Jesse wrote:
Quit a lot of maintenance people turn the prop backwards for this reason if possible. That way you can’t trigger the impulse coupling, and reduce the risk. Some don’t agree with that though, mainly due to the fair of damaging the vacuum pump vanes.
Changes of damaging them is quite small IMHO, as the engine will kick back at unsuccesfull starting as well and sometimes during switch off as well. Also most manufacturers don’t consider this to be an issue.
Quite. I flew the personal aircraft of the man considered THE Cessna guru in southern Africa on two trips and that’s exactly the advice he gave me.
With a Rotax, I have been told it has to be avoided at all cost to turn it backwards as it would be damaged. Does anyone know why? Or is that not true?
Rwy20 wrote:
Rotax, I have been told it has to be avoided at all cost to turn it backwards
Rotax uses dry sump forced flow lubrication system (no oil pan). The prop must be turned top/left counterclockwise before each flight to ‘burp’ the engine by forcing oil from the reservoir into the crankcase and lifters. If you turn the prop backward you force air into the crankcase and lifters and ruin them.
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This was one of the factors I considered whilst designing the RV10 switches, whether to use the classic key or separate Mag switches.
I chose separate. This way you have a little more control, and the switches should be a higher reliability.
Using separate you can choose to spin the starter with mags off, to ensure they are grounded and safe.
As was pointed out earlier: mags only have a thin ground wire! A few years back I found a broken wire and nobody had noticed as the engine was always lean cut.
carlmeek wrote:
Using separate you can choose to spin the starter with mags off, to ensure they are grounded and safe.As was pointed out earlier: mags only have a thin ground wire! A few years back I found a broken wire and nobody had noticed as the engine was always lean cut.
Not sure that “to spin the starter with mags off,” is a conclusive test that the mags are indeed grounded since starts are not 100% even with hot mags.
Much better to test the mag grounding before shut-down, which is 100% conclusive.
Michael wrote:
Much better to test the mag grounding before shut-down, which is 100% conclusive.
Agree. Having a key is another advantage, as you can check from outside the aircraft that the magneto’s are at least switch off, by placing the keys on the glareshield, visible from the outside. This is also quite common practice at many maintenance facilities. Having the keys visible. Even with the keys visible the prop is rotated backwards for safety . And perform the magneto grounding test at every shut-down.
For a compression check, where you will rotate forward, I would also remove the sparkplug caps from the sparkplugs which don’t get removed for the compression check.
USFlyer wrote:
Rotax uses dry sump forced flow lubrication system (no oil pan). The prop must be turned top/left counterclockwise before each flight to ‘burp’ the engine by forcing oil from the reservoir into the crankcase and lifters. If you turn the prop backward you force air into the crankcase and lifters and ruin them.
Poppycock !
First off, we are talking about just pushing the prop a few degrees or half a turn – there is never any reason to do more so.
You do not force air into the crankcase from the oil pump (the cylinders do a great job of that!)
You will NOT ruin the lifters by pushing the prop back, period.
That said, Rotax SB states:
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 .. “engines which have had the prop spun for more than 1 turn in reverse direction allowing air to be ingested into the valve train.”
carlmeek wrote:
As was pointed out earlier: mags only have a thin ground wire! A few years back I found a broken wire and nobody had noticed as the engine was always lean cut.That should have been noticed at the mag check done before every takeoff!! If people had been flying the aircraft without noticing (or bothering), I would have a serious talk with them about engine check procedures.
And one of the reasons the engine is lean cut is to reduce the risk of it firing when you turn the prop by hand.
Airborne_Again wrote:
And one of the reasons the engine is lean cut is to reduce the risk of it firing when you turn the prop by hand.
Unfortunately, Lycomings (in particular) are notorious for dumping fuel in the cylinders during cool down, hence why starting when still hot can be problematic.
Airborne_Again wrote:
That should have been noticed at the mag check done before every takeoff!!
Does that not depend on the aircraft and how the mag switch is wired?
For example, on a Cessna type, would a mag check during run-up (checking L and R) reveal a broken wire at the OFF position?
