Damaging an engine by turning the prop backwards

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Damaging an engine by turning the prop backwards

Post by ahramin » Mon Jun 10, 2019 12:47 pm

If I need to move the propeller by hand I always turn it backwards so that the mags do not fire. Recently a pilot told me that this could damage the engine. Has anyone ever heard of a small Lycoming or Continental engine being damaged by turning the prop backwards?
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Re: Damaging an engine by turning the prop backwards

Post by linecrew » Mon Jun 10, 2019 1:09 pm

I think the pilot was thinking of hydraulic lock which only happens with downward facing pistons like the bottom one on a radial engine, not a horizontally opposed engine...but I'm no mechanic.
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Re: Damaging an engine by turning the prop backwards

Post by tired of the ground » Mon Jun 10, 2019 1:29 pm

I believe there are certain vacuum pumps and that can be damaged by being rotated backwards.
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Re: Damaging an engine by turning the prop backwards

Post by Daigo » Mon Jun 10, 2019 2:07 pm

Maybe they got it mixed up with turboprops which can have detrimental damage if spun backwards
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Re: Damaging an engine by turning the prop backwards

Post by iflyforpie » Mon Jun 10, 2019 2:11 pm

It is the dry vacuum pump that can be damaged by turning it backwards. This is because the brittle carbon vanes are “swept” opposite the direction of rotation (which is why you have CW and CCW pumps vs just changing inlet and outlet.

However, if the pump cavity is so chattered and your vanes are so short that it breaks in this manner at low rpm... your pump is on borrowed time anyways.
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Re: Damaging an engine by turning the prop backwards

Post by youhavecontrol » Mon Jun 10, 2019 4:42 pm

tired of the ground wrote:
Mon Jun 10, 2019 1:29 pm
I believe there are certain vacuum pumps and that can be damaged by being rotated backwards.
That's what I've heard too.. but I often wonder about that. Sometimes when the engine shuts down, the engine compression will push the prop slightly backwards before it rests to a stop. ..always wondered if that would do something to the Vac pump brushes if damage like that was even possible.
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Re: Damaging an engine by turning the prop backwards

Post by goingnowherefast » Mon Jun 10, 2019 4:44 pm

So what happens if the start attempt fails, the engine comes up against a compression stroke and "bounces" back. Same thing on shut down.

Free turbines spin the prop the wrong way all the time on windy days
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Re: Damaging an engine by turning the prop backwards

Post by oldtimer » Mon Jun 10, 2019 5:19 pm

It is partially the vacuum pumps as already mentioned plus the starter/generator brushes plus old wives tales. Used to do it all the time to clear a flooded engine. Never broke anything nor had a premature failure that I knew of.
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Re: Damaging an engine by turning the prop backwards

Post by PilotDAR » Mon Jun 10, 2019 5:45 pm

Yes, the vacuum pump may be damaged by turning the engine backward, though for my experience, this is low probability. The more serious problem is for larger continental engines which have a newer style starter adapter. In this case, by turning the engine backward, you're also trying to turn a worm and worm wheel backward, which does not work in these. When you try, you'll feel resistance, don't fight it. Also, these engines don't like kickback during starts. My friend trashed his continental with a new style adapter with a mismanaged jump start. The engine was filled with starter drive gear bits. Do regular live mag checks, and pull the engine through forward with immense caution
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Re: Damaging an engine by turning the prop backwards

Post by corethatthermal » Mon Jun 10, 2019 7:38 pm

Yes, the vacuum pump may be damaged by turning the engine backward, though for my experience, this is low probability. The more serious problem is for larger continental engines which have a newer style starter adapter. In this case, by turning the engine backward, you're also trying to turn a worm and worm wheel backward, which does not work in these. When you try, you'll feel resistance, don't fight it. Also, these engines don't like kickback during starts. My friend trashed his continental with a new style adapter with a mismanaged jump start. The engine was filled with starter drive gear bits. Do regular live mag checks, and pull the engine through forward with immense caution
This is the correct answer! As usual DAR is a treasure trove of information !

