Maybe dumb engine start question

[Joe Larussa]

So my understanding is that starting the engine puts bad things into your oil, but flying burns it off. So you go fly it then taxi to the pump and fuel it up. I have bladders so I like to keep it full. You restart the engine to taxi to the hangar and put it away till you can fly again. Did I just  help to create a corrosion issue?

[ArtVandelay]

I leave the dipstick pulled out for 10 minutes or so after a flight to allow plenty of water vapor to escape, so I don’t know how much you burn off.
I don’t think a few minutes of idle will add that much. Now ground running for 20-30 minutes would be different.

[PeteMc]

As I understand it, but will defer to a true engine-maniac...

The oil acidity will build up as the oil sits.  Also, as the engine warms and cools (surrounding air temperature) while the plane sits for days, not minutes, it can build up humidity inside.  Some of that moisture is just in the engine case, some gets in the oil.

So those guys that start their planes for 10-15 minutes in the winter thinking they're doing good are really spreading that acidic oil and moisture around in their engine.  That's the stories you've heard about the quick starts of the engine.

In your case, after flying for a while, you have probably boiled off all the moisture and a lot of the acidity (old oil always is more acidic than new).  In addition, your engine parts are still covered with oil from the flight.  So there is no problem restarting the engine for your short taxi.  The oil is as good as it's going to get until your next change.

NOTE: There are a lot of people that say never start up again for a short taxi.  They say it is the starting that causes a lot of damage, separate from the oil question you raised.  So I try not to unnecessarily start my engine (and I used to be able to hand tow my plane the 4 spots over to where the self-serve used to be), but I too think keeping the tanks full is important.  And it's better to restart the plane after it has run for a while vs. running the engine for 3-5 minutes to get the fuel and then shutdown.

[201Steve]

So, there are people who say not to ground run your engine too hard because it doesn’t get adequate cooling. Engine gets too hot. They also say don’t ground run your engine because it doesn’t get hot enough; to burn off moisture. 
 

E’splain that, Lucy

[MikeOH]

WARNING!!!  Heretical opinion follows:

I think a lot of this 'moisture'/acid start-up/shut-down procedure thing falls into OWT territory.  Frankly, the important thing, IMHO, is to just fly frequently for close to an hour.  If you do that, I don't think you'll have corrosion issues.  I don't think twice about starting up just to taxi over to the pumps and back; I sure as hell wouldn't tow the plane there and back over this issue!!

/heretical opinion off

[PT20J]

I think we worry too much about this sort of stuff. I fly, land, taxi to the pump, shut down, fill up, start up, taxi to the hangar, shut down and put the airplane away. I see absolutely no harm in that except for highly theoretical conjecture unsupported by any actual data. 

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[MikeOH]

I believe, with Lycomings, the major risk is the camshaft; the oil drains off exposing the lobes directly to the environment and thus corrosion potential.  Therefore, if I was not flying for a protracted period I would see benefit in starting up every week just for a minute or two and re-coating the cam with oil.  I'd worry far more about the lack of oil than some moisture/acid concern over not running the engine at a high power setting for an hour.

[PT20J]

I don’t know that anyone has ever figured out the root cause of Lycoming cam problems. But, my guess is that it is some sort of design issue. This is supported by the fact that Lycoming has changed the design to roller lifters for new and rebuilt engines and DLC lifters for overhauls. It is certainly better to fly often — that’s one reason Lycoming TBO also has the often ignored calendar time limit. Frequent oil changes are also good. But I’ll bet most other things we do have only a minor effect on cam life. 

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[ArtVandelay]

11 hours ago, MikeOH said:

WARNING!!!  Heretical opinion follows:

I think a lot of this 'moisture'/acid start-up/shut-down procedure thing falls into OWT territory.  Frankly, the important thing, IMHO, is to just fly frequently for close to an hour.  If you do that, I don't think you'll have corrosion issues.  I don't think twice about starting up just to taxi over to the pumps and back; I sure as hell wouldn't tow the plane there and back over this issue!!

/heretical opinion off

The real question is, after short taxi the engine hasn’t had a chance to get warm let alone hot, so which method do you use to restart: cold or hot?

[ArtVandelay]

9 hours ago, PT20J said:

I don’t know that anyone has ever figured out the root cause of Lycoming cam problems. But, my guess is that it is some sort of design issue. This is supported by the fact that Lycoming has changed the design to roller lifters for new and rebuilt engines and DLC lifters for overhauls. It is certainly better to fly often — that’s one reason Lycoming TBO also has the often ignored calendar time limit. Frequent oil changes are also good. But I’ll bet most other things we do have only a minor effect on cam life. 

Skip

The design issue is the cam is high so gets only splash oil and no oil dripping from above.

I always let it start up at 1200 RPMs for 10-15 seconds to get the oil distributed before leaning and reducing RPMs.

[Yetti]

14 hours ago, ArtVandelay said:

I leave the dipstick pulled out for 10 minutes or so after a flight to allow plenty of water vapor to escape, so I don’t know how much you burn off.
I don’t think a few minutes of idle will add that much. Now ground running for 20-30 minutes would be different.

