in oil quantity in the pumped in than can be pumped out. There are severaltank. When the aircraft levels off, the accumulated oil possible causes for this problem. One is a plugged backflows to the main pump, which usually pumps most of it vent on the main oil pump (Fig. 11). When the backback to the tank. The oil quantity then partially recov- vent becomes plugged, the scavenge element of theers. The reason the main pump can handle the extra oil pump tends to cavitate, especially at high altitudes. Thisat this point is that after a long descent, the aircraft has reduces the pump’s effectiveness and oil begins tocome to a lower altitude and the pump is more efficient accumulate inside the accessory drive housing.than at cruise altitudes.

Another cause for main oil pump inefficiency can beSometimes, during extremely long descents, there is found within the pump itself. Both the pressure element

so much oil accumulation that after leveling off, the main and the scavenge element share the same drive shaft. Ifoil pump cannot handle the added oil.In this case, the the shaft seal between the two elements wears abnormal-oil again flows out through the torquemeter and dumps ly , oil from the pressure element begins to flow to theoverboard.A uniquething about this situation is that the scavenge element, Unfortunately, the scavenge elementoil will dump for a brief period of time after leveling off, is not capable of pumping both the scavenge oil and thethen stop. additional oil from the pressure side.

The reason for this is that only the excess oil that the In this case, the scavenge pump is working to itsmain scavenge pump could not pump is being dumped maximum capacity, but oil is still not being pumped outoverboard. As soon as the excess oil is dumped, the of the accessory drive housing fast enough. The result issystem returns to the situation where the main scavenge that the accessory drive housing fills with oil, andpump is carrying the load of both pumps. And again, eventually an oil dumping situation arises. In this case,since the aircraft is usually at a much lower altitude after oil will usually dump when the aircraft is on the grounda long descent, the pump will be more effective and able as well as at altitude.

14 Lockheed SERVICE NEWS V18N3

Figure 11. Accessory drive housing oil accumulation is often caused by wear, obstructions. or seal problems affecting the mainpressure scavenge pump.

Another internal problem with the pump is that themain oil pump can become worn to the point where it isno longer effective. When this occurs, the problem isusually first indicated by oil dumping at altitude only.As we have seen, a scavenge pump normally becomesless effective at higher altitudes. A worn pump will bemost noticeable at these altitudes first. The main oilpump differs from the external scavenge pump in thatwhen the drive shaft shears, both the scavenge andpressure pumps stop working. In this situation, therewill be no oil pressure indication on the flight deck.

One more thing to remember about accessory drivehousing oil accumulation is the possibility of the “figure8” seal causing the problem (Fig. 11). Oil from theexternal scavenge pump flows through one side of thisseal and oil from the pressure pump flows through theother side. If either of these seal sides leak, the oil fromthe leak will flow to the bottom of the accessory drivehousing. This creates a reservoir too large for thescavenge pump to handle and accumulation begins.

It is worth repeating that all these things can happenand the scavenge back pressure and lighthouse pressurestill be within prescribed limits. However, excessivescavenge back pressure can, and often does, cause oilaccumulation in the accessory drive housing.


Excessive Scavenge Back Pressure

Excessive scavenge back pressure is probably themost commoncause of oil accumulation. Unfortunately,this can be a rather complex subject. There are manyfactors, acting singly or in concert, that can lead to back-pressure problems.

The oil flowing back to the oil tank in the 501/T56oil system has much in common with water flowingthrough a garden hose. If you squeeze a garden hosewith a pair of pliers at its midpoint, the flow to the outletdecreases, and the pressure above the restriction in-creases. This is the key point for troubleshooters toremember. The pressure inside the hose will increaseonly on the upstream side of the pliers, between thepliers and the faucet.

This fact of fluid dynamics holds true for the 501-IT56 oil return system as well. Any restriction betweenthe pump and the oil tank will cause a decrease in theflow of oil to the tank, and an increase in pressure on theupstream side of the restriction; in other words, betweenthe restriction and the pump. Such restrictions have tobe dealt with; the good news is that we can use this in-creased pressure as a troubleshooting aid to find wherethe restriction is located in the system.

15

Remember that we are not directly concerned withthe pressure itself, but with what is causing the increasedpressure. When the flow of oil to the oil tank is reduced,the oil must go somewhere else. This is how oil accumu-lation in all three major areas can begin.