I have an older 182 with the older design adapter, had to go through all the information and learn everything ( even as an AME who should know better? ) Not a great design, but my engine can be turned backwards, do not worry about brushes or vacuum pumps too much but IF the starter drive does not allow backwards rotation do not do it!
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Re: Damaging an engine by turning the prop backwards

Post by Ki-ll » Mon Jun 10, 2019 8:07 pm

goingnowherefast wrote:
Mon Jun 10, 2019 4:44 pm
Free turbines spin the prop the wrong way all the time on windy days
That’s not a good thing.
At least on the Garret that’s a big no no since starter-generator brushes are worn in only one way, spinning them the other way is not good.
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Re: Damaging an engine by turning the prop backwards

Post by iflyforpie » Mon Jun 10, 2019 8:18 pm

Kind of like the vacuum pumps... I find it very curious that something that spins at tens of thousands of RPM with up to 1000 amps of current running through the series circuit brushes and 300 amps running through the shunt circuit brushes would be harmed by moving it backwards slowly when cold.

Sounds like another OWT where maybe someone damaged some paper thin brushes that were going to go anyways.
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Re: Damaging an engine by turning the prop backwards

Post by Diadem » Mon Jun 10, 2019 8:41 pm

Ki-ll wrote:
Mon Jun 10, 2019 8:07 pm
goingnowherefast wrote:
Mon Jun 10, 2019 4:44 pm
Free turbines spin the prop the wrong way all the time on windy days
That’s not a good thing.
At least on the Garret that’s a big no no since starter-generator brushes are worn in only one way, spinning them the other way is not good.
Garretts ain't free turbines. Free turbines can have the props spin back and forth all day long without the engine turning at all.
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Re: Damaging an engine by turning the prop backwards

Post by Ki-ll » Mon Jun 10, 2019 8:59 pm

Diadem wrote:
Mon Jun 10, 2019 8:41 pm
Ki-ll wrote:
Mon Jun 10, 2019 8:07 pm
goingnowherefast wrote:
Mon Jun 10, 2019 4:44 pm
Free turbines spin the prop the wrong way all the time on windy days
That’s not a good thing.
At least on the Garret that’s a big no no since starter-generator brushes are worn in only one way, spinning them the other way is not good.
Garretts ain't free turbines. Free turbines can have the props spin back and forth all day long without the engine turning at all.
My bad, should have read better.
I can’t speak about all free turbines but if iirc PT-6 has no lubrication when prop spins the gearbox alone so probably not a great thing to leave it spinning in the wind all day?
iflyforpie wrote:
Mon Jun 10, 2019 8:18 pm
Kind of like the vacuum pumps... I find it very curious that something that spins at tens of thousands of RPM with up to 1000 amps of current running through the series circuit brushes and 300 amps running through the shunt circuit brushes would be harmed by moving it backwards slowly when cold.

Sounds like another OWT where maybe someone damaged some paper thin brushes that were going to go anyways.
Not too sure. I have it from a pretty reliable source I believe. Whether a couple of revolutions in opposite direction of normal does any lasting damage - I don’t know.
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Re: Damaging an engine by turning the prop backwards

Post by goingnowherefast » Tue Jun 11, 2019 4:45 am

I haven't met anybody who would let a PT6 or other free turbine windmill all day. Maybe 5 or 10 minutes while after taking the prop tie off to hitting the starter.

Just watch a Q400 next time you see it on a windy day, the PW150 is a free turbine.
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Re: Damaging an engine by turning the prop backwards

Post by Ki-ll » Tue Jun 11, 2019 5:19 am

goingnowherefast wrote:
Tue Jun 11, 2019 4:45 am
I haven't met anybody who would let a PT6 or other free turbine windmill all day. Maybe 5 or 10 minutes while after taking the prop tie off to hitting the starter.

Just watch a Q400 next time you see it on a windy day, the PW150 is a free turbine.
For sure. They do feather it before shutdown though, don’t they?
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Re: Damaging an engine by turning the prop backwards

Post by PilotDAR » Tue Jun 11, 2019 5:36 am

Nothing on an aircraft should be allowed to flap nor freewheel needlessly. If it is parked and might move, it should be tied. Brief windmilling of PT6 props won't hurt them, prolonged windmilling introduces lubrication concerns - long long would you like to leave it windmilling to learn it was damaged due to lack of lubrication, when you could tie it?

