The blades twist and flex. The stresses imposed on the prop are more concentrated in the small areas that are nicked or cut. These nicks and scratches act as stress risers, which can weaken the blade enough to eventually cause blade failure.

When an engine quits, the airplane can glide to a safe landing. When a propeller blade is lost, the resulting imbalance can tear the entire engine from the aircraft, putting the center of gravity far beyond limits — and rendering the aircraft uncontrollable.

Treat the propeller with caution.

In 1994, 15 accidents occurred that were blamed on the propeller; an additional eight accidents showed the prop to be a cause or factor. Previous years’ numbers were worse, with a high of 24 accidents in 1991.

In 1994, there were four accidents caused by the blade portion of the prop only — usually separation of part or all of a blade, one after contact with the ground — and another three in which blade failure was an accident factor.

These accidents break down into some general groups:

• A precautionary landing because of abnormal vibration. Often caused by a missing balance weight or deteriorated spinner.

• A forced landing from a catastrophic prop failure.

• A precautionary landing due to propeller overspeed or runaway.

• An accident from a botched hand-propping episode.

• Ground personnel, passengers, or bystanders walking into spinning propellers.

The root cause of mechanically induced accidents is almost always neglect.

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

The spinner backing plate is fragile.

• Avoid pulling the airplane around by the prop. Yes, this seems the perfect solution to a vexing problem of how to change the airplane’s position without having to walk around and get the tow bar, but it’s worthwhile to make the extra effort. Neither the engine nor the prop particularly benefit from the loads imposed by horsing the whole airplane around.

• Avoid pushing the airplane by the spinner. The spinner and backing plate are built to be light, so they’re quite fragile. Pushing on them can cause the backing plate to crack and can lead to spinner failure.

• Avoid contact with prop deice boots and associated wiring. If you want your hot prop to be toasty when it counts, stay clear of the boots.

Do not attempt hand propping without proper training. This is poor form — he is leaning into the prop.

Hand Propping

If you already know how to hand prop, remember to secure neckties, silk scarves, and loose clothing; remove rings, watches, and bracelets; and have a qualified person at the controls.

Always shut down the engine when boarding passengers.

Constant-speed propeller

As soon as aircraft developed wide speed ranges — the difference between the slowest climb and the fastest cruise — it 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 and Piper Cherokee designer Fred Weick.

A separate mechanism is used to alter each blade’s angle of attack, with the goal to maintain a constant engine speed. In a fixed-pitch prop, as the airplane accelerates, the engine, given a fixed throttle position, will follow suit. With a constant-speed arrangement, the blades’ angle of attack increases as the engine tries to accelerate, loading the engine and maintaining the set speed.

This setup provides two main benefits over fixed-pitch propellers. First, you get a more optimum blade pitch setting, and second, the engine can be made to run at a set speed, greatly reducing pilot work load and making precise power settings possible.

Feathering props are found on most twins.

Potential areas of oil or grease leakage

A cracked aluminum hub

Corroded steel hub

As the fleet ages, it’s becoming more common to hear of prop-hub distress in addition to the more prevalent blade maladies. For the most part, failures of hubs are comparatively rare. Typically, the hub problems involve cracking prior to failure. Corrosion pits inside the hub can cause cracks to form when the hub is subjected to operating stresses. Such incidents are particularly distressing because they are preventable with proper maintenance and overhaul. Prior to failure a cracked hub or blade retention component may provide a warning with the sudden onset of grease or oil leakage or vibration.

Look carefully at the propeller when you preflight.

General condition: Is the prop clean or covered in grime? (You can’t tell much about the condition of the blades if you can’t see them.)

Are the blades scratched, pockmarked, or nicked? This is a critical question. Blade separations start with small stress concentrators in the metal. These are formed by scratches, gouges, or corrosion that allow stress to concentrate in a very small area. These stress concentrators weaken the metal, which can then crack. Constant flexing of the blade makes cracks wider until the part fails. Generally, a nick that is less than 1/32-inch wide or deep can be deferred to the next maintenance cycle, but anything larger — or if there is a preponderance of nicks, say from a recent departure from a gravel strip — should be dressed out immediately by a professional A&P mechanic.

Typical gravel damage

Are the blades tight in their sockets? Constant-speed props depend upon a certain amount of centrifugal force to seat the blades, but there should not be more than the slightest bit of movement.

