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Props: What to Look For and Maintenance

Safety Spotlight: Aircraft Systems

close up on the weathering on an aircraft propeller blade

Many pilots seem to take the prop for granted, so the next time you fly, take time to carefully check it. Here’s what to look for:

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? 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 are numerous nicks, say from a recent departure from a gravel strip) should be dressed out immediately by an A&P mechanic.

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 on the ground. 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. Investigate any evidence of red oil 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 the 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.

Tip: Props can sustain significant damage when operating over loose gravel. If the airplane is parked on a gravel surface, move it to hard ground before starting the engine. Likewise, when taxiing to the tiedown spot—if it is on gravel, shut the engine down on hard ground and use a tow bar to move the airplane.

In the Cockpit

Prior to flight, take a few precautions to ensure that your prop will perform as needed. For fixed-pitch props, listen and feel for unusual noises and vibration. Because there’s so little to go wrong with a fixed-pitch propeller, you’re basically on the lookout for gross problems like loose bolts or a tip that departed during the start and taxi sequence.

During the Runup

For constant-speed applications, there are additional considerations. Keep an eye on the oil pressure and temperature. Pressure should be in the normal, green-arc range, and the temperature should be rising according to outside conditions. Because the constant-speed prop needs both good oil pressure to do its job and oil thin enough to be pumped through the smaller passages of the prop, it’s important to keep these parameters in mind, particularly for cold-weather departures. A takeoff with cold oil will result in a poorly governed prop and a possible overspeed event. In subfreezing conditions, it could take 15 to 30 minutes to get minimum oil temperature. Storing the airplane overnight in a heated hangar or calling for an engine preheat will help reduce that time. 

On the Takeoff Roll

Takeoff is a busy time but ensure that the prop and its governing systems are functioning properly.

  • 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 a major 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 stop safely) or continue around the pattern to a normal landing using low power settings.

In Flight

There is no clear pattern of propeller failures: they can happen during any phase of flight. Warning signs in flight may be the sudden onset of grease or oil leakage or vibration. Should those occur, you should investigate immediately.

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 stick at the set rpm or to spin rapidly beyond the redline.

Blade Failures

It’s almost too obvious to state, but the departure of any part of the prop will get your attention in a hurry. If that happens, stay calm.

In the event of any type of propeller failure, always remember to fly the airplane.

In the event of a propeller issue:

  • Immediately reduce power with the throttle.
  • Slow the airplane to best glide speed 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 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.
  • Unless you are sure that the engine roughness is something other than the prop, do not advance the throttle again. The tremendous 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.

In the event of any type of propeller failure, always remember to fly the airplane.

‘I just knew what to do’

Spencer Suderman was cruising over Southern California on April 27, 2019, on his way to Arizona for a crack at beating his own inverted flat spin world record, when the propeller departed his Pitts. The veteran instructor and airshow pilot spoke to AOPA about how training and experience proved crucial. Read full article

Governor Failures—Overspeed

The following information applies to single-engine aircraft with loss of governor oil pressure to the prop; i.e. overspeed. A governor failure that causes engine overspeed or poor prop control is also a possibility. Wild changes in prop rpm in flight can signal loss of governing control, forcing the blades into the fine or high-rpm pitch settings. When this happens, get the engine speed down before it does any damage.

If a governor fails to supply oil to the propeller, the failure effects are different depending on whether the propeller is a pressure-to-decrease-pitch or pressure-to-increase-pitch design. On multiengine aircraft, loss of pressure will cause the prop to feather. On most single-engine aircraft, loss of pressure will cause an overspeed.

Whenever something causes the prop to overspeed, there’s a good bet its oil pressure related.

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.
  • Set best-glide airspeed, and start looking for a place to land.
  • Check the oil-pressure gauge. Many prop overspeeds 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. Still, remember that it’s better to fly to an airport with an overspeeding engine than to land in a field because you cut the throttle.
  • Plan for a landing as soon as practical. You may have a broken oil-supply line to the governor 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 may also be loss of oil pressure and subsequent seizure of the engine. Whenever something causes the prop to overspeed, there’s a good bet it is oil pressure related.

 

One strike, you’re out!

It’s a sad tale that’s repeated several times a year. The overworked pilot, perhaps coming home from a difficult day of flying terminated by a challenging instrument approach, gets distracted and forgets to put the gear down. At the first sound of crunching metal and the strange stroboscopic blur of the prop taking chunks out of the runway, many pilots’ first reaction is to add power and try to salvage the situation. Two words: Do not!

In the case of a gear-up landing, 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 gear-up landings are extremely rare. However, executing a go-around with damaged blades is very risky. As soon as the first blade tip hits the concrete, that prop is ruined and unable to carry the aerodynamic and structural loads imposed by a go-around. If the prop stays on the airplane long enough to make it around for a wheels-down landing, you’d be beating the odds.

Maintenance Matters

Many aircraft owners believe the prop is a no-maintenance item. That is not the case.

Have a more detailed look at the prop at the normal oil-change interval. Have the mechanic file out any nicks now, while you have the time. Dressing prop blades is an art: just enough must be filed to remove the nick but not so much that the integrity of the blade is sacrificed to do it. 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.

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 with many four-cylinder Lycomings—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.

Respect overhaul periods. What kills most props are not external defects, but unseen internal corrosion.

Hours or Years?

Student embarrassed by instructor requesting logbook information.  Reproduction of this image is prohibited without written consent on photographer's invoice, stating rights granted, and payment in full of said invoice.Props, like engines, 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.

Corrosion is the Culprit

Respect overhaul periods. 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, and 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.

Internal corrosion can develop 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.