Thanks everyone for the valuable input.
Interesting that Cessna includes it in new generation Cessnas, given the other risks of hand propping.
The POH for the Cessna 172S (the in-production version) recommends hand-turning the prop in the opposite direction before cold-weather staring. (It also recommends preheat, of course.)
Turning the propeller backwards greatly reduces the risk of an accidental start, as the magneto impulse couplings will be disabled.However, that means that the vacuum pump will also be being turned backward, and that’s less good for them. If the vanes are worn, or there’s some oily crud in there, it can break it inside, or shear the drive. I avoid turning engines backward as much as possible.
Is there still a vacuum pump on modern Cessnas?
There is, because they are one of the very few manufacturers to still use classic steam gauges as backups. Hence, one vacuum-driven AI.
“Turning the propeller backwards … However, that means that the vacuum pump will also be being turned backward, and that’s less good for them.”
That’s what I always been told too. But from here:
“Some people say you can break a vacuum pump vane by turning the prop backwards, but I’ve never actually seen that in 25 years of maintenance.”
On our Grob 109B there is no primer, neither manual nor electric. In the winter I always set throttle idle, choker ON, and pull the prop through 5-7 times before starting the engine. The starter and/or battery is a little tired in cold weather, and it greatly helps starting. I believe it is not so much the “limbering” of the oil, but the “priming” of the engine by sucking in fuel/air to the cylinders even before starter is first activated that does it. Of course that requires the prop to be pulled through in the normal direction.
I hardly ever do it on Lyco/Cont airplanes with primers or injection.
I understand the Grob has a large displacement VW based engine, maybe with Bing CV carbs… If it has Bings it’s much like starting a BMW motorcycle with same carbs. I can imagine pulling through with choke on and throttle completely closed would work well to get some fuel in the intake.
An old A-65 or C-85 Continental with Stromberg carb was specifically designed to be primed in the same way, and the engines that were Stromberg equipped often had no electric starter, so pulling them through to prime is a natural part of the process before making the mags hot.
The reason I have heard for this hand turning was to get the oil pump to feed a bit of oil to the bearings. That is all that could possibly do; just a little bit. I can’t believe it does anything useful because the starter motor – which is turning the engine quite slowly too – will achieve more in a few seconds.
So I think this is nonsense, other than a “pre start test” i.e. if you can’t turn the prop then you should not try starting the engine.
Folklore & myths!
So much of this stuff is half understood truth perpetuated under the heading of airmanship, the quality of what is filed under airmanship depends if it is common sense applied to aircraft or something your instructor half understood while training and repeats.
Just like Huv Commented above I have never In forty + years in the business ever seen a vac pump damaged by pulling the prop backwards by hand.
It has been said above that turning the engine backwards might damage a badly worn or contaminated vac pump……. good ! If it breaks it should long ago have been changed and the breakage should be seen as a flight safety enhancement.
As to the pre-start test Peter writes about this comes from radial and inverted engine practice to prevent the cylinders hydraulic locking if one or more are full of oil and is of course not applicable to the opposed engines found in most GA aircraft.