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

With a spinner fitted, it’s often difficult to determine the condition of the propeller hub itself, but you should be able to see the mounting hardware — look for loose nuts or backed-out bolts. Some props on GA aircraft have oil-filled hubs to assist you in finding hairline cracks that might otherwise go unnoticed. Any evidence 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 not corrected. If part of the spinner departs the airplane in flight, it can feel as dramatic as if a very small part of a blade itself had jumped ship. Grasp the tip of the spinner firmly and try to move it in a circular pattern. Just don’t overdo it. The spinner and backing plate are easily damaged. Never move the aircraft or lift the nose by pushing on the spinner.

A prop in the process of failing. Note crack.

In the cockpit

Keep an eye on oil pressure and temperature.

• Watch the tachometer and listen for signs of surging or overspeeding. Know what is normal and what is excessive needle swing for your aircraft. If there is much of a problem, you’ll likely hear it before noticing it on the tach.

• Note that the prop has achieved redline (or near redline) speed.

• Listen for abnormal noise and vibration.

It is perfectly normal for a constant-speed prop to spin up just short of maximum rpm during the initial takeoff roll. As the airplane accelerates, the prop will unload slightly and the speed should come up. If it does not reach the redline value at climb airspeed, have the governor and/or tachometer checked.

If you see a drop in oil pressure or experience abnormal noise or vibration, you should either abort the takeoff (if there’s room to safely stop) or continue around the pattern to a normal landing using low power settings.

Stay alert to signs of prop distress.

There are two main in-flight failure modes — departure of part of a blade, balance weight, or spinner that causes strong (sometimes extreme) vibration, and governor maladies that can cause the prop to stay at the set rpm or to spin rapidly beyond the redline.

• Immediately reduce power with the throttle.

• Slow the airplane to best-glide speed — trading airspeed for altitude if you have it — and start looking for somewhere to land.

• Conduct the normal power-loss troubleshooting. Did you just run a tank dry? Are the mags acting up? Usually, running a tank dry gets you a bump or two as the fuel pressure fluctuates, then light sputtering followed by silence. Similarly, a mag problem typically manifests itself in inconsistent misfiring and possible backfiring. A prop-blade failure, on the other hand, is much more rhythmic and undiminishing in amplitude until the throttle is pulled back.

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

• Unless you are sure that the engine roughness is something other than the prop, do not advance the throttle again. The tremendous centrifugal forces created by a grossly unbalanced prop can tear the engine from its mounts. With the engine idling and the prop windmilling, the forces will likely be small enough to let you get the airplane on the ground before a major structural component comes unglued. If you have the time and the presence of mind, shut down the engine if you are sure some of the prop has departed. But, first things first, fly the airplane.

If a governor is not doing its job and a propeller overspeeds, the amount of overspeed is controlled by two things: engine power output and airspeed. Reducing throttle and airspeed will minimize the amount of overspeed. In an overspeed condition on a single-engine aircraft, it would be better to fly to an airport while overspeeding than to shut down the engine and risk an off-airport landing. If you are faced with propeller overspeed, take the following steps:

• Immediately reduce the throttle to idle.

• Reduce airspeed, set up for best-glide airspeed, and start looking for a place to land.

• Check the oil-pressure gauge. Many prop runaways result from broken oil lines or oil starvation. Dropping oil pressure and increasing temperature are the classic indications.

• If oil pressure is good (cross-check with temperature if you have unusual gauge indications), then slowly advance the throttle and note the prop’s reactions.

• Chances are good that you will be able to maintain some power before the prop rpm reaches the redline. With good oil pressure and temperature, pick a throttle setting that will allow you to maintain sub-redline engine speeds. Better to fly to an airport with an overspeeding engine than to land in a field because you cut the throttle.

• Plan for landing as soon as practical. Do not try to nurse the airplane home. You may have a broken oil-supply line to the governor (this was common for a while on some engines that used external prop-supply lines), which will exhaust the oil supply and seize the engine. A spun bearing or other internal leakage may be restricting flow to the prop.

There also may be loss of oil pressure and subsequent seizure of the engine. It may not happen, but something caused the prop to overspeed, and it’s a good bet it’s oil pressure related.

Don't try to salvage this situation.

Do not!

As soon as the first blade tip hits the concrete, that prop is ruined, unable to carry the aerodynamic and structural loads imposed by the go-around.

As soon as the first blade tip hits the concrete, that prop is ruined, unable to carry the aerodynamic and structural loads imposed by the go-around. If it stays on the airplane long enough to make it around for a wheels-down landing, you’ll be beating the odds.