The common scavenge return system has some built-in restrictions (Fig. 12). For example, the external oilfilter creates a certain amount of back pressure, depend-ing on how contaminated it is. The more contamination,the more force required to push the oil through the filter.

Other components also contribute to system backpressure. As oil flows through the fuel heater, the re-quirement for pressure increases, depending on howrestricted the cooler core has become. When the oilbecomes hot enough to flow through the oil cooler,additional energy is again required, this time to push theoil through the core. The result is another increase inpressure. The exact amount of increase depends on justhow contaminated the oil cooler has become. There aremany variables to contend with when troubleshootingthis system.

We also must not lose sight of the fact that the entiresystem has to be held under a maximum pressure of 30psi for the pumps to pump against. The total systemback pressure represents the sum of the pressure increas-es caused by each component’s resistance to oil flow allalong the way The significance of this cumulative effectcannot be overstated. If each individual component inthe oil return system were to offer its maximum allow-

able flow resistance, the combined total would push theback pressure beyond the overall 30 psi maximum.

If the scavenge back pressure is shown to be above30 psi, it becomes a matter of finding the location of therestriction that is forcing the system pressure to above-normal levels. This can be somewhat involved becausethe cause may not be just one component.Quite possi-bly, problems in two or more units may be involved.The main thing fortroubleshooters to remember is to tryto get the scavenge back pressure as far below 30 psi aspossible. Note that it will be very unusual to get the totalpressure below about 20 psi, so there is little point indevoting a lot of effort to trying to get it lower thansomewhere in the 20 psi to 25 psi range.

On the other hand, if a troubleshooter takes onlyenough action to reduce the total pressure to about 29psi, the problem will return when the total systemcontamination level pushes the oil pressure up only 1 or2 psi; in other words, to the dumping threshold. Thelower the scavenge back pressure, the longer it will taketypical contamination processes to raise the pressure tothe dumping stage.In the interest of preventive mainte-nance, it always makes good sense to keep the totaloverall scavenge back pressure as low as possible.

Using the Test Set

The Allison PN 23008696 test set has been designedto check scavenge oil pressures at four locations in theoil return system. These key locations are the external

Figure 12. Nacelle-mounted oil system components such as the oil cooler, fuel heater, and external scavenge oil filter are importantcontributors to scavenge system back pressure.

16 Lockheed SERVICE NEWS V18N3

scavenge filter inlet, the external scavenge filter outlet,the oil cooler inlet, and the oil cooler outlet.

(1) External Scavenge Filter Inlet

The reading here indicates the overall pressure towhich the scavenge pumps are subjected (Fig. 4). Thispressure should be maintained as far below the 30 psimaximum as possible.

(2) External Scavenge Filter Outlet

The value obtained at this point is used to check thepressure differential (AP) across the external scavengefilter; in other words, the difference in pressure betweenthe filter inlet and the filter outlet.This can help deter-mine the condition of the filter itself. Contamination willcause an increased resistance to oil flow. In normaloperation, a clean filter will have a AP of 3 to 8 psi. Anyamount over 8 psi can be considered excessive.

I Figure 73. Even in full operation at 80°C, the AP of the oilcooler should not be more than 9 psi.

(3) Oil Cooler Inlet

heater usually causes oil dumping at altitude only, andthat scavenge back pressure without the special toolinstalled-will often be within limits when measured onthe ground.

The pressure reading at this location can be com-pared with the external scavenge filter outlet pressure

That is not to say that the oil bypass valve could not

value to determine the AP across the fuel heater strainerbecome stuck in the closed position, causing high

assembly (Fig. 14). Under normal operations, a cleanscavenge back pressure both at altitude and on the

fuel heater in full bypass will have a AP of 7 to 12 psi.ground. This condition is very rare, but one must

Remember, this is in the full bypass condition. In otherremember that the limits given above, fully open or fully

words, the temperature of the fuel flowing through theclosed, are extremes. The engine usually operates some-

exchanger is not cold enough to begin closing the bypasswhere between these two settings. Unfortunately, this is

valve, and all of the oil is bypassing the exchanger.yet another variable that the troubleshooter must contendwith.

But the fuel is not always this warm. Fuel can getvery cold, especially at altitude. During a typicalmission, the decreasing fuel temperature is sensed by athermostat in the fuel heater as the aircraft climbs towardcruise altitude. The thermostat in the fuel heater thenbegins to close the bypass valve, which forces the return-ing oil up through the heat exchanger.