I'd like to think the water vapor escapes though the crankcase vent tube while running.   as the engine cools and contracts, I would think you are pulling more moisture into the now open crank case.    https://www.aopa.org/news-and-media/all-news/2018/april/flight-training-magazine/how-it-works-breather-tube

 

Edited January 2, 2022 by Yetti

[Boilermonkey]

A hot start after stopping for gas I wouldn't think could cause any problems.  A cold start and taxi to fill up before T/O could essentially be two cold starts, but at lest you've moved the oil around.

In either case, I don't think it matters in the end.  Don't let your airplane sit...that's the root of all evil.

[StevenL757]

14 hours ago, PeteMc said:

...but I too think keeping the tanks full is important.

I disagree...unless you want to pay for installation of new shock discs every year or so.

This isn't a Cessna...where many love to "top off" after every flight.  Your shock discs will compress badly in short order due to added fuel weight, and eventually become unairworthy.  I see this periodically...the latest being this past week in San Marcos.  Of the 7 Mooneys in the McKee hangar...mainly Ovations and Acclaims...2 of them were in need of new discs within a year because the owners all made a habit of storing excessive fuel in their tanks for extended periods.  Avoid this by aiming to carry no more than tanks no more than 1/2 full if you regularly plan to not fly for more than a couple of days.

[Will.iam]

1 hour ago, Yetti said:

I'd like to think the water vapor escapes though the crankcase vent tube while running.   as the engine cools and contracts, I would think you are pulling more moisture into the now open crank case.    https://www.aopa.org/news-and-media/all-news/2018/april/flight-training-magazine/how-it-works-breather-tube

 

As much water vapor that goes out the breather tube it’s still shocking to me to see how much steam/ water vapor that comes out the oil filler tube when i take off the cap after engine shutdown. I have heard (can’t find the reference now) that 45 mins after shutdown is optimal vent time for allowing the vapor to escape out the oil filler tube. I usually wipe down the leading edges and windshield of bugs and clean the belly of oil and then close the oil filler cap. I try to fly once every 10 days any longer and i put a dehumidifier blowing down the oil filler tube to keep moisture away until i can fly again. 

[Yetti]

18 minutes ago, Will.iam said:

As much water vapor that goes out the breather tube it’s still shocking to me to see how much steam/ water vapor that comes out the oil filler tube when i take off the cap after engine shutdown. I have heard (can’t find the reference now) that 45 mins after shutdown is optimal vent time for allowing the vapor to escape out the oil filler tube. I usually wipe down the leading edges and windshield of bugs and clean the belly of oil and then close the oil filler cap. I try to fly once every 10 days any longer and i put a dehumidifier blowing down the oil filler tube to keep moisture away until i can fly again. 

Are you sure that is steam?  and not oil vapor?

[Will.iam]

7 minutes ago, Yetti said:

Are you sure that is steam?  and not oil vapor?

When i forget or don’t remove the cap, and open it say an hour later it has water droplets all over the inside of the cap. When i remove the cap after shutdown and let it vent, checking the cap later after i have put it back on never shows any water droplets on the inside of the cap. I’m sure there is some oil in the vapor but what is in the inside of the cap is far more watery than oily to the touch. 

[skydvrboy]

3 hours ago, ArtVandelay said:

The real question is, after short taxi the engine hasn’t had a chance to get warm let alone hot, so which method do you use to restart: cold or hot?

After a short run to the fuel pumps, use the hot start procedure.  That always works in my plane, but if it doesn't, you can then go to the cold start procedure.  If you try the cold start procedure first and it doesn't work, you've flooded it and now must use the flooded start procedure.  It still works, but in my opinion the flooded start procedure is the most awkward and challenging.

1 hour ago, StevenL757 said:

I disagree...unless you want to pay for installation of new shock discs every year or so.

I think this is mainly a problem in the long bodies.  My F had shock discs from 3/77 that I replaced recently and they still met airworthy specs.  I have no idea how the tanks were filled during those 40 years, but I'm guessing you won't get 40 years out of your discs in a long body regardless of how much fuel you store.

[ArtVandelay]

I'd like to think the water vapor escapes though the crankcase vent tube while running.   as the engine cools and contracts, I would think you are pulling more moisture into the now open crank case.    https://www.aopa.org/news-and-media/all-news/2018/april/flight-training-magazine/how-it-works-breather-tube
 

I can see the water vapor escaping, I gotta believe the low pressure due to cooling and moist air that enters is a small fraction of what escapes. Try it yourself.

[PT20J]

3 hours ago, ArtVandelay said:

The design issue is the cam is high so gets only splash oil and no oil dripping from above.

I always let it start up at 1200 RPMs for 10-15 seconds to get the oil distributed before leaning and reducing RPMs.

I am aware that this is one of the things that many people believe. Here are some things to think about:

1. The hydraulic lifters are supplied with pressurized oil which provides a source of lubrication to the lifter bores and the camshaft during operation.

