AOPA ASF Propeller Safety

On takeoff, propeller tip speeds approach the speedof sound. The blades must absorb not only the pun-ishing vibration of the engines power pulses, butalso vibration caused by the oncoming airstream.Centrifugal loadsthose forces that try to pull theblade out of the hubamount to 10 to 20 tons perblade.

The blades twist and flex. The stresses imposed on theprop are more concentrated in the small areas that arenicked or cut. These nicks and scratches act as stress ris-ers, which can weaken the blade enough to eventuallycause it to fail.

When an engine quits, the airplane can glide to a safelanding. When a propeller blade is lost, the resultingimbalance can tear the entire engine from the aircraft,putting the center of gravity far beyond limits and ren-dering the aircraft uncontrollable.

Statistically speakingAlthough accidents and human injuries from propellersare not widespread, they are serious and most are easi-ly avoidable.

In 2003, 14 accidents were blamed on the propeller;five were prop strikes resulting in two fatalities andthree serious injuries. The remaining nine were classi-fied as propeller system failures. Forty-four percent ofthese took place in homebuilt airplanes.

Four accidents involved propeller blade or hub fatigue,failure, or separation. Three were due to prop pitchchange mechanism failure. Two accidents were attrib-uted to a failed oil line to the propeller governor, whichcaused a loss of oil pressure.

Propeller Safety

S A F E T Y A D V I S O RTechnology No. 3

The root cause of mechanicallyinduced accidents isalmost alwaysneglect.

Propeller Safety Pg. 2

Prop accidents break down into some general groups:

Precautionary landings because of abnormal vibra-tion. Often caused by a missing balance weightor deteriorated spinner.

Precautionary landings due to propeller overspeed or runaway.

Forced landings following a catastrophic prop failure.

Accidents from a botched hand-propping episode.

Ground personnel, passengers, or bystanders walkinginto spinning propellers.

The root cause of mechanically induced accidents isalmost always neglect.

Working around the propellerBecause it is attached to the engine, the propellerdeserves respect on the ground. The single mostimportant concept you should understand is this: thepropeller must always be treated as though the ignitionhas been left on and the engine is just a hairsbreadthfrom starting. Even though most pilots are careful tooccasionally check that the mags properly shut downthe engine, not all do. Assume the worst and youllnever be surprised.

Magneto CheckPeriodically ensuring that your magnetos p-leads havenot broken is a good defense against unexpected startson the ground. These leads, which are connected to theignition (or magneto) switch in the cockpit, are responsi-ble for grounding the mags to keep the engine from run-ning. Sometimes the wires or connections between the

switch and the magnetos break or come loose. In thiscase, one or both mags may be hot, or ready to deliverspark whenever the prop is turned. To test them, insteadof shutting down the engine in the usual manner withthe mixture control, use the key. Allow the engine tocool normally and idle down. Move the key slowlythrough both Right, Left, and then to the Off position.The engine rpm should drop slightly at both of the indi-vidual mag positions and shut down completely in theOff detent. Allow the prop to stop and then move themixture to the idle-cutoff position. Do not try to catchthe engine before it comes to a stop because a danger-ous backfire might occur. If the engine does not stopwhen the key is in the Off position, shut it down with themixture, prominently mark the prop as being hot, andcontact maintenance personnel immediately.

There are a few other caveats to consider when in thevicinity of the prop.

Avoid pulling the airplane around by the prop. Yes,this seems the perfect solution to a vexing problem ofhow to change the airplanes position without having towalk around and get the tow bar, but its worthwhile tomake the extra effort. Neither the engine nor the propparticularly benefit from the loads imposed by horsingthe whole airplane around.

Avoid pushing the airplane by the spinner. The spin-ner and backing plate are built to be light, so theyrequite fragile. Pushing on them can cause the backingplate to crack and can lead to spinner failure.

Avoid contact with prop deice boots and associatedwiring. If you want your hot prop to be toasty when itcounts, stay clear of the boots.