PT6's and Allison C250 turbine engines are free turbine, and can be turned either direction briefly without harm. I have been trained with the Allison C250 to turn the rotor backward to assure that there are no carbon deposits on the free turbine ('cause you'll here a grinding sound if there is).

As for starter motors in piston engines, all Lycomings have a starter whose pinion moves to engage the ring gear when energized. With one exception, a non energized Lycoming starter is not engaged, and the starter is not harmed by turning the prop backward. The exception for Lycomings is that some of the older direct drive starters pinions will remain engaged to the ring gear following an aborted start (when your battery went flat). In that case, you'll feel that the pinion is engaged when you turn the prop (and depending upon the cowl, you may be able to see it). An electric boosted start will be required.

For Continentals (vacuum pump considered), as I mentioned, older starter adapters on O-300D's, IO-360's, 470's, 520's & 550's can be turned backward with caution. Newer starter adapters mechanically will not turn backward, and you'll feel that they won't if you try - don't force them. The O-300's and Older O-200's with a pull type starter don't care, the pinion is not engaged if the handle is not pulled. For the "key start" O-200's, if it has the original starter clutch, you can turn the engine backward gently. You can feel the starter clutch engage as you start to move the prop backward. This is one way to check that the clutch is working. While doing this, you will be turning the starter motor backward, and you'll feel the drag of doing that. Don't do it much, and you won't hurt anything. Many key start O-200's now have a new starter motor, which is Lycoming like inside the engine. You'll know this because it too removes the pinion engagement when the starter is not energized. The prop will turn either direction with equal freedom. This is one way of knowing the type of starter that engine has.

While we're talking Continental starters other than pull clutches and new starters on O-200's, if when the starter is engaged, if the clutch slips meaning it's more than a few seconds before the prop begins to turn normally, abandon the start, and have the starter serviced. Over the years I saw so many O-200 key start clutches, and larger starter adapters which were ruined because they obviously slipped during the start, and the pilot just held the starter. They got hot inside, welded themselves into engagement, and the engine started. The pilot flew, and did nothing for starter maintenance. The starter was ruined the next attempt to start. Do not "live with" a slipping Continental starter.
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Re: Damaging an engine by turning the prop backwards

Post by Ki-ll » Tue Jun 11, 2019 6:18 am

PilotDAR wrote:
Tue Jun 11, 2019 5:36 am
Nothing on an aircraft should be allowed to flap nor freewheel needlessly. If it is parked and might move, it should be tied. Brief windmilling of PT6 props won't hurt them, prolonged windmilling introduces lubrication concerns - long long would you like to leave it windmilling to learn it was damaged due to lack of lubrication, when you could tie it?

PT6's and Allison C250 turbine engines are free turbine, and can be turned either direction briefly without harm. I have been trained with the Allison C250 to turn the rotor backward to assure that there are no carbon deposits on the free turbine ('cause you'll here a grinding sound if there is).

As for starter motors in piston engines, all Lycomings have a starter whose pinion moves to engage the ring gear when energized. With one exception, a non energized Lycoming starter is not engaged, and the starter is not harmed by turning the prop backward. The exception for Lycomings is that some of the older direct drive starters pinions will remain engaged to the ring gear following an aborted start (when your battery went flat). In that case, you'll feel that the pinion is engaged when you turn the prop (and depending upon the cowl, you may be able to see it). An electric boosted start will be required.

For Continentals (vacuum pump considered), as I mentioned, older starter adapters on O-300D's, IO-360's, 470's, 520's & 550's can be turned backward with caution. Newer starter adapters mechanically will not turn backward, and you'll feel that they won't if you try - don't force them. The O-300's and Older O-200's with a pull type starter don't care, the pinion is not engaged if the handle is not pulled. For the "key start" O-200's, if it has the original starter clutch, you can turn the engine backward gently. You can feel the starter clutch engage as you start to move the prop backward. This is one way to check that the clutch is working. While doing this, you will be turning the starter motor backward, and you'll feel the drag of doing that. Don't do it much, and you won't hurt anything. Many key start O-200's now have a new starter motor, which is Lycoming like inside the engine. You'll know this because it too removes the pinion engagement when the starter is not energized. The prop will turn either direction with equal freedom. This is one way of knowing the type of starter that engine has.

