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Accidental diesel in Rotax 912S

PostPosted: Sun May 09, 2010 6:59 am
by everett
I accidentally added 20 litres of diesel to 20 of avgas already in the tank and flew one circuit. Taxying back I found that the oil pressure had dropped to the red line and the temperature was rising rapidly. I shut down. Having identified the problem we drained and flushed the tank and fuel lines and changed the oil: the engine started and ran well. However, the oil consumption immediately rose to one litre per six hours as compared with one litre to 22 hours previously. The usual oil pressure dropped from four bar to two bar and we now found it very difficult to 'gurgle' the engine. We have discovered small amounts of debris in the filter, whereas there was none previously and a differential cylinder pressure test shows two of the cylinders down to 62 psi against a reference of 86 psi. This is below the Rotax minimum. The engine will therefore have to be stripped down and appropriate repairs made.

As this results from a single event accident it is covered by insurance but the insurers' engineer, who admits to no previous knowledge of Rotaxes, maintains that running on a diesel/avgas mix cannot damage an aircraft engine. As I understand it, the introduction of diesel could result in a reduction in the blow by gasses that are essential to keeping the oil circulating back to the oil reservoir.

Does anyone have any useful advice or information?

Re: Accidental diesel in Rotax 912S

PostPosted: Fri May 14, 2010 5:00 pm
by alan
By mixing diesel with avgas 50/50 you lowered the octane rating by maybe half. You were, without a doubt, experiencing extreme detonation. Detonation will erode the inside of the combustion chamber quickly away putting lots of metal past the rings and throughout the oil supply. Your CHTs spiked as well, overheating the rings and valve seats. You were lucky you didn't erode a hole in a piston which would probably have resulted in an inflight engine fire and catastraphic engine failure. I'm happy the insurance paid up and even happier you made it make on the ground safe and sound.

When doing a L/D test, check the exhaust for excessive valve leakage. If the valves (intake or exhaust) are not seating properly you will not have enough ring blow-by at very low RPM to purge the engine sump to the remote tank. Low oil pressure is likely caused by a worn oil pump caused by compustion chamber debris.

Alan