During pre-flight which direction should I be rotating the propeller of a Cessna 172 if I need to reach the alternator belt? I thought I was told "In the direction it rotates during flight"; however I feel there was some misunderstanding. Isn't there the possibility of accidentally hand propping if I do it that way on a warm engine? Today I rotated it clockwise in the direction of the blade and it kicked over and popped me in the knee (leg shouldn't have been there - lesson learned!) Please help me clarify!
Almost everyone in the industry has opinions about this, sometimes with good reason. I know knowledgeable people with good arguments for why you should only turn the prop backwards, never turn the prop backwards, or why it doesn't matter either way.
In any case, the issues of concern are primarily:
- The engine firing and rotating the propeller, or
- The dry vacuum pump sustaining damage.
Those who say "you should never turn a prop forwards," will typically argue that:
- Rotating the prop in the firing direction can cause the engine to fire, if:
- A magneto (with an impulse coupling) is live (faulty ground or actually switched on), and
- The cylinder still has a burnable fuel/air mixture (particularly if recently not properly shut-down).
- Rotating the prop backwards eliminates the possibility of the impulse coupling catching and sending a spark to any cylinder.
Those who say "you should never turn a prop backwards," will typically argue that:
- Dry vacuum pumps are fragile and only meant to be turned in one direction (with normal engine rotation), and
- Rotating the prop backwards introduces the possibility of damaging the vacuum pump's (relatively) delicate carbon vanes leading to pump failure.
Those who hold that either direction is potentially dangerous but can be done with caution will typically argue that:
- Backward movement of the prop—especially small, gentle, cautious movement—introduces little likelihood of damage to the vacuum pump vanes;
- Good engine shutdown and magneto switch policing will very nearly eliminate the possibility of an inadvertent cylinder firing, and
- Carefully moving the prop forward while treating the engine as live will give you a good margin of safety.
One may present a variation on the above, but I think those three perspectives capture the essence of any argument on this topic. Inadvertent engine firing can indeed be very dangerous, and a piston prop should be treated as live and dangerous. Dry vacuum pumps are indeed built with carbon or composite vanes which are far more brittle and delicate than steel parts. However, there are differing takes on just how delicate these are.
"Healthy dry air pump, left, showing graphite vanes angled in carbon rotor. When an air pump fails, its rotor and vanes often shatter" — Photo and caption as seen in Aviation Consumer
The Australian Civil Aviation Safety Authority (CASA) states that:
Simply pulling the propeller backwards to “test” cylinder compressions may fail the pump by forcing the vanes to rotate in a direction opposite to their intended design. In order to prevent the vacuum pumps from being operated in reverse, never turn the propeller backwards.
However, Aviation Consumer states in this article that:
One of aviation’s old wives’ tales is that you’ll wreck your vacuum pump if you move the prop backward. Fortunately, that’s not true. While most vacuum pumps are unidirectional—they either turn clockwise or counterclockwise in service based on the direction of rotation of the accessory shaft on the engine (and have CC or CW as a suffice to their model number)—it takes more than a few turns in the wrong direction to cause damage.
Mike Berry, whose opinion on such matters I respect, seems to think that turning the prop backwards will not hurt it, as seen in this quote from his May 2002 article in Light Plane Maintenance, reprinted by AvWeb.com:
By the way, pulling a prop through backwards will not hurt a dry pump.
I know people that operate and maintain several Cessnas with dry vacuum pumps and are not very gentle when they move the prop either forward or backward they don't seem to have issues either with the engine firing or with low vacuum pump replacement intervals. The vacuum pumps replacements that I have done were typically brought about by normal long-term wear, as far as I can tell.
My take is: firstly, know your aircraft and it's systems. Secondly, be careful; respect the prop. Personally, I turn the prop either direction, but not usually a lot, and I do so gently and cautiously.
Avoid rotating the propeller through a compression of one of the cylinders. You can easily rotate the propeller in either direction safely about 45 degrees before you feel a cylinder in compression.
If you want to pull the propeller completely through a compression stroke you must be prepared for it to snap around after reaching the top of the compression stroke and that is how you must have whacked your knee.
Normally after an engine shutdown the mixture is pulled to idle cutoff which would prevent fuel reaching the cylinders. Also if procedures are followed the magnetos that generate ignition sparks should be off after the end of a flight. So in theory it should be safe to pull the propeller thru during preflight. But still you need to consider that it is not completely without some small risk that the engine could start simply because humans make errors and machines can need maintenance.