My engine coughed!

[asra]

Wondering why my engine coughed yesterday.. Big drop in engine sounds and power for a moment then back to normal. I didn't notice the rev drop... was looking outside ...for landing spots []. First time I've had this sort of event since my last engine [xx(]Running a 582 Bluehead, recent 100 hourly, new fuel filter 10 hrs ago, all fuel 'Mr Funneled' and it was tested for water at the lowest point before the flight. Running at 5600rpm on slow decent from 3000' which was close to the inversion layer. This was about 40mts into the flight, did another 15mts before landing back at the field. Checked the carby bowels which were super clean but the fuel was very cold (as was I! brrrr). Engine water temp in flight was a nice 65C. Exhaust about 1150 as usual. I understood that carby icing is usually humidity related, it was probably around 10C at height yesterday, wondering if its a possibility or maybe I should be checking something else? Maybe it was just crook fuel or a momentary pilot brain explosion?Dave

[asra]

Sounds like ice to me Dave! With that inversion layer you mention the relative humidity on the ground may have had little relationship to it at 3000', it may have been quite high. As you know reduced throttle for descent provides the best conditions for ice.John EvansThink logically and do things well, think laterally and do things better.

[asra]

Thanks John,So next time, whats the best thing to do? Little bit more throttle? Louder praying? [] Do nothing? If I had a choke circuit which just dumps a bit of raw fuel would that be a good response? Dave

[asra]

without a source of carby heat ya just dont fly in carb ice conditions

[asra]

As Albert points out it's better not to fly in icing condition if you don't have carb heat. The trouble is you don't always know because conditions vary, you mentioned the inversion layer, this may have had a lot of moisture associated with it.A few years ago I was trapped by this when flying over a dam in my old VW, She lost power and ran rough, it actually makes the mixture rich, so I don't think choke would help. My friend flying with a 582 flew on, oblivious to my problem, while I landed on a turf farm.It seems the 2 strokes are less susceptible, but still capable of getting into trouble. We had flown into a patch of very moist air over the dam, while elsewhere the air was relatively dry.My guess is that all you could do was open the throttle, a little awkward when you want to descend! Perhaps flight on the backside of the power curve would allow safe descent!John EvansThink logically and do things well, think laterally and do things better.

[asra]

Hi DaveIn the past I have been either very lucky, or my instructors teachings help me avoid this condition.During a decent in suspect conditions, always give your engine a burst of power, say every 20 seconds, this is suppost to do 2 things, keep your engine warm, and break up and suck through any ice building up.Not sure if it's been test proven, but I have always done this and never had icing.Regards Sam. []

[asra]

I agree sam i never go any more than about 30 secs with out giving it full noise for a couple of secs, even if its fine and hot.Remember -- A good friend will come and bail you out of jail...BUT a true friend will be sitting next to you saying, "Damn...that was fun!"Mark

[asra]

They say for carby ice to form or even airframe ice to acumulate(not relative unless your flying above freezing level)there has to be VISIBLE moisture....cloud...mist...rain...fog...but im not sure if high humidity alone is sufficient unless it was very cold.

[Tim]

As John said, "the 2 strokes are less susceptable" to icing, in fact it is vary rare - but I do believe I have seen it happen on one instance. Often other problems are blamed on carby ice in the 2 strokes because it is the most convenient or obvious thing at the time. Such is the case during a long descent with low throttle setting. Ice would be a plausable explanation with a Subaru or a VW but not necessarily thr Rotax. Ice in cruise with the 4 stroke slowely closes the ventury, causing an over rich mixture and a loss of top end rpm's, with some restriction of the butterfly movement. The engine will cut out if reduced to idle (Idle jet discharge hole frozen over). In an extended low throttle descent the throttle butterfly often freezes and cannot be opened or closed to change rpms. In both cases the engine will ultimately cut out.In the case of the 2 stroke, an extended descent with low throttle setting causes an over rich mixture to build up in the crankcase, although the engine still runs - albeit roughly. When the throttle is given a 'blip', the engine can flood and die. This is not icing.As far as an intermittent miss is concerned, it doesn't sound like ice to me. Ice build up in the carby throats should cause an over-rich mixture which would result in rough running or 4-stroking. I would not expect that injested ice would wet both plugs at the same time. Another good reason for giving your engine a burst every X seconds during a descent is for peace of mind - knowing you still have your engine.Cruise ice is predictable in a Subaru because it takes time to form, and a regular hit to full throttle will show if there is any drop in max rpm's, giving you time to land with power. PS "Idle jet discharge hole frozen over". The Subaru has a small hot water hose to the base of the carby near the idle discharge hole to minimise this possibility.Tim McClure

[asra]

Thanks for the input. Done another 10 hours since the blip with the engine running fine. At the time my 'descent' was only 200 revs or so off cruise and the 3000' was indicated, not agl, I needed height to get over some hills, sorry if I was unclear. I always notice inversion layers as a matter of habit from gliding, but no mist and CAVOK. If not carby ice which, from what you guys say, seems unlikely.. do 2 strokes 'miss' occasionally? Bit of water in the fuel (doubtful)? It was ordinary unleaded rather than premium - so maybe just plain bad fuel? Anything I should be looking for?Dave

[asra]

Dave I think the reason 2 strokes don't ice up is the oil mixed in the fuel.Safe Flying ... ding Insanity is hereditory, I got it from my kids!

[Tim]

I think you are right Ding, plus in a carby, the lower the throttle setting, the higher likely hood of ice forming. A Carby butterfly severly changes the direction and speed of the airflow, creating high and low pressures and making conditions ideal for 'refridgeration'to occurr. The Bing carbys found on Rotaxes have a "slide" instead of a butterfly and the airflow is straight through, especially at full throttle, where a pressure and speed reading at each end of the carby would be the same.Tim McClure

[asra]

Geoff I also agree, but most new Blue Head 99 model 582 now come with oil injection (pump) and I know Dave's is also injected, so there is no oil in the fuel when deliverd by the carbies.Sam.

[asra]

Dave just out of interest what is the fuel consumption of your engine on an average lts per hour?

[asra]

SamL ...I'm down in flames again ...Safe Flying ... ding Good judgment comes from experience and experience comes from bad judgment