While we're talking Continental starters other than pull clutches and new starters on O-200's, if when the starter is engaged, if the clutch slips meaning it's more than a few seconds before the prop begins to turn normally, abandon the start, and have the starter serviced. Over the years I saw so many O-200 key start clutches, and larger starter adapters which were ruined because they obviously slipped during the start, and the pilot just held the starter. They got hot inside, welded themselves into engagement, and the engine started. The pilot flew, and did nothing for starter maintenance. The starter was ruined the next attempt to start. Do not "live with" a slipping Continental starter.
Thanks! That’s a wealth of knowledge.
What about impulse coupling magnetos? Can they be damaged by turning the engine backwards?
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Re: Damaging an engine by turning the prop backwards

Post by iflyforpie » Tue Jun 11, 2019 7:11 am

Turning an engine backwards is actually easier on the impulse couplings because they won’t snap through. The spring loaded arms simply slide by the pins rather than getting stopped by them (which in normal rotation will retard the timing and load the spring so it will snap and produce a hot and late spark for starting). Whenever we’re doing internal timing or positioning it for install, we always rotate mags with impulse couplings backwards.
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Re: Damaging an engine by turning the prop backwards

Post by Ki-ll » Tue Jun 11, 2019 7:22 am

Good to know.
Thanks!
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Re: Damaging an engine by turning the prop backwards

Post by corethatthermal » Tue Jun 11, 2019 11:40 am

Another thing to be concerned about is lack of spin-back on the large continental starter adapters. Putting a newer geared starter or having an old non-geared but hard(er) to turn starter motor will not allow the adapter spring to fully release and the starter motor will be engaged during your flight !
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Re: Damaging an engine by turning the prop backwards

Post by PilotDAR » Tue Jun 11, 2019 2:41 pm

Putting a newer geared starter or having an old non-geared but hard(er) to turn starter motor will not allow the adapter spring to fully release and the starter motor will be engaged during your flight !
Though not associated with turning the prop backwards, this is certainly a very important aspect. Do not use geared starters on starter adapters. The drive tang of the starter motor should move freely enough that you can turn it either way with your thumb and fingers (not a wrench). If it's too stiff for this, have the motor serviced. The starter motor must turn backward about a quarter turn for the starter adapter clutch spring to release for the shaft gear. Otherwise, it'll get really hot during your flight! (And costly after you land).
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Re: Damaging an engine by turning the prop backwards

Post by bodyflyer2 » Wed Jun 12, 2019 7:12 pm

For another very engine-specific case:
Shouldn't turn a Rotax 912 etc prop backwards, well, not more than 1 turn they say.
Don't know the details but something about air getting ingested into the valve train -- have to keep oil filling the hydraulic valve tappets

One ref I found online:
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.”
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Re: Damaging an engine by turning the prop backwards

Post by Jungle Jim » Thu Jun 13, 2019 6:09 am

Guys,

I pull my prop through in the forward direction. I also do mag checks, so I know I won't have a live mag. My logic in this is that it is better to have the oil pump gears pushing oil into the engine in the normal direction as opposed to having the oil sucked out, like it would if the prop were turned backwards.

Jim
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Re: Damaging an engine by turning the prop backwards

Post by PilotDAR » Thu Jun 13, 2019 7:05 am

The effect of turning the prop backwards relative to the operation of the oil pump is minor. The important film of oil [hopefully] remaining between all the moving parts when you turn the engine over will be there whichever direction the engine is turned. Yes, if you turned the engine over forward by hand a lot, you would move some oil into the engine. Conversely, the oil pump could not create suction so as to remove oil from all the passages. Of course, we want a good oil film where it should be in the engine, the best way to get that is to assure that the oil is warmed prior to a cold start, then get the engine running, and confirm oil pressure is present. There are pre-oil systems available, but they're not common.

Live mag checks are really important. I once hand propped a friend's 150. I called "mags off" he repeated "mags off" and I pulled it through. It started and ran first pull. I stood glaring at him through the prop disc. He held up the keys for me to see. Always treat mags as though they are live.
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