In this instance, the choice, though certainly not palatable to many aircraft owners, is simply to keep the throttle at idle and ride the belly landing to the bitter end. Fatalities and injuries from inadvertent wheels-up landings are extremely rare. However, executing a go-around with damaged blades is very risky. Even if the propeller stays intact enough not to cause serious vibration, damage could be so severe that climb performance is seriously degraded.

Missing blade tip

Maintenance matters

A big part of preventing in-flight emergencies is to keep on top of propeller maintenance. Many aircraft owners believe the prop is a no-maintenance item. Not so.

Overhaul periods deserve respect because what kills most props are not surface defects, but unseen internal corrosion.

Take the opportunity at the normal oil-change interval to have a more detailed look at the prop. Have the mechanic file out any nicks now, while you have the time. This is not, incidentally, a procedure a pilot can legally undertake, nor should you attempt to file the prop with supervision unless you’ve been specifically trained. Dressing prop blades is an art, because just enough must be filed to remove the nick but not so much that a lot of blade is sacrificed to do it. The idea is to get completely to the bottom of any nick so that there can be no stress riser that can later cause blade failure. There are blade minimum dimensions, and if too much is filed off, it must be replaced. Any propeller overhauler can provide the dimensions, or you can request service information from the propeller manufacturer.

Dressing the prop — a job for an A&P mechanic.

Tachometers

Have the tachometer checked annually. Mechanical tachs are notoriously inaccurate and subject to drift over their lifetimes. If your engine and prop combination has one or more yellow arcs or red restricted arcs within the normal operating range — common to a lot of four-cylinder Lycomings — then it’s vital that the tach accurately guides you out of these trouble spots. These limitations are in place because of vibration characteristics of the engine and prop combination and can lead to long-term trouble if routinely ignored.

Overhaul at the time or calendar limit — whichever occurs first.

Hours or Years?

Props have recommended overhaul intervals based on calendar time and flight hours. Depending upon the prop model, this could be 1,500 or 2,000 flight hours, but there’s also a calendar limit (typically five years) that too many pilots ignore. This is a serious problem in a fleet that flies, on average, fewer than 100 hours per year. At 100 hours per year, a typical 2,000-hour prop might not get checked for 20 years! This is clearly imprudent, so the calendar limit applies.

Have the prop overhauled at either the time or calendar limit, whichever occurs first. If the engine comes up for overhaul before the prop reaches either limit, most shops will recommend removing the prop and governor and having them overhauled anyway. This will get the times in sync.

Corroded bearings

Corrosion Is the Culprit

Overhaul periods deserve respect because what kills most props are not external defects, but unseen internal corrosion. Dissimilar metals in the prop and hub create an environment ripe for corrosion, and the only way to properly inspect many of these areas is through a teardown. Extensive corrosion can dramatically reduce the strength of the blades or hub. Even seemingly minor corrosion may cause a blade or hub to fail an inspection. Because of the safety implications, this is clearly not an area in which to skimp.

Corroded blade clamp

Corrosion can be internal in critical blade retention components. Such conditions present both a hidden defect and a potential safety-of-flight issue. This is the primary reason that calendar limits are an important inspection requirement. Also, the overhaul needs to include more than just a corrosion inspection. Restoration of paint and plating are important to assure future corrosion protection until the next overhaul.

A badly corroded blade

Notification

It’s vitally important to keep up-to-date with airworthiness directives (ADs) or service bulletins (SBs) for your prop. Compliance with ADs is, of course, required to make the airplane legally airworthy, but it’s also good form to follow the SBs — particularly those marked “mandatory” — because there may be prop maladies that manifest themselves only under certain conditions, such as aerobatic flight or basing the airplane in a salt-intensive environment. All work performed on the prop — including AD and SB compliance — should be noted in the propeller logbook.

Airworthiness directives and service bulletins Aircraft owners are seemingly besieged by ADs and SBs. Although some owners see only an outlay of cash, paying attention to ADs and SBs, particularly where propellers are concerned, might just save your life. For Part 91 operators, only ADs are mandatory. An airplane is not considered airworthy unless all ADs have been complied with, either by proving by model or serial number that the AD does not specifically apply, or by showing that an inspection or replacement of parts has taken place. Compliance with ADs is required to be noted in the aircraft, engine, and propeller logbooks. It's vitally important to keep up-to-date with airworthiness directives (ADs) or service bulletins (SBs) for your prop. SBs, even those marked by the manufacturer as “mandatory,” are purely optional for Part 91 operators. Does this mean you should ignore them? Hardly. Many ADs are simply rewritten SBs, and in many cases, compliance with a previously optional SB will cover the requirements of a new AD. SBs can also provide useful service information. A list of SBs can be ordered from the propeller’s manufacturer. Current AD information can be downloaded from the AOPA Web site, www.aopa.org.