Prop InjuriesNever attempt to load or unload the airplane withthe engine running unless there are significant exten-uating circumstances. If you must allow passengers orcrew to board or otherwise approach the airplane,be certain that they understand the areas to avoid.Keep your hand on the mixture control and monitorthe movement of the individuals without fail. Ifsomeone begins to walk in the direction of the spin-ning prop, kill the engine immediately. Worry abouthow to restart it later. Remember that passengers areunlikely to hear your cautionary shouts over theengine. Almost every year, there are injuries or fatali-ties from someone walking into a turning prop.

Treat the propeller with caution.


Hand-ProppingWere not going to tell you how to hand prop an air-plane because it is NOT something you learn from abooklet and it is inappropriate for nose-gear airplanes.

If you already know how to hand prop, remember tosecure neckties, silk scarves, and loose clothing.Remove rings, watches, and bracelets, and have a qual-ified person at the controls.

Prop mechanicsPropeller technology is considered to be mature, likemuch of what we use in aviation. The basic designshave changed little in the past 30 years or so, althoughincremental advances in blade aerodynamics haveimproved efficiency slightly (at best, a prop is about 85percent efficient at converting torque to thrust). Mod-ern production techniques have also helped reduceoverall weight. Nonetheless, pilots ought to be familiarwith a few basic types of props.

Fixed-PitchUsed when low weight, simplicity, and low cost areneeded, the fixed-pitch prop is a compromise.Because in most cases the pitch setting is ideal for nei-ther cruise nor climb, the airplane suffers a bit in eachperformance category. Fixed-pitch props simplifypower management and cost less to overhaul than aconstant-speed version.

Constant-SpeedBefore there were constant-speed props, there wereadjustable-pitch models. By altering each blades

angle of attack, the prop can be better optimized forboth climb and cruise performance. Early models usedmanual adjustment of the prop pitch, while a few pio-neer inventors played with automatic pitch-changemechanisms.

As soon as aircraft developed wide speed rangesthedifference between the slowest climb and the fastestcruiseit was clear that a better system was needed.By the early 1930s, the groundwork for the constant-speed prop had been laid, in large part by Ercoupe andPiper Cherokee designer Fred Weick.

A separate mechanism is used to alter each bladesangle of attack, with the goal to maintain a constantengine speed. In a fixed-pitch prop, as the airplaneaccelerates, the enginegiven a fixed throttle posi-tionwill follow suit. With a constant-speed arrange-ment, the blades angle of attack increases as theengine tries to accelerate, loading the engine andmaintaining the set speed.

This setup provides two main benefits over fixed-pitchpropellers. First, you get a more optimum blade pitchsetting, and second, the engine can be made to run ata set speed, greatly reducing pilot work load and mak-ing precise power settings possible.

Do not attempt hand-propping without proper training.

Governor Propeller

SpeederSpring

Pilot Valve

PressurizedOil Cavity

Flyweights

PilotControl

Constant-speed propeller


GoverningIntegral to the constant-speed setup is a device calledthe governor. It is geared to the engine and takes oilfrom the main engine supply. An internal pumpincreases the pressure of this oil and directs it, throughthe hollow nose of the crankshaft, to the propeller. Insingle-engine applications, oil pressure serves toincrease the pitch of the blades (called coarse pitch),which in turn reduces engine rpm. Twins and someaerobatic aircraft operate so that engine oil forces theblades to fine pitch. Pilots should follow manufacturersrecommendations for feathering props and securingengines after failures.

FeatheringA feathering propeller is simply a constant-speed unitthat can rotate the blades until they are nearly alignedwith the relative wind. This provides reduced drag inthe event of an engine failure. Feathering props arefound on most twin-engine airplanes.

Blade MaterialsToday, the vast majority of props in general aviation aremetalspecifically aluminum. In the days beforesophisticated metallurgy and precision metalworkingtools, the wooden propeller reigned supreme, and itsstill popular for ultralights, small experimental aircraft,and antique models. There are also a few composite(fiberglass, Kevlar, and graphite) props in circulation;most of these use either a foam or wood core wrappedwith fiberglass cloth. Potentially, the composite propcan be lighter than a metal prop, and its stiffness-to-weight ratio is better, but so far the certificated com-

posite propeller has proven to be too expensive formost GA aircraft.

