In case it helps anyone else I had just this scenario (rough running, losing power etc) in a small Cessna. It was a little irregular insofar as it would come and go a bit and the rough running itself wasn't constant
.
Being a bit of a spanner I could tell that as it wasn't regularly missing it was less likely to be ignition. This was more of a 'feel' thing but FWIW the logic is that a fouled plug or failed valve/piston/cylinder would be more likely to have a regular miss than say carb ice. If you want an example try pulling a plug lead off a running 4 or 6-cyl car engine sometime.
Anyway I did the first thing one should do and pulled the carb heat, miraculously it made quite a difference ... for a short period of time after which the problem returned. Bearing in mind this was a hot, dry, day with temp/dewpoint miles apart (I checked!) it seemed rather unlikely to be real icing but I ran with that view for a bit.
After a while (probably not that long although it seemed ages) I decided it was more likely to be a mechanical problem with the carb and/or air intake. Careful manipulation of the carb heat control would restore the engine for a period of time, after which it would return to the original problem. At times I had to mess with it continually, other times it would operate ok for several minutes.
Given I was some distance from a 'drome, over rough(ish) terrain, or sea, I elected to continue on rather than attempt a precautionary landing. It would have been do-able, if necessary, in some places but at its worse the engine would run a max of 2200/2300 rpm (with EGT vy hot) which was enough (just) to maintain a couple of thousand feet.
Incidentally I also checked the fuel tap was in the correct position and tried running on individual mags once I'd established an ability to keep going, at least for a bit. What I didn't do was mess with the mixture, my logic being that I didn't want to play with anything more than was necessary - especially carb related - in case I exacerbated the situation and/or introduced another problem.
Eventually I landed and a subsequent check showed that the carb intake flap select lever cable clamp had become loose meaning that the engine was in fact alternating between full carb heat to normal air intake and something in between from time to time. By moving the carb heat control I was occasionally able to move the flap when the clamp momentarily grabbed.
The crunch to this long-winded tale is in the decision-making really, IMO. Not every situation is the same and, if possible, some logic should be brought to bear on the issue. If the fan stops completely then between soundbarrier & k1w1 chick you've got it mostly covered <grin> but if it's still running, albeit roughly, then there could be other options. Most important, methinks, is to DO ONE THING AT A TIME, and, if possible, give that action a few moments to see if it makes a difference. If it is icing carb heat won't necessarily work immediately, in fact it could even become worse momentarily, likewise changing tanks or whatever could take a few moments to make a difference. The main thing is not to panic (yeah
) and start pulling and pushing things haphazardly. If in the end nothing works and you've insufficient power to keep in the air then you better go through the drill and pick your landing spot smartly, however if you are able to keep it going, at least for a bit, it widens your options considerably - at least insofar as where you're going to land. This could make all the difference, particularly if you're over inhospitable terrain.
No doubt there's other things I could say from this experience, I'm not convinced myself that I made the right decision
- perhaps I should have landed immediately, perhaps I should have tried the mixture too (theoretically at least it may have helped), but I've lived to tell the tale which has allowed me to ruminate about the situation somewhat. I don't think I'll ever quite reach a conclusion, mostly in fact I think I didn't do quite what I should have, but I'm not sure if thinking this would actually help another time because the chances are it would be a different situation with different parameters to deal with. So perhaps the moral of the story is to by all means follow the drill but don't do so blindly, do it deliberately. Particularly if you've height and/or partial engine in hand try and think things through both in terms of what you're about to do and if perhaps you (or pax!) have just recently done something that may have introduced the problem...
Finally, once down, don't be afraid to make an arse of oneself on proon and tell all - it could well help someone else one day as could (some of!) the subsequent comments. Bring 'em on
!