Overhaul: What Do They Do?

This prop actuating cylinder burst from pressure created by a rusted governor pressure relief valve that stuck in the closed position.

Upon receipt for overhaul, a document is prepared that will track the propeller components throughout the overhaul process. All applicable ADs, current specifications, and manufacturers’ SBs are researched for incorporation during the overhaul process. The serial number is double-checked, and notes are made on the work order regarding the general condition in which the propeller was received.

As the unit is disassembled and cleaned, a preliminary inspection is accomplished on all related parts. Those revealing discrepancies requiring rework or replacement are recorded in the overhaul record by part number, along with the reason for the required action.

Dissimilar metals in the prop and hub create an environment ripe for corrosion, and the only way to properly inspect many of these areas is through a teardown.

All threaded fasteners are discarded during disassembly and, with a few exceptions permitted by the manufacturer, are replaced with new components. Many specialized tools and fixtures are required in the disassembly and proper reassembly of propellers. These tools are generally model specific and range from massive 15-foot torque adapter bars and 100-ton presses down to tiny dowel pin alignment devices.

The Hub

Nonferrous hubs and components are stripped of paint and anodize and inspected for cracks using a liquid penetrant inspection (LPI) procedure. The parts are etched, rinsed, dried, and then immersed in a fluorescent penetrant solution. After soaking in the penetrant, they are rinsed again and blown dry. Developer is then applied, which draws any penetrant caught in cracks or defects to the surface. Under an ultraviolet inspection lamp, the penetrant clearly identifies the flaw. Certain models of hubs are also eddy-current inspected around critical, high-stress areas. Eddy current testing passes an electrical current through a conductive material that, when disturbed by a crack or other flaw, causes a fluctuation on a meter or CRT display. This method of inspection can detect flaws that are below the surface of the material and not exposed to the eye.

A cracked hub

Magnetic particle inspection (MPI) is used to locate flaws in steel parts. The steel parts of the propeller are magnetized by passing a strong electrical current through them. A suspension of fluorescent iron oxide powder and solvent is spread over the parts. While magnetized, the particles within the fluid on the parts surface immediately align themselves with the discontinuity. When examined under black light, the crack or fault shows as a bright fluorescent line.

Components that are subject to wear are dimensionally inspected to the manufacturer’s specifications. After passing inspection, aluminum parts are anodized and steel parts are cadmium plated for maximum protection against corrosion.

The Blades

Disassembly work stand

The first step in blade overhaul is the precise measurement of blade width, thickness, face alignment, blade angles, and length. The measurements are then recorded on each blade’s inspection record and checked against the minimum acceptable overhaul specifications established by the manufacturer.

Blade overhaul involves surface grinding and re-pitching, if necessary. Occasionally, blade straightening is also required. The manufacturer’s specification dictates certain allowable limits within which a damaged blade may be cold straightened and returned to airworthy condition. Specialized tooling and precision measuring equipment permit pitch changes or corrections of less than 1/10 of 1 degree. To ensure accuracy, face alignment and angle measurements are taken repeatedly during the repair process.

The older wooden props need thorough inspection.

Precision hand grinding of the blade airfoil is done to remove all corrosion, scratches, and surface flaws. When all stress risers and faults have been completely removed, final blade measurements are taken and recorded on each blade’s inspection record. The propeller blades are balanced to match each other and are anodized and painted for long-term corrosion protection.

Prop Reassembly

When both the hubs and the blades have completed the overhaul process, the propeller is ready for final assembly. Part numbers are re-checked with the manufacturer’s specifications. The parts are lubricated and installed per each unit’s particular overhaul manual. After final assembly, both high- and low-pitch blade angles on constant-speed propellers are checked for proper operation and leaks by cycling the propeller with air pressure through its blade angle range. The assembled propeller is then checked for static balance. If necessary, weights are placed on the hub areas of each “light” blade socket to bring about its proper balance. These weights should be considered part of the basic hub assembly and should not be moved during subsequent dynamic balancing to the engine. As with most aircraft components, all of the hardware on the propeller assembly must be safety wired unless secured by self-locking devices. Maintenance release tags reflecting the work accomplished, applicable ADs, and all incorporated service documents are then filled out and signed by the final inspector. These documents certify that the major repairs and/or alterations that have been made meet established standards and that the propeller is approved for return to service.