(4) Oil Cooler Outlet

This action increases the AP across the exchangereven more. The problem is that it is difficult to duplicatethis situation on the ground. However, the locallymanufactured tool shown in Figure 14 on page 18 canhelp in isolating this problem. In use, the fuel heater oilbypass (slide) valve is removed and the special toolinstalled in its location. With the tool installed, all of thereturning oil is forced through the exchanger.

This pressure reading, coupled with that of the oilcooler inlet, can help determine the condition of theinterior of the oil cooler.Remember that the cooler doesnot come into play until the oil gets hot enough to requirecooling. At 60°C oil temperature, the oil is bypassingthe cooler core. The normal AP for the cooler at thistemperature is 0 to 6 psi.

When the tool is in place, normal operation of theexchanger will increase the AP to about 12 to 18 psi.Any more than 18 psi should be considered excessive.However, as long as the total overall scavenge backpressure does not exceed 30 psi with the special toolinstalled, the exchanger and the entire scavenge returnsystem can be regarded as serviceable. An importantthing for the troubleshooter to remember is that the fuel

As the oil begins to heat up, the thermostat in the oilcooler starts to route some of the oil through the coolercore. At 75°C oil temperature and above, the bypass isfully closed and all of the oil is routed through the coolercore. This increases the AP, but even at 8O'C, the APshould not be more than 9 psi. Any more than 9 psi canbe regarded as excessive and suggests that the unit isbecoming contaminated. Once again, however, if theoverall total back pressure is below the maximum of 30psi, the system can be considered serviceable.

It is clear from the foregoing that the way the oilcooler affects scavenge back pressure is closely relatedto oil temperature. This fact can prove useful duringtroubleshooting activities.If the scavenge back pressureis below 30 psi when the oil is cold and it then increases


The tool is locally manufacturedusing mild steel stock (or equiv-alent). Do not use aluminum.

Use appropriate caution in re-moving the oil bypass valve.It is strongly spring-loaded inthe installed position.

BACK-PRESSURE TESTING TOOL

TOOL IN TEST POSITION(OIL BYPASS VALVE REMOV

FUEL HEATER

‘ED)

Figure 14. Installed in the fuel heater, this locally manufactured tool will allow the unit�s maximum back pressure to be measured.

to above 30 psi when the oil temperature reaches be-tween 65°C and 75”C, the likelihood is that the oilcooler is the culprit.

It is important not to lose sight of the total backpressure of the system as a whole, however.Consideran example in which the total overall back pressure is 29psi when the oil is cold, but has risen to 33 psi at 75°Coil temperature, at which point dumping occurs.Systempressure is now above normal, but the oil cooler has onlyraised the total back pressure by 4 psi.A 4 psi increaseis within the normal operating range of a good cooler, sowe must look elsewhere for the cause of the problem.

Continuing with our example, further investigationshows that the AP across the external scavenge filter isfound to be 10 psi. By replacing the external scavengefilter and bringing its AP down to 4 psi, the total scav-enge back pressure will be reduced to 23 psi. This isbelow the dumping threshold even when the extra 4 psiof the oil cooler in full operation is added. Now theoverall back pressure reading will be 27 psi, and nodumping should occur. Remember to look not only atindividual components, but also at the total system andwhat effect each component is having on it.

If it is determined that a certain component iscausing the high scavenge back pressure, and thatcomponent is replaced but little change in the pressurereadings results, check the condition of the oil lines

18

connecting the affected component. These oil lines canbreak down on the inside and cause restrictions. Some-times it is simply a matter of a line being kinked.

Altitude Effects

Misconceptions about oil loss at altitude are notuncommon. The first thing that must be determined iswhether altitude actually has anything to do with theproblem. We have already discussed some of the manyways in which altitude can affect the oil system, but it isnot uncommon for troubleshooters to get the impressionthat a problem is altitude-related when it is not.

Sometimes the problem involves a time factor inwhich oil accumulation occurs for a while before dump-ing begins. An example might be an engine with arestricted oil filter which raises the scavenge backpressure to, let us say, 32 psi. This creates an oil accu-mulation problem, but even though the pump is pumpingagainst excessive back pressure, it is stillpumping. Theaccumulation in this case will be very slow.

By the time the oil has accumulated to the dumpingstage, the aircraft may have reached 20,000 feet. Thenoil dumping begins, and is noted visually by the flightcrew. You can see how easily a troubleshooter might bemisled. Of course, flight crews can help by closelymonitoring the oil quantity indicators during climbout.