HubsThe function of the hub is to fasten the propeller bladesto the engine. For most fixed-pitch props, the hub isintegral to the blades. Constant-speed props need toallow the blades to rotate in the hub. Many differentblade retention systems have been used throughout theyears, with the more recent designs intended to be long-wearing and corrosion resistant.

As the fleet ages, its becoming more common to hearof prop-hub distress in addition to the more prevalentblade maladies. Failures of hubs are comparatively rare.Typically, the hub problems involve cracking prior tofailure. Corrosion pits inside the hub can cause cracksto form when the hub is subjected to operating stress-es. Such incidents are particularly distressing becausethey are preventable with proper maintenance andoverhaul. Prior to failure, a cracked hub or blade reten-tion component may provide a warning with the sud-den onset of grease or oil leakage or vibration.

Potential areas of oil or grease leakage

Corroded steel hub

A cracked aluminum hub


Preflight considerationsMany pilots seem to take the prop for granted, so thenext time you fly, take time to carefully check it. Hereswhat to look for:

General condition: Is the prop clean or covered ingrime? You cant tell much about the condition of theblades if you cant see them.

Are the blades scratched, pockmarked, or nicked? Thisis a critical question. Blade separations start with smallstress concentrators in the metal. These are formed byscratches, gouges, or corrosion that allow stress to con-centrate in a very small area. These stress concentratorsweaken the metal, which can then crack. Constant flex-ing of the blade makes cracks wider until the part fails.Generally, a nick that is less than 1/32-inch wide ordeep can be deferred to the next maintenance cycle,but anything larger (or if there are numerous nicks, sayfrom a recent departure from a gravel strip) should bedressed out immediately by an A&P mechanic.

Are the blades tight in their sockets? Constant-speedprops depend upon a certain amount of centrifugalforce to seat the blades, but there should not be morethan the slightest bit of movement.

With a spinner fitted, its often difficult to determinethe condition of the propeller hub itself, but you shouldbe able to see the mounting hardwarelook for loosenuts or backed-out bolts. Some props on GA aircrafthave oil-filled hubs to assist you in finding hairlinecracks that might otherwise go unnoticed. Any evi-dence of red oil must be investigated before flight.

Is the spinner secure? Remember that a loose or off-center spinner can self-destruct in very few hours if notcorrected. If part of the spinner departs the airplane inflight, it can feel as dramatic as if a very small part of ablade itself had jumped ship. Grasp the tip of the spin-ner firmly and try to move it in a circular pattern. Justdont overdo it. The spinner and backing plate are easi-ly damaged. Never move the aircraft or lift the nose bypushing on the spinner.

Prop Tip: Props can sustain a lot of damage whenoperating over loose gravel. If the airplane isparked on a gravel surface, move it to hard groundbefore starting the engine. Likewise, when taxiingto the tiedown spotif it is on gravel, shut theengine down on hard ground and use a tow bar tomove the airplane.

In the cockpitPrior to flight, take a few precautions to ensure thatyour prop will perform as needed. For fixed-pitchprops, listen and feel for unusual noises and vibration.Because theres so little to go wrong with a fixed-pitchpropeller, youre basically on the lookout for grossproblems like loose bolts or a tip that departed duringthe start and taxi sequences.

During the RunupFor constant-speed applications, however, there areadditional considerations. Keep an eye on the oil pres-sure and temperature. Pressure should be in the nor-mal, green-arc range, and the temperature should berising according to outside conditions. Because theconstant-speed prop needs both good oil pressure todo its job and oil thin enough to be pumped throughthe smaller passages of the prop, its important to keepthese parameters in mind, particularly for cold-weatherdepartures. A takeoff with cold oil will result in a poorlygoverned prop and a possible overspeed event. In sub-freezing conditions, it could take 15 to 30 minutes toget minimum oil temperature. Storing the airplaneovernight in a heated hangar or calling for an enginepreheat will help greatly.