Accident Briefs

From the NTSB Files

The pilot of the Cessna 150 stopped at the terminal after a local flight. A person standing on the ramp came over to the airplane and talked to the pilot through the open door. The engine was left idling. After the conversation had concluded, the bystander waved goodbye and walked into the propeller, sustaining serious injuries.

It’s always good form to shut down the engine in the vicinity of onlookers, even if they are experienced pilots.

Anytime you hit a solid object with the prop — whether it's during a takeoff on a rough strip or the misguided attempt to salvage a gear-up landing — you should immediately discontinue the takeoff.

From the NTSB Files

The Taylorcraft was in cruise when a sudden severe vibration went through the airframe. The pilot shut down the engine and noted that the brass abrasion strip and part of one wooden blade had departed. Although the pilot landed the airplane in a field, it could not be stopped before plunging down a ravine. A mechanic who recovered the aircraft reported seeing dry rot in part of a remaining blade.

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

From the NTSB Files

During the takeoff roll, the Piper Super Cub’s right main gear hit a hole. The pilot said he heard the propeller strike a rock. He continued the takeoff, but after liftoff, the airplane began to vibrate. The pilot reduced engine power and landed in a river. Examination of the propeller disclosed approximately three inches of one blade missing.

Anytime you hit a solid object with the prop — whether it’s during a takeoff on a rough strip or the misguided attempt to salvage a gear-up landing — you should immediately discontinue the takeoff. This pilot was lucky that the imbalanced prop did not do serious secondary damage to the engine or airframe. In the worst case, the engine can be shaken from its mounts; the loss of engine weight will, in most cases, make the airplane uncontrollable.

From the NTSB Files

The pilot of the Air Tractor noticed serious engine vibration during a spraying run. He reduced power, then reapplied power, but the vibration remained. The aircraft was landed in wet terrain and nosed over. Inspection revealed that a bolt in the propeller counterweight had broken.

Something as seemingly simple as a counterweight caused this pilot to sit up and take notice. The moral here is that quick action is the key to salvaging a prop problem; the pilot walked away without injuries.

From the NTSB Files

A factory-overhauled engine had been installed in the Cessna Cardinal RG. The pilot departed on a

Training: If you don't have it, don't try hand-propping the airplane.

post-maintenance test hop. Eleven minutes into the flight, the pilot reported a runaway prop. The engine seized shortly thereafter. A forced landing on a soccer field ended in a collision with two vehicles on an adjoining road; the pilot was not injured. Later inspection revealed that one of two required gaskets at the base of the prop governor pad had been omitted, allowing the engine oil to be vented overboard.

Good work on the part of the pilot for keeping this dead-stick Cardinal under control. It serves as a reminder, though, that any flight after major maintenance should be conducted as close to a suitable runway as possible.

From the NTSB Files

During cruise flight, about half of one prop blade departed the Beech Musketeer, resulting in substantial engine and airframe damage. The airplane landed uneventfully at a nearby airport. Inspection later revealed that the prop blade had failed from fatigue that started from a single corrosion pit on the camber side of the blade. There were no records in the aircraft’s logbooks that the prop had been serviced since it was manufactured in September 1963.

Wine may improve with age, but metal does not. That the prop had apparently been untouched for 33 years when the accident occurred is nothing short of amazing. Again, most prop makers specify, in addition to the hours-in-service limits, a calendar limitation of, on average, five years between overhauls. This one was way overdue.

From the NTSB Files

The pilot of the Piper Cherokee 140 was hand-propping the engine when it started. The unmanned airplane taxied away and struck a building, with the pilot’s four-year-old daughter riding in the right front seat. This was the pilot’s first attempt at hand-propping an airplane; he had attempted the start about 20 times before the engine ran.

One word here: Training. If you don’t have it, don’t try hand-propping the airplane. Moreover, tying the aircraft down or having an experienced pilot at the controls of this Cherokee would have prevented this accident.

Keep on top of propeller maintenance.

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© Copyright 1998, AOPA Air Safety Foundation
421 Aviation Way, Frederick, Maryland 21701; Telephone: 800/638-3101
Internet: www.aopa.org/asf
Publisher: Bruce Landsberg
Editors: Kathy Dondzila, John Steuernagle
Consultant: Marc Cook

Sponsored by Hartzell Propeller, Inc.
A partner in safety.

Special thanks to:

Hartzell Propeller, Inc.
Aircraft Owners and Pilots Association
Aircraft Propeller Service, Inc.
Worldwide Aircraft Propeller Association
Frederick Aviation

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Posted Friday, March 27, 1998 4:25:06 PM