Engine 'missing' during flight, in very hot weather.
I have a Rotax 912UL (TT: 350hrs) in my 912 Mark 1 , 'Classic'.
I have a great deal of confidence in this engine. Although underpowered
for the aircraft, I am convinced it was the right choice at the time.
A couple of months back, I flew to Blair Atholl in Scotland, from S.E.
England.
It was a very warm day (circa 28 C on the ground).
Around half way to my destination, an hour and a half into the flight
the engine
coughed and stuttered repeatedly. I performed the standard engine
problem checks
and began to seek out a diversionary airfield. Paradoxically, the only
thing which helped,
and it helped instantly, was to swap fuel tanks, using the selector. I
carried on cautiously.
Around 1 1/4 hours later, 10 mins from my destination, the problem
returned. I climbed
higher (the aircraft could still climb on the spluttering engine) to
give me extra scope,
should I be unfortunate enough to loose power altogether. Greater
altitude didn't help,
and swapping tanks was now proving to be ineffective. I carried out a
safe landing at my
destination.
I have only had a similar experience once before, when returning from
Scotland, nearly
3 hours into a flight, again on a very hot day. I have never experienced
it in the air at any
other time, and it has not returned since. (at least 25 hours ago)
I checked my double fuel filter system (as per Europa mod.) Both filters
were dirty
with crud from the bottom of the tank, and with a few tiny 'Jerry Can'
paint flecks. (I strain
my fuel through a fine mesh funnel, but occasionally the contaminants
still get as far
as the filters). However, fuel was still able to get through, and it is
my judgement that
this was not the cause of the engine problem.
My favourite theory, unproven, is that with a hot engine block and after
a number of hours
of continuous operation, unleaded fuel was vaporising in the fuel pipes.
Although I have
fitted the mandatory fuel return mod, and this seemed to cure hot start
ground mis-firing, I
have a suspicion that this may have occurred in the air, due to a number
of contributory
factors. I cannot easily explain why swapping tanks helped on the first
occasion.
Opening up the bottom cowl forward of the wheel well, using thick penny
washers
allows a greater air flow (and thus heat dissipation) over the engine
block. I have some
confidence that this may prevent re-occurrence.
I have always used unleaded petrol, from a reputable source, as per
recommendation.
Anyone else had a similar experience ??
Alan
Alan. D. Stewart
14 Goddard Way phone: +44 (0)1245
264186
Chelmer Village mobile: +44 (0)7860
720257
Chelmsford, Essex CM2 6UR email:
alan.stewart@blueyonder.co.uk
office - email : alan.d.stewart@bt.com
phone : +44 (0)1473 605163
fax : +44 (0)1473 622594
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