A prop in the process of failing. Note crack.

Blade erosion: Bare metal corrodes rapidly. Keeping blades properly painted is inexpensive routine maintenance.


On the Takeoff RollTakeoff is a busy time, but ensure that the prop and itsgoverning systems are functioning properly.

Watch the tachometer and listen for signs of surgingor overspeeding. Know what is normal and what isexcessive needle swing for your aircraft. If there is amajor problem, youll likely hear it before noticing it onthe tach.

Note that the prop has achieved redline (or near red-line) speed.

Listen for abnormal noise and vibration.

It is perfectly normal for a constant-speed prop to spinup just short of maximum rpm during the initial takeoffroll. As the airplane accelerates, the prop will unloadslightly and the speed should come up. If it does notreach the redline value at climb airspeed, have the gov-ernor and/or tachometer checked.

If you see a drop in oil pressure or experience abnormalnoise or vibration, you should either abort the takeoff (iftheres room to stop safely) or continue around the pat-tern to a normal landing using low power settings.

In FlightThere is no clear pattern of propeller failures: they canhappen on the takeoff roll, in steady cruise flight, oreven in the traffic pattern. Stay alert to signs of prop dis-tress in the cruise phase of flight. The sudden onset ofgrease or oil leakage or vibration in a failing propellerhas been frequently reported. A timely and thoroughinvestigation of such conditions is prudent.

There are two main in-flight failure modesdepartureof part of a blade, balance weight, or spinner that caus-es strong (sometimes extreme) vibration, and governormaladies that can cause the prop to stick at the set rpmor to spin rapidly beyond the redline.

Blade FailuresIts almost too obvious to state, but the departure ofany part of the prop will get your attention in a hurry.Equally obvious is the advice to stay calm.

Immediately reduce power with the throttle.

Slow the airplane to best glide speedtrading air-speed for altitude if you have itand start looking forsomewhere to land.

Conduct the normal power-loss troubleshooting. Didyou just run a tank dry? Are the mags acting up? Usually,running a tank dry gets you a bump or two as the fuelpressure fluctuates, then light sputtering followed bysilence. Similarly, a mag problem typically manifestsitself in inconsistent misfiring and possible backfiring. Aprop-blade failure, on the other hand, is much morerhythmic and undiminishing in amplitude until thethrottle is pulled back.

Prop ExerciseWhen exercising a constant-speed prop, payattention to several items:

Does the prop control move freely? Excessive frictioncould indicate a frayed cable or poor lubrication. It alsomakes precise setting of the prop lever difficult.

Does the prop respond promptly with warm oil?The prop should take no more than two or threeseconds to respond to cycling when the oil is warm. Ifit fails to cycle at all, even when the oil temperaturegauge indicates green-arc conditions, do not attemptto take off. There may be a broken cable, failedgovernor, or plugged oil passage. The latter two ofthese possibilities could lead to an overspeed event.

Does the prop return to the set runup speed aftercycling?

Does the oil pressure fluctuate during cycling? Oilis pulled out of the engine during cycling, so thepressure should drop slightly when the prop is cycledand recover as prop rpm is restored.

A catastrophic hub failure that allowed the blades to depart and puncture the nose.

Unless you are sure that the engine roughness issomething other than the prop, do not advance thethrottle again. The tremendous centrifugal forces creat-ed by a grossly unbalanced prop can tear the enginefrom its mounts. With the engine idling and the propwindmilling, the forces will likely be small enough to letyou get the airplane on the ground before a major struc-tural component comes unglued. If you have the timeand the presence of mind, shut down the engine if youare sure some of the prop has departed. But, first thingsfirst, fly the airplane.

Governor FailuresOverspeedThe following information applies to single-engine air-craft with loss of governor oil pressure to the prop; i.e.,overspeed. A governor failure that causes engine over-speed or poor prop control is also a possibility. Wildchanges in prop rpm in flight can signal loss of governingcontrol, forcing the blades into the fine or high-rpmpitch settings. When this happens, get the engine speeddown before it does any damage.

