Don’t Touch That Propeller!

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 normalstartup or after shutdown for the following suggested reasons:

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

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

  3. To align the prop in a vertical position for outside storage so asto 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 precautionsthat need to be taken before touching that propeller. Few CFIs or pilotexaminers provide guidance about propeller safety, apparently assumingthe 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 reasonsto 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…asCessna still suggests, though Lycoming does not.

  • If your aircraft is in storage, I would not rotate the propeller unlessI knew the spark plugs were removed and/or the magneto ignition completelydisconnected. (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 forice and water drainage or to attach your tow bar is it worth risking yourlife? Propellers are most likely to start spinning when the engine is warmwith residual fuel in the carburetor!

Some mechanics suggest rotating the propeller in the reverse directionwhich will prevent an inadvertent start as the impulse coupling is notactivated with reverse propeller rotation. However, Airborne says thatpossible 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 thesewarnings, at least be mindful of the following. There have been severalthousand recalls of faulty ignition switches, starting capacitors, starters,and magnetos in all types of aircraft, that will allow a propeller to startspinning 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 orcut (from vibration or whatever), once again you have what is known asa "hot mag" situation. This "P" lead wire can comeloose 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" situationis to carefully watch your RPM drop either during your initial run up orbefore a shutdown, assuming your tachometer is working properly to reportthis RPM change. If, when alternating from left to right magneto by switchingwith your ignition, you do not notice a magneto drop, this may indicatea "hot mag" situation, which could be caused by a disconnected"P" lead. Or, before shutdown, while the engine is running andyou are stationary and secure, you can briefly switch the ignition keyto an off position and listen if the engine wants to stop. (Do this onlyat idle RPM.) If it does, your ignition system is most likely working correctlyat that moment. Also, make sure your ignition key can not exit the ignitionwith the ignition switched in the on position!

Expert advice

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

Assuming you have a working ignition system, and you still feel thathand rotation of the propeller is essential, here are some additional precautionsto implement. First, always assume that the propeller may start and thenimplement 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. (Bewareeven if this control is in the off position, the engine can start if thereis 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 addedsafety to shut down the aircraft if it inadvertently starts.

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

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