The age old question, can you turn the prop backward without damage to the vacuum pump. 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.
It also takes enough speed to actually throw the vanes against the case which you're not going to get by hand turning.
I knew a guy who had a mechanic that took care of a pilot in a backwoods airport that turned his prop the wrong direction and broke the vacuum pump and locked up the whole engine. So, there’s that. YMMV. Sent from my iPad using Tapatalk
Look at this: See the angle of those vanes with respect to the rotational axis? They're not unidirectional and haven't been for a long time. The pump model numbers have CW or CCW in them to indicate the rotation, which also indicates the slant of those vanes. See how that vane at the 7 o'clock position pretty much fills the slot when retracted? That pump has few hours on it. When it wears to the point where it's short enough to cock and jam in the slot if the pump is turned backward, it will break the carbon rotor. If the rotor doesn't break, the plastic drive coupling will shear. The old straight-vane pumps will break their drives, too, not lock the engine. If you break it by turning the propeller backwards, it was long past its best-before date. Long past. Rapco and Tempest pumps all have vane wear inspection provisions top tell you when to replace that pump. Buy one and get it inspected when they tell you to and you will never have pump failures unless you fly 30 hours a year and let that plastic drive coupling get old. It has a six-year recommended replacement. I don't know why anyone would buy a pump that doesn't have the vane wear inspection port, but people do. Edit: that pump in the picture above turns clockwise as seen from the camera. This minimizes the cocking forces on the vanes during normal rotation, but also increases them if turned backward. And that prop can turn backward if the engine kicks back, or as the prop stops it sometimes rocks back and forth a bit, and when a mechanic checks compression or sets the mag timing he's moving the prop both ways. Like I say, if it breaks when you turned it backwards, it was shot anyway and one should be glad it failed on the ground and not in flight at night or in IMC.
Don't turn the prop backwards too fast or you might turn back time, and I think we all had enough of the year 2020.....
In the cold weather starting section of my POH, one of the suggestions is to turn the prop a few times before starting. And it states that it SHOULD be turned backwards. I do have a dry vacuum pump.
It is a well known fact if you don’t all the sap runs to one end and the prop will be out of balance. If you leave it like that long enough the thrust will be asymmetric as well. Very bad.
If you have a hydraulic lock on a radial engine..do not turn the prop backwards... Most dry vacuum pumps will shear the plastic coupler if they lock up...only affects the vacuum pump from spinning...engine runs as normal.
The Tempest pump vane wear inspection is explained here. Pictures, too: https://tempestplus.com/wp-content/uploads/2019/01/AIA114-100110.pdf Rapco's inspection: https://www.rapcoinc.com/pdf/Service letters/Rapco Service Letter RASL-002_rev_B.pdf And this page shows how their wear measurement works: https://www.rapcoinc.com/pdf/Service letters/Rapco Service Letter RASL-002_rev_B.pdf
