I have always been told never to turn the Rotax engine in the wrong direction as it will cause damage, but recently seeing someone turn a prop backwards a few degrees I challenged the person who said “what damage could it do!”, this got me Googling it and I found many hits saying it will damage the engine and as many saying it won’t, below is a summary of what I found, I have also emailed Rotax but not had any response!
- Turning an engine backwards will only cause an issue if it has a dry vacuum pump.
- Turning an engine backwards to re position the prop is preferred as the engine will not fire even if the mags are on as they only work when rotated in the correct direction.
- Turning the engine in the wrong direction causes oil to be squirted into wrong places.
- Turning the engine backwards or forwards by hand causes no issues.
- Turning the engine backwards causes the engine to suck in air, all you need to do is re burp the engine in the correct direction and all is OK.
- Turning the engine backwards causes the engine to suck in air and if turned for more than one revolution this may enter the valve train, thus the engine must be vented.
What is the truth and what should one do if the engine has been turned in the wrong direction?
While there would seem to be some truth in a few of the above and others may be pure fiction, I’m not an aircraft engineer and I’m not going to comment on which are correct and which are not, but I what I have found out from 2 Rotax bulletins is this:
A rotation on less than one revolution in the reverse direction of the prop appears to require no corrective action, while a rotation of one or more revolutions in the wrong direction requires the engine to have the venting checked as layout by Rotax. The bulletin says that the rotation of the prop in the wrong direction may cause the ingestion of air into the valve chain.
Service Bulletin: Inspection for correct venting of the oil system for Rotax engine type 912 and 914 (series)
SB-912-036 R1 SB-914-022 R1
Cited in Compliance 1.5 – “engines which have had the prop spun for more than 1 turn in reverse direction allowing air to be ingested into the valve train.”
Service instruction: Venting of lubrication system for Rotax engine type 912 and 914 (series) SI-04-1997 R3
Cited in 1.3 Reason – “and/or had the prop spun in the reverse direction allowing air to be ingested into the valve train.”
I have not included the SB or SI documents as I have not had permission from Rotax, but just google SB-912-036 or SI-04-1997 and you will find the PDF files!