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.
April 28, 1997
Most of us have been taught to hand rotate the propeller before normal
startup or after shutdown for the following suggested reasons:
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.
If your aircraft is stored longer than 10 days to lubricate the internal
To align the prop in a vertical position for outside storage so as
to allow rain and ice to evacuate the spinner area.
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.
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.
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!
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:
The ignition key should be out of the ignition and the switch in the
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.)
Make sure the aircraft is secured by wheel chocks and/or tiedowns.
Engage the emergency brake.
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.
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
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