Governor FailureIf a governor fails to supply oil to a propeller, the fail-ure effects are different depending on whether thepropeller is a pressure-to-decrease-pitch or pressure-to-increase-pitch design. On multiengine aircraft,loss of pressure will cause the prop to feather. Onmost single-engine aircraft, loss of pressure will causean overspeed.

If a governor is not doing its job and a propeller over-speeds, the amount of overspeed is controlled by twothings: engine power output and airspeed. Reducingthrottle and airspeed will minimize the amount of over-speed. In an overspeed condition on a single-engine air-craft, it would be better to fly to an airport while over-

speeding than to shut down the engine and risk an off-airport landing. If you are faced with propeller over-speed, take the following steps:

Immediately reduce the throttle to idle.

Set best-glide airspeed, and start looking for a placeto land.

Check the oil-pressure gauge. Many prop overspeedsresult from broken oil lines or oil starvation. Droppingoil pressure and increasing temperature are the classicindications.

If oil pressure is good (cross-check with temperature ifyou have unusual gauge indications), then slowlyadvance the throttle and note the props reactions.

Chances are good that you will be able to maintainsome power before the prop rpm reaches the redline.With good oil pressure and temperature, pick a throttlesetting that will allow you to maintain sub-redlineengine speeds. Still, remember that its better to fly toan airport with an overspeeding engine than to land in afield because you cut the throttle.

Plan for landing as soon as practical. You may have abroken oil-supply line to the governor (this was com-mon for a while on some engines that used externalprop-supply lines), which will exhaust the oil supply andseize the engine. A spun bearing or other internal leak-age may be restricting flow to the prop.

There also may be loss of oil pressure and subsequentseizure of the engine. It may not happen, but somethingcaused the prop to overspeed, and its a good bet its oilpressure-related.

Prop StrikesIts a sad tale thats repeated several times a year. Theoverworked pilot, perhaps coming home from a difficultday of flying terminated by a challenging instrumentapproach, gets distracted and forgets to put the geardown. At the first sound of crunching metal and thestrange stroboscopic blur of the prop taking chunks outof the runway, many pilots first reaction is to add powerand try to salvage the situation. Two words: Do not!

As soon as the first blade tip hits the concrete, that prop isruined, unable to carry the aerodynamic and structuralloads imposed by the go-around. If it stays on the air-


Missing blade tip


plane long enough to make it around for a wheels-downlanding, youll be beating the odds.

In this instance, the choice, though certainly not palat-able to many aircraft owners, is simply to keep thethrottle at idle and ride the belly landing to the bitterend. Fatalities and injuries from inadvertent wheels-uplandings are extremely rare. However, executing a go-around with damaged blades is very risky. Even if thepropeller stays sufficiently intact to prevent seriousvibration, damage could be so severe that climb perfor-mance is seriously degraded.

Maintenance mattersA big part of preventing in-flight emergencies is to keep ontop of propeller maintenance. Many aircraft owners believethe prop is a no-maintenance item. That is not the case.

Take the opportunity at the normal oil-change intervalto have a more detailed look at the prop. Have themechanic file out any nicks now, while you have thetime. This is not, incidentally, a procedure a pilot canlegally undertake, nor should you attempt to file theprop with supervision unless youve been specificallytrained. Dressing prop blades is an art: just enough mustbe filed to remove the nick but not so much that a lot ofblade is sacrificed to do it. The idea is to get completelyto the bottom of any nick so that there can be no stressriser that can later cause blade failure. There are blademinimum dimensions, and if too much is filed off, itmust be replaced. Any propeller overhauler can providethe dimensions, or you can request service informationfrom the propeller manufacturer.

TachometersHave the tachometer checked annually. Mechanical tachsare notoriously inaccurate and subject to drift over their

lifetimes. If your engine and prop combination has one ormore yellow arcs or red restricted arcs within the normaloperating rangecommon with many four-cylinderLycomingsits vital that the tach accurately guides youout of these trouble spots. These limitations are in placebecause of vibration characteristics of the engine andprop combination and can lead to long-term trouble ifroutinely ignored.

