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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!

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I've never had the prop block access to the alternator belt. – Dawn Feb 14 at 16:20
I've also never had the prop block access to the belt, even with the prop in the 45° over the cowling opening, I can usually still see/reach it. – Ron Beyer Feb 14 at 16:24
I mean in the circumstances where it may need to be moved. – Crikit Feb 14 at 16:50
Can you give an example of a situation where you need to move the prop? – Dawn Feb 14 at 16:54
Let's not get caught up on why @Crikit needs to move the prop backwards, here, folks. The question is a valid one, details aside. – egid Feb 14 at 20:21
up vote 12 down vote accepted

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.

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45 degrees is in a 4-cylinder engine. for 6 or 9 cylinders, its a much smaller amount. – rbp Feb 15 at 18:51
many shutdown checklists have an item like "check mags for proper grounding," which suggests turning off the mags with the engine still running, to make sure that the engine coughs, and then immediately back on. This is to ensure that the mags are properly grounded, and they won't provide a spark if the engine is pulled through by the prop. It is still particularly important with rotary engines which must be pulled through to circulate oil in the top cylinders before starting. Most ops at schools/rentals won't allow this, but in a privately owned aircraft it should be done periodically. – rbp Feb 15 at 18:54
that's a completely different check. the wiring of a mag switch only has one grounding wire, so turning off only one or the other doesn't check the grounding. and if you're going to "make sure the mags are off" before pulling the prop through, its not enough to check the switch position. you'd better be sure the mag is grounded. – rbp Feb 15 at 19:08
I wish I could edit my earlier comment. The RPM would drop about 50 rpm or more. Not 500 rpm. I looked at the manual for the Cessna 172. Preflight does not offer any advice to move the propeller. Run up before takeoff mentions that the absence of an rpm drop when turning off one of the magnetos can indicate a faulty ground wire. Also it the manual is the caution to expect the small chance that the engine can fire if you pull the prop thru a compression. Source: skywarriorinc.com/downloads/POH%20BOOKS/172M%20POH.pdf – Craig K Feb 15 at 19:35
you misread what that document says. it says "An absence of RPM drop may be an indication of faulty grounding of one side of the ignition system." You can't check the grounding unless you move the switch to OFF. "No drop in rpm could be an indication of a broken P-lead. You can check this yourself by bringing the engine to idle rpm and placing the ignition switch to Off." Here's a good article to educate yourself: flighttraining.aopa.org/magazine/2002/January/… . You – rbp Feb 16 at 0:17

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.

Dry Vacuum Pump "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.

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