Piston Seizure - Avgas???

[Lemoncake]

We had a piston seizure in our Cessna 150 the other day. A friend of ours suggested that Avgas can cause the pistons to stick, due to lead deposits around the piston. He is certain we should be filling up with Mogas. Any thoughts?

Cheers,


PS Many thanks to the Dunkeswell engineer who came out to Farway to help us out.

[ormonde]

Surely Avgas contains anti icing additive which is not normally associated with Mogas and what about the octane rating ?

[FlyingForFun]

Seems to me that there are two issues here: legal, and practical.

It is illegal to use MoGas in an aircraft with a CofA. It is only legal (in the UK) in PFA aircraft, and then only if the aircraft has been specifically authorised to use it. There are further requirements once you get this authorisation: you can't use MoGas above 6000', and you can't use MoGas when the tank temperature exceeds 20 degrees C. The instrument panel must have a placecard to inform the pilot of these restrictions, and there must be a label near the filler cap indicating that the aircraft runs on MoGas. There are probably some other legal requirements I've forgotten about or never knew about, too.

Now, let's put aside the legal side of things. I'm not an engine expert, but I can't see how the use of AvGas would cause a piston to stick. Assuming there was no prior damage to the engine, I would guess that the cause is most likely overheating, which would cause the oil to loose effectiveness. Lead deposits in engines generally occur on the crown of the piston, and although this can cause its own problems, it wouldn't cause the piston to stick.

If you were to switch to AvGas, you might potentially encounter several problems. The main one would be the lack of lead. Lead is required to protect the valve seats in an engine. Engines which run on unleaded fuel have their valve seats made out of a much harder material so that they can survive without this protection. Also, as ormonde says, the lower octane rating might cause problems - I know that when converting old cars to run on unleaded fuel, it's often necessary to adjust the ignition timing to account for the difference in octane rating.

So I guess the answer, in a nutshell, is No. But I'm no mechanic, so please don't listen to anything I say - speak to your mechanic instead.

Anyway, glad it all worked out ok for you - I assume the problem was noticed on the ground???

FFF
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[dirkdj]

I don't think Avgas has anything to do with 'sticking pistons'.

Piston scuffing is when a piston comes into contact with the cylinder walls, not protected by a layer of oil.

Probable causes could be: overheating because of bad baffling or
bad magneto timing, improper rich mixture during full power climbs, etc

Overheating will lead to detonation (explosive burning of the fuel/air mixutre) (TEL lead is there to postpone detonation, no other function in Avgas), which will heat up the cylinder heads very quickly leading to out-of round cylinder with a round piston pumping away to destruction. Once started, this scuffing condition will boost itself.

On a C150 engine instrumentation is marginal I suppose.

Dirk

[big pistons forever]

C-150 engine is the low compresion Continental O-200, an engine designed to run on 80/87 avgas. Here in North America the only available avgas is 100 Low Lead. Despite the name this gas has 4 times the lead of 80/87. There has been many problems with exhaust guides and pistons rings particularly if is not run at cruise power for extended periods on a regular basis.

[max roll rate]

Quick note about legality not so long ago the Oxford air training school used to use duel fuel in there pa28 fleet we ran one tank on avgas and one tank on mogas , this is a large fleet of public transport c of a aircraft its not so illegal me thinks .

[Flyin'Dutch']

MRR,

You are correct (how could you not be ;) ) and it is perfectly legal to operate a mount on Mogas as long as it is CAA approved and you stick to the operational limitations.

More detail on the CAA website.

FD

[146fixer]

Gents for info on using mogas check your AWN's.There is load of info on it in there.As for Avgas causing your seizure it is more likly to be something else,lack of lubrication or a failure of a component

[Lemoncake]

Gents,

I have to apologise for leading you up the garden path. Not being atall engineering minded, it seems I made a cock up in my original explanation of the engine problem. A seized piston it was not!!!! It was a stuck valve :confused: Completely and utterly different I know.

Thanks all anyway


FFF - Yes it was on the ground. Rough running engine discovered in power checks.