Hours or Years?Props have recommended overhaul intervals based oncalendar time and flight hours. Depending upon theprop model, this could be 1,500 or 2,000 flight hours,but theres also a calendar limit (typically five years) thattoo many pilots ignore. This is a serious problem in afleet that flies, on average, fewer than 100 hours peryear. At 100 hours per year, a typical 2,000-hour propmight not get checked for 20 years! This is clearlyimprudent, so the calendar limit applies.

Have the prop overhauled at either the time or calendarlimit, whichever occurs first. If the engine comes up foroverhaul before the prop reaches either limit, mostshops will recommend removing the prop and governorand having them overhauled anyway. This will get thetimes in sync.

Corrosion Is the CulpritOverhaul periods deserve respect because what killsmost props are not external defects, but unseen internalcorrosion. Dissimilar metals in the prop and hub createan environment ripe for corrosion, and the only way toproperly inspect many of these areas is through a tear-down. Extensive corrosion can dramatically reduce thestrength of the blades or hub. Even seemingly minorcorrosion may cause a blade or hub to fail an inspec-

Overhaul at the time or calendar limitwhichever occurs first.

Dont try to salvage this situation.


tion. Because of the safety implications, this is clearlynot an area in which to skimp.

Internal corrosion can develop in critical blade retentioncomponents. Such conditions present both a hiddendefect and a potential safety-of-flight issue. This is the pri-mary reason that calendar limits are an important inspec-tion requirement. Also, the overhaul needs to includemore than just a corrosion inspection. Restoration ofpaint and plating are important to assure future corrosionprotection until the next overhaul.

NotificationIts vitally important to keep up-to-date with airworthi-ness directives (ADs) or service bulletins (SBs) for yourprop. Compliance with ADs is, of course, required tomake the airplane legally airworthy, but its also goodform to follow the SBsparticularly those markedmandatorybecause there may be prop maladiesthat manifest themselves only under certain conditions,such as aerobatic flight or harsh environments. All workperformed on the propincluding AD and SB compli-anceshould be noted in the propeller logbook.

Overhaul: What Do They Do? Upon receipt for overhaul, a document is prepared thatwill track the propeller components throughout the over-haul process. All applicable ADs, current specifications,and manufacturers SBs are researched for incorporationduring the overhaul process. The serial number is double-checked, and notes are made on the work order regardingthe general condition in which the propeller was received.

As the unit is disassembled and cleaned, a preliminaryinspection is accomplished on all related parts. Thoserevealing discrepancies requiring rework or replacementare recorded in the overhaul record by part number,along with the reason for the required action.

All threaded fasteners are discarded during disassemblyand, with a few exceptions permitted by the manufac-turer, are replaced with new components. Many spe-cialized tools and fixtures are required in the disassem-bly and proper reassembly of propellers. These tools aregenerally model specific and range from massive 15-foottorque adapter bars and 100-ton presses down to tinydowel pin alignment devices.

Corroded blade clamp

Corroded bearings

Airworthiness Directives and Service BulletinsAircraft owners are seemingly besieged by ADs andSBs. Although some owners see only an outlay ofcash, paying attention to ADs and SBs, particularlywhere propellers are concerned, might just saveyour life.

For Part 91 operators, only ADs are mandatory. Anairplane is not considered airworthy unless all ADshave been complied with, either by proving bymodel or serial number that the AD does notspecifically apply, or by showing that an inspection orreplacement of parts has taken place. Compliancewith ADs is required to be noted in the aircraft,engine, and propeller logbooks.

SBs, even those marked by the manufacturer asmandatory, are purely optional for Part 91operators. Does this mean you should ignore them?Hardly. Many ADs are simply rewritten SBs, and inmany cases, compliance with a previously optionalSB will cover the requirements of a new AD. SBs canalso provide useful service information.

A list of SBs can be ordered from the propellersmanufacturer. Current AD information can bedownloaded from the AOPA Web site, www.aopa.org.


