Pitch Imperfect

A constant-speed propeller takes a good airplane and makes it great. Varying the pitch angle of a prop reduces fuel consumption, enhances performance in all phases of flight, and can reduce strain on the engine compared to a fixed-pitch prop. 

But, just as with other systems that improve aircraft performance, like retractable gears, a constant-speed prop is another thing on the airplane that can break.

Regardless of the cause of a failure, the seriousness of a stuck or uncontrollable prop pitch can represent anywhere from a mere nuisance to an immediate emergency. As such, it’s important to be able to correctly identify the failure, the prop’s pitch and degree of controllability (if any), and how to respond. 

The following advice on identifying and responding to prop problems (drawn from the AOPA Air Safety Institute) applies to single-engine constant-speed props adjusted by hydraulic pistons.

During the runup

You’re still on the ground, so the analysis is pretty simple here. If any of the following are true, taxi back to the hangar and find an A&P.

• Prop control moves freely with no significant changes in required force as you advance or retract the prop pitch handle (although it’s normal if different amounts of effort are needed to advance the handle versus pulling it back).
• Prop responds quickly to commands from the prop pitch handle.
• Prop returns to the set runup speed after fully cycling it.
• Oil pressure fluctuates when exercising the prop.

On takeoff

There are a few main indicators of prop pitch failures.

• Audible overspeeding (if able, consider leaving the ANR function on your headset turned off during takeoff).
• RPM does not reach or approach redline RPM shortly after applying full throttle for takeoff.
• Uncharacteristic vibration.

If you have enough runway to do so, reject the takeoff. Otherwise, fly the pattern and return to the airport.

In flight

For a prop pitch failure in cruise, we need to look at the most likely cause in addition to simply identifying a symptom. That cause will most likely be some form of governor failure (not to get political).

Once again, overspeeding can tip you off to an issue, as identified by sound or an increase in RPM. If you identify such an issue:

• Throttle to idle; airspeed to best glide; look for a landing spot.
• Check oil gauges. If there's low pressure and high temp, you’ve got bigger problems than the governor; the engine is about to seize from a lack of oil. If normal pressure and temp, add throttle slowly to see what the prop does. If you can take it to redline to produce at least a little power, do so.
• If you can get to an airport, but it would require overspeeding the engine, do it. It's better to land at an airport with a toasted engine than in a field where you might become toast.

Other failures can happen, such as a loose or snapped prop pitch cable, in which case you need to determine quickly the state of the prop. If it’s overspeeding, it doesn’t matter if it’s due to a snapped cable or a failed governor. Follow the overspeed procedure. 

Otherwise, determine what kind of power you can still generate and land at an airport as soon as you can safely.

Of course, these recommendations from ASI don’t outrank anything found in your airplane’s POH. That is always the first, best authority.

It goes without saying (but we'll say it, anyway): Prevention is preferable to any cure. Proper maintenance is the best way to keep your plane and yourself from peril. The root cause of mechanically induced accidents is almost always neglect, and prop pitch failures are no exception.

Know Before You Go