The actual accumulation rate can be calculated fromthe difference between the flow of oil being pumped intothe area versus the flow being pumped out. It all de-pends on how severe the leak or restriction is. With thisin mind, it is easy to see why it may take several hoursfor the oil to accumulate to the dumping stage.

There are three components in the oil system that aredirectly affected by altitude (Fig. 1). The oil tankpressurization valve is a spring-loaded valve that con-trols the amount of pressure in the oil tank. The springtension on the valve may become too weak and possiblycause a change in oil tank pressure, or the valve couldstick open or closed and cause problems. As a practicalmatter, there is not too much that can go wrong with thisvalve because of its simplicity.Replacing it does notoften correct an oil dumping problem. It is a factorwhich should be considered, however.

The oil tank cap should not be overlooked. Some-times the tank cap is improperly seated or the seal maybe defective. Normally there are no problems until thepressure on the outside of the tank is reduced, which ofcourse happens at altitude. The oil may then flow fromthe cap and very possibly out the scupper drain.This isdue to the venturi effect of the drain mast saber, whichcreates suction at the scupper. But there may also be noevidence of an external oil leak at all.

As noted earlier, the operation of the fuel heaterstrainer assembly is indirectly related to altitude in thatit responds to fuel temperature, which usually becomeslower at altitude. When the fuel cools, oil is directedinto the heat exchanger.If this unit is overly contami-nated, high scavenge back pressureat altitude can result.

One more thing to consider. There have been casesrecorded where changing the oil fixed an altitude dump-ing problem. As strange as it may seem, this can anddoes occur. Engine oil has an anti-foaming agent added.When oil is left undisturbed for a long period of time, theagent separates from the oil and goes to the bottom of thecontainer. If the container is not shaken or the oil mixedprior to servicing, the agent may get left in the container.When this happens, the oil foams excessively at altitudeand exits through the venting system. Mixing incompati-ble oils can also cause excessive foaming and oil loss.

Preventive Maintenance

One of the keys to successful 501/T56 engine opera-tion is a sound preventive maintenance program. Thishas been proven throughout the world as one of the mostimportant elements in the satisfactory completion ofHercules aircraft missions. It holds true for the oilsystem as well.

The primary thing for the maintainer to be con-cerned with is identifying a potential problem ahead of


time. Although a little extra effort must be exerted,there are a number of things he can do to prevent oilsystem problems from developing.

Check for accumulation. Engines are operatedperiodically throughout the maintenance program,usually on a scheduled basis: test cell, pre-ISO, backline, or compressor washes. It is during theseoperational checks that inspection for oil accumula-tion can be made. Simply drain and measure theamounts of oil from the reduction gear assembly andaccessory drive housing after the run. If too muchoil is drained from either location, a potential prob-lem may exist and troubleshooting should begin. Itmakes little sense to wait until oil begins to dump inflight, possibly interfering with a priority mission.

Keep records. It is a good idea to keep records ofstatic oil leakers. This way, transient problems caneasily be separated from repeat offenders. Repeatoffenders need to be thoroughly checked out and themalfunctioning components replaced.

Monitor lighthouse pressure. Lighthousepressurecan be taken during the operational checks men-tioned above. If the pressure is getting close to the1 l-inch Hg limit, the engine should be scheduled forremoval and repair at the earliest possible time.

Keep critical components clean. During the in-shop repair cycle, many components of the commonoil return system are removed. This is a good timeto make the effort to clean these components so thatwhen they are reinstalled they will not create prob-lems with back pressure. Ensure that new, cleanfilters are installed. Oil coolers and fuel heatersshould be cleaned before installation.

Go all the way with maintenance.When an enginehas been removed for oil accumulation in the light-house area, there are certain extra steps that can betaken in the shop that may avoid future failures.Forinstance, we know that oil is used to keep bearingscool. If lighthouseaccumulation has occurred, therea good chance the bearings have been overheated.So while the engine is disassembled, replace boththe turbine front and the compressor rear bearings.

While the material offered in this article is by nomeans all-inclusive, we hope it will prove helpful anduseful to the many 501/T56 engine maintainers through-out the world.

Service News wishes to express its appreciation tothe Allison Gas Turbine Division Training Departmentand the Lockheed (LASC) Customer Training SystemsDepartment for the use of the illustrations accompanyingthis article.

Documents

Focal · The engine will, of course, smoke and vent abnormally during the first few minutes after starting when the sumps have been flooded. Start the engine in low-speed ground idle