The HubNonferrous hubs and components are stripped of paint andanodization and inspected for cracks using a liquid pene-trant inspection (LPI) procedure. The parts are etched,rinsed, dried, and then immersed in a fluorescent penetrantsolution. After soaking in the penetrant, they are rinsedagain and blown dry. Developer is then applied, whichdraws any penetrant caught in cracks or defects to the sur-face. Under an ultraviolet inspection lamp, the penetrantclearly identifies the flaw. Certain models of hubs are alsoeddy-current inspected around critical, high-stress areas.Eddy current testing passes an electrical current through aconductive material that, when disturbed by a crack orother flaw, causes a fluctuation on a meter or CRT display.This method of inspection can detect flaws that are belowthe surface of the material and not exposed to the eye.

Magnetic particle inspection (MPI) is used to locateflaws in steel parts. The steel parts of the propeller aremagnetized by passing a strong electrical currentthrough them. A suspension of fluorescent iron oxidepowder and solvent is spread over the parts. Whilemagnetized, the particles within the fluid on the partssurface immediately align themselves with the disconti-nuity. When examined under black light, the crack orfault shows as a bright fluorescent line.

Components that are subject to wear are dimensionallyinspected to the manufacturers specifications. Afterpassing inspection, aluminum parts are anodized andsteel parts are cadmium plated for maximum protectionagainst corrosion.

The BladesThe first step in blade overhaul is the precise measure-ment of blade width, thickness, face alignment, bladeangles, and length. The measurements are then record-ed on each blades inspection record and checkedagainst the minimum acceptable overhaul specificationsestablished by the manufacturer.

Blade overhaul involves surface grinding and re-pitch-ing, if necessary. Occasionally, blade straightening is alsorequired. The manufacturers specification dictates cer-tain allowable limits within which a damaged blade maybe cold straightened and returned to airworthy condi-tion. Specialized tooling and precision measuring equip-ment permit pitch changes or corrections of less than1/10 of one degree. To ensure accuracy, face alignmentand angle measurements are taken repeatedly duringthe repair process.

Precision hand grinding of the blade airfoil is done toremove all corrosion, scratches, and surface flaws. Whenall stress risers and faults have been completely removed,final blade measurements are taken and recorded oneach blades inspection record. The propeller blades arebalanced to match each other and are anodized andpainted for long-term corrosion protection.

Prop ReassemblyWhen both the hubs and the blades have completedthe overhaul process, the propeller is ready for finalassembly. Part numbers are re-checked with the manu-facturers specifications. The parts are lubricated andinstalled per each units particular overhaul manual.After final assembly, both high- and low-pitch bladeangles on constant-speed propellers are checked forproper operation and leaks by cycling the propellerthrough its blade range with air pressure. The assembledpropeller is then checked for static balance. If necessary,weights are placed on the hub areas of each lightblade socket to bring about its proper balance. Theseweights should be considered part of the basic hubassembly and should not be moved during subsequentdynamic balancing to the engine. As with most aircraftcomponents, all of the hardware on the propellerassembly must be safety wired unless secured by self-

A cracked hub


locking devices. Maintenance release tags reflectingthe work accomplished, applicable ADs, and all incor-porated service documents are then filled out andsigned by the final inspector. These documents certifythat the major repairs and/or alterations that have beenmade meet established standards and that the pro-peller is approved for return to service.

From the ASF Accident DatabaseInjury to BystanderThe pilot of the Cessna 150 stopped at the terminal aftera local flight. A person standing on the ramp came overto the airplane and talked to the pilot through the opendoor. The engine was left idling. After the conversationhad concluded, the bystander waved goodbye andwalked into the propeller, sustaining serious injuries.

Its always good form to shut down the engine in thevicinity of onlookers, even if they are experienced pilots.

Wooden Prop FailureThe Taylorcraft was in cruise when a sudden severevibration went through the airframe. The pilot shutdown the engine and noted that the brass abrasionstrip and part of one wooden blade had departed.Although the pilot landed the airplane in a field, itcould not be stopped before plunging down a ravine. Amechanic who recovered the aircraft reported seeingdry rot in part of a remaining blade.

