Don't Touch That Propeller!

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Most piston-engine pilots habitually hand-rotate their propeller for various reasons: to orient the blades horizontally for tiedown in close quarters or vertically for parking in the rain, to "loosen the oil" prior to starting in cold weather, etc. The author used to do this, too, until his prop unexpectedly started to spin after hand rotation one cold morning...with nearly catastrophic results. After researching the subject carefully, and came to the conclusion that there are no compelling reasons ever to hand-rotate a propeller. Here's why.

Watch that prop!SafetyMost of us have been taught to hand rotate the propeller before normal startup or after shutdown for the following suggested reasons:

  1. When the outside temperature is below 20 degrees F to loosen and limber the oil before startup, which will make normal starting easier, distribute oil on bearing surfaces, and conserve battery life.

  2. If your aircraft is stored longer than 10 days to lubricate the internal engine components.

  3. To align the prop in a vertical position for outside storage so as to allow rain and ice to evacuate the spinner area.

  4. To move the propeller out of the way for attaching a tow bar.

But few of us have been properly trained about the proper precautions that need to be taken before touching that propeller. Few CFIs or pilot examiners provide guidance about propeller safety, apparently assuming the aircraft manual provides this information. In most cases, it doesn't.

Shiny Hamilton-Standard

Hands off

I have come to the conclusion that there are no compelling reasons to ever hand rotate the propeller. Consider this:

  • With the advent of multi-viscosity oils, which most of us now use, it is not necessary for anyone to pre-rotate the propeller for oil limbering...as Cessna still suggests, though Lycoming does not.

  • If your aircraft is in storage, I would not rotate the propeller unless I knew the spark plugs were removed and/or the magneto ignition completely disconnected. (Unless the mag has an internal grounding spring, a disconnected "P" lead leaves the mag hot.)

  • As far as rotating your prop to place it in a vertical position for ice and water drainage or to attach your tow bar is it worth risking your life? Propellers are most likely to start spinning when the engine is warm with residual fuel in the carburetor!

Some mechanics suggest rotating the propeller in the reverse direction which will prevent an inadvertent start as the impulse coupling is not activated with reverse propeller rotation. However, Airborne says that possible damage to dry vacuum pump vanes may result by reverse rotation.

Hot mag

If you are one of those adventurous pilots who still ignores all these warnings, at least be mindful of the following. There have been several thousand recalls of faulty ignition switches, starting capacitors, starters, and magnetos in all types of aircraft, that will allow a propeller to start spinning inadvertently when rotated, even if the keys are out of the ignition. Even with a properly working ignition switch, if the wire known as a "P" lead, running from the ignition to the magnetos, becomes disconnected or cut (from vibration or whatever), once again you have what is known as a "hot mag" situation. This "P" lead wire can come loose at anytime without notice. All of the above scenarios may be intermittent, making it difficult, if not impossible, to locate the problem.

One very simple check for a potential "hot mag" situation is to carefully watch your RPM drop either during your initial run up or before a shutdown, assuming your tachometer is working properly to report this RPM change. If, when alternating from left to right magneto by switching with your ignition, you do not notice a magneto drop, this may indicate a "hot mag" situation, which could be caused by a disconnected "P" lead. Or, before shutdown, while the engine is running and you are stationary and secure, you can briefly switch the ignition key to an off position and listen if the engine wants to stop. (Do this only at idle RPM.) If it does, your ignition system is most likely working correctly at that moment. Also, make sure your ignition key can not exit the ignition with the ignition switched in the on position!

Expert advice

Following my propeller incident, I wrote letters to the FAA and NASA concerning inadvertent propeller starts. On March 9,1994, the FAA issued "Priority Letter Airworthiness Directive 96-06-09," followed by "Owner Advisory SEB94-5A" issued on March 18,1994, alerting every registered pilot concerning a potential defective capacitor within the magneto that could cause an inadvertent propeller start. Following this directive, I contacted Tim Davis (expert from Teledyne Continental Motors) regarding Bendix magnetos, and discovered several other reasons that supported my belief to never trust your ignition system.

Assuming you have a working ignition system, and you still feel that hand rotation of the propeller is essential, here are some additional precautions to implement. First, always assume that the propeller may start and then implement the following safety precautions:

  1. The ignition key should be out of the ignition and the switch in the "off" position.

  2. The mixture control should be in the mixture shutoff position. (Beware even if this control is in the off position, the engine can start if there is residual fuel in the carburetors from priming or whatever.)

  3. Make sure the aircraft is secured by wheel chocks and/or tiedowns.

  4. Engage the emergency brake.

  5. If a pilot is available, place him or her in the front seat for added safety to shut down the aircraft if it inadvertently starts.

  6. Make sure you have been taught the proper way to evacuate the propeller area from an inadvertent start. Your survival rate is minimal without proper training, especially when you re not prepared for the propeller to start spinning.

Older pilots seem to be well aware, from experience, of the dangers surrounding propellers. But we newer pilots, taught by new instructors, may not be fully aware of the threat. If this information reaches just one pilot and saves his life, I am thankful. Meanwhile, happy flying and be proud of the fact that we are still part of the most well organized aviation community in the world, and we shall stay that way if we continue to communicate.