-
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.
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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.
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Propeller Safety Pg. 3
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
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Propeller Safety Pg. 4
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
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Propeller Safety Pg. 5
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.
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Propeller Safety Pg. 6
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-
Propeller Safety Pg. 7
Missing blade tip
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Propeller Safety Pg. 8
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.
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Propeller Safety Pg. 9
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.
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Propeller Safety Pg. 10
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
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Propeller Safety Pg. 11
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
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Propeller Safety Pg. 12
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.