Constant inspection and maintenance is a necessity,particularly for the older wooden props. The pilot andflight instructor in this accident are to be lauded forkeeping their cool.

Prop StrikeDuring the takeoff roll, the Piper Super Cubs right maingear hit a hole. The pilot said he heard the propeller strikea rock. He continued the takeoff, but after liftoff, the air-plane began to vibrate. The pilot reduced engine powerand landed in a river. Examination of the propeller dis-closed approximately three inches of one blade missing.

Anytime you hit a solid object with the propwhetherits during a takeoff on a rough strip or the misguidedattempt to salvage a gear-up landingyou shouldimmediately discontinue the takeoff. This pilot waslucky that the imbalanced prop did not do serious sec-ondary damage to the engine or airframe. In the worstcase, the engine can be shaken from its mounts; theloss of engine weight will, in most cases, make the air-plane uncontrollable.

Broken CounterweightThe pilot of the Air Tractor noticed serious engine vibra-tion during a spraying run. He reduced power, then reap-plied power, but the vibration remained. The aircraft waslanded in wet terrain and nosed over. Inspection revealedthat a bolt in the propeller counterweight had broken.

Something as seemingly simple as a counterweightcaused this pilot to sit up and take notice. The moralhere is that quick action is the key to salvaging a propproblem; the pilot walked away without injuries.

Runaway PropA factory-overhauled engine had been installed in theCessna Cardinal RG. The pilot departed on a post-maintenance test hop. Eleven minutes into the flight,the pilot reported a runaway prop. The engine seizedshortly thereafter. A forced landing on a soccer fieldended in a collision with two vehicles on an adjoiningroad; the pilot was not injured. Later inspectionrevealed that one of two required gaskets at the base ofthe prop governor pad had been omitted, allowing theengine oil to be vented overboard.

Good work on the part of the pilot for keeping thisdead-stick Cardinal under control. It serves as areminder, though, that any flight after major mainte-nance should be conducted as close to a suitable run-way as possible.

Lost BladeDuring cruise flight, about half of one prop bladedeparted the Beech Musketeer, resulting in substantial

Disassembly work stand


engine and airframe damage. The airplane landeduneventfully at a nearby airport. Inspection laterrevealed that the prop blade had failed from fatiguethat started from a single corrosion pit on the camberside of the blade. There were no records in the air-crafts logbooks that the prop had been serviced sinceit was manufactured in September 1963.

Wine may improve with age, but metal does not. Thatthe prop had apparently been untouched for 33 yearswhen the accident occurred is nothing short of amaz-ing. Again, most prop makers specify, in addition tothe hours-in-service limits, a calendar limitation of, onaverage, five years between overhauls. This one wasway overdue.

Hand-ProppingThe pilot of a Piper Comanche (PA-24-250) was struckin the head and elbow by the propeller as he wasattempting to start the aircraft in cold weather. Thepilots operating handbook suggests pulling the propthrough four to six times before attempting a coldweather start, but in this case the pilot had alreadytried to start the airplane without success. Leaving hisnon-pilot passengers on board, he exited the aircraft to

pull the prop through but neglected to ensure that themagneto switch had been returned to the Off position.

A couple of thoughts here: Be sure that any time youtouch a propeller the magnetos are in the Off position preferably with the key in your pocket. Even then youmust assume the engine could start. That means keepingall parts of your body out of the prop arc. Had theengine started, the non-pilots in the airplane would beunlikely to help. Having a qualified pilot at the controlswould have been the safest option.

Safe Pilots. Safe Skies.

Copyright 2005, AOPA Air Safety Foundation421 Aviation Way, Frederick, Maryland 21701 Phone: 800/638-3101 Internet: www.asf.org

Publisher: Bruce Landsberg Editors: Kathy Dondzila, Brian Peterson, John Steuernagle

Consultant: Marc Cook

Special Thanks to:Hartzell Propeller, Inc.

McCauley Propeller Systems

SA06-12/05

Keep on top of propeller maintenance.

Documents

AOPA ASF Propeller Safety