2. Many IC engines utilize overhead cams without issues.

3. After an engine has been sitting for a time, oil will drain off all internal parts without regard to their position within the crankcase.

4. Flight school engines are started frequently and often reach TBO and beyond, so lack of lubrication at start up doesn't seem to be a problem.

5. Spalling is caused by metal fatigue. 

6. Corrosion is not always simply a chemical reaction -- it can have mechanical causes (called frictional or fretting corrosion).

7. Someone on MS posted a picture of a camshaft that had been sitting around in a hangar for years and looked almost new.

I didn't design the thing and I'm an electrical engineer, not a mechanical engineer. But, I would bet that the lifter faces have operating pressures near the limit for the hardness of the material and some just fail. Based on flight school experience, it seems frequent operation is beneficial. Everything else has likely a second or third order effect with only anecdotal evidence to suggest it has any effect at all. 

Skip

 

[philiplane]

4 hours ago, ArtVandelay said:

The design issue is the cam is high so gets only splash oil and no oil dripping from above.

I always let it start up at 1200 RPMs for 10-15 seconds to get the oil distributed before leaning and reducing RPMs.

The cam is oiled by what oozes out of the cam bearings, and lifter bores, in addition to what throws off the crankshaft. There is plenty of oil on a Lycoming cam, same as there is on a Continental cam which sits at the bottom of the case. And Continental is the current leader in spalled lifters and ruined camshafts. Most are on engines less than ten years old.

[Shadrach]

4 hours ago, ArtVandelay said:

The real question is, after short taxi the engine hasn’t had a chance to get warm let alone hot, so which method do you use to restart: cold or hot?

Try hot first. You can always add more fuel if needed. My plane hot starts well after just 45 sec operation to the self serve fuel farm.

Edited January 2, 2022 by Shadrach

[y2kiah]

14 hours ago, PT20J said:

I don’t know that anyone has ever figured out the root cause of Lycoming cam problems. But, my guess is that it is some sort of design issue. This is supported by the fact that Lycoming has changed the design to roller lifters for new and rebuilt engines and DLC lifters for overhauls. It is certainly better to fly often — that’s one reason Lycoming TBO also has the often ignored calendar time limit. Frequent oil changes are also good. But I’ll bet most other things we do have only a minor effect on cam life. 

Skip

As explained to me (by my IA) some of the lobes on the shaft do double-duty in spots where the horizontally opposed valves align, and those lobes are subject to increased wear and are usually the ones found worn prematurely. If you throw pitting due to corrosion into the mix, it will only accelerate the effect.

[Shadrach]

38 minutes ago, y2kiah said:

As explained to me (by my IA) some of the lobes on the shaft do double-duty in spots where the horizontally opposed valves align, and those lobes are subject to increased wear and are usually the ones found worn prematurely. If you throw pitting due to corrosion into the mix, it will only accelerate the effect.

The only metal to metal contact between lobe and lifter is in the first second of operation. I don’t buy the double duty theory nor have I seen evidence that one lobe failing is more typical than others. Lifters typically fail before the cam. When the lifter stops rotating, cam failure is imminent.  My cam is 54 years old and has just under 3300hrs. Engine was overhauled in 1999-2000 after 33 years in service and 18XX hrs. The cam was serviceable and sent out to be serviced. Engine was IRAN’d in 2010 because of a case crack.  The cam was visually inspected and showed minimal wear then as I expect it would if the case was split today.  My engine has gone as long as 2 years (prior to me being in the partnership) without activity.  I think there is a metallurgy issue not a design problem. I’ve used CAMGAURD for the last 12 years but that does not explain it surviving the previous 43 years on mineral oil alone. I live in MD, not AZ and the plane has certainly had periods of inactivity. There must be more going on than luck.

Edited January 2, 2022 by Shadrach

[Yetti]

1 hour ago, ArtVandelay said:

I can see the water vapor escaping, I gotta believe the low pressure due to cooling and moist air that enters is a small fraction of what escapes. Try it yourself.

but is'nt that why we are told to fly our planes alot so we burn off all the moisture in the oil?

[PT20J]

Physicist Richard Feynman described the scientific method as a two step process: Guess at a solution and then prove or disprove it by experiment. The problem with all these supposed causes of cam problems that we hear and read about is that they are just guesses that have never been validated. After they get repeated long enough they become "Old Wives Tales" -- a term I believe was first applied to aviation beliefs by Randy Sohn and popularized by John Deakin. I'm not pointing any fingers: I have believed a lot of things over the years that have turned out not to be true, and I probably still believe things that aren't true.

While I have no firsthand knowledge, I believe it reasonable to assume that Lycoming knows more about its engines than anyone outside the company. Besides seeing engines that come back for overhaul and rebuild, Lycoming has all the design and test data and a failure analysis lab. The fact that Lycoming changed the lifter design tells me that they knew something needed fixing. The change to roller tappets required redesigning the case haves to accept them as well as the tappets themselves and probably a ton of testing. Changes like this are very expensive and not undertaken without good reason.

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