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Cold starting 0-360 (does manual propeller rotation do anything?)

Engine preheat posts moved to the Engine preheat thread

Administrator
Shoreham EGKA, United Kingdom

It’s unlikely that you’ll turn the prop by hand enough to prime the oil pump, and thereafter supply any meaningful amount of oil to the bearings under pressure. It’s more about breaking the sticktion of very cold oil, particularly single weight, on the high friction surfaces like the cylinder walls. When the engine is freed up a little inside, the starter does less work to start the engine. If the oil is that cold, so is the battery. Some older large continental powered Cessnas had oil dilution systems for this purpose. As you operated the system, some gasoline was injected into the engine oil to reduce its viscosity. Once the engine was running, the gasoline would burn off. I’ve only used one of these once, in my early days, and have not seen one at all in decades. Probably multigrade oils made these systems seem unnecessary, and preheating became a lot easier. Many pilots I know will carry a very small generator to run the preheat, when they know that they’re landing away from electricty.

That said, I still do not advocate this, unless you know the airplane very well, and there is no possibility of an effective preheat. If you know that plane that well, you should have made a plan for preheat. When I was the second pilot of the Aztec in far northern Ontario in the winter, I was assigned to take a cab out to the airport during a layover, unwrap the engines, and run them to keep them warm. Every three hours at the longest, while we waited for our passengers. After a few years of this, they bought the PT-6 powered Cheyenne, then it did not matter any more!

A brand new vacuum pump should be fine turning backwards, but as the vanes wear, they are more sloppy in the grooves of the hub, and can cock over sideways and break, even breaking the hub too. I have seen this. Usually when that happens, the vacuum pump jams, and the drive shears too, but you were already replacing it anyway. My first vacuum pump on my 150 lasted more than 3500 hours in service (as did the engine), as the previous owner, and I were both careful with it.

Home runway, in central Ontario, Canada, Canada

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.

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.

Administrator
Shoreham EGKA, United Kingdom

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.

Last Edited by Silvaire at 11 Oct 02:30

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.

Last Edited by huv at 10 Oct 22:32
huv
EKRK, Denmark

“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.”

huv
EKRK, Denmark

There is, because they are one of the very few manufacturers to still use classic steam gauges as backups. Hence, one vacuum-driven AI.

Last Edited by boscomantico at 10 Oct 12:07
Mainz (EDFZ) & Egelsbach (EDFE), Germany

Is there still a vacuum pump on modern Cessnas?

France

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.

Home runway, in central Ontario, Canada, Canada
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