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Partial power loss

(seen on Gasco) Australian TSB information on partial power loss accidents

I found this interesting. The main assertions ring very true : partial power loss is 3x more likely as total power loss; still, we train for the latter but not for the former. In that type of accident, the time you spend trying to decide what to do is what increases the risk; do you try to get back to the airport, do you treat it as a total power loss and just glide?

The main conclusions are obviously true as well, but I’m going to think about a specific pre-take-off brief that could improve the engine out brief I currently do.


Very true. A partial power loss is often a failure in one cylinder which prevents power being developed in that cylinder, while all others continue to develop power. Hopefully, if you have the misfortune to suffer this, it’s a stuck open exhaust valve, and then at least the remaining engine power is not also having to compress air in a non functioning cylinder. In such a case, diagnosing that you still have some power, and flying the aircraft accordingly may get you home, but it’s a difficult call to make. I’ve had several friends die because they made an aggressive turn back, with a rough engine which would have continued to develop some power.

Even more challenging is an engine which dies, and comes back to life (particularly short final into your chosen forced approach). It temps one to abandon the forced approach, but that can lead to a crash when the engine quits again. Tough decisions to make, and yes, more understanding of failures, and training is a great idea!

Home runway, in central Ontario, Canada, Canada

This is a really good question to read. I am really surprised it hasnt received more contributions.

I have had one engine failure (but that was in a twin) and a number of partial failures in a single – but never a total failure.

In a way, be it single (with a partial) or twin (with a single failure) the thought process come to think about it is not that different.

I think carrying any engine that isnt performing satisfactorily for any distance between airfields is ill advised – you are just going to feel stupid if it quits on you (or the other engine does). The exception with a twin I guess is if you are absolutely certain you know what the cause is. For example if there is oil every where, no pressure, over temp, and safely and completely shut down and feathered I guess it is a really bad day if the other engine quits with a problem that isnt going to be related.

I guess with that single engine developing partial power the similarity is you are naturally going to assess whether you have a reasonbale idea what is the cause and whether it is likely to end in total failure. I had an issue with an injection problem which was intermittent, had occurred a few times, and which the engineers had a pretty reasonable idea as to the cause – they didnt think if it was as suspected, it would end in failure. I limped back on that partially running engine in the expectation it would continue to produce enough power.

If the engine is clearly over temping and oil pressure is falling then other indications aside I guess there is a reasonable expectation of catastrophic failure. Are you more comfortable landing with some power or do you continue in the hope it doesnt but in such a way as to give you as many options as possible. Do you go for height to give you an advantage while you can, or do you get yourself below the base if this is a factor. Do you immedaitely divert to whatever is available regardless. I think these are all good questions.

I think for me I am reasonably conservative and I am going anywhere I can as soon as I can in terms of airfields. I am especially avoiding any built up areas (of course). However, I am probably going to be happy enough hoping it doesnt fail than making an early off field landing using what power the engine is still producing unless it is really banging and clanking and spewing oil in which case any field will do.

Everyone should definitely watch this video (intermittent power loss):

I had partial power loss (twice) a couple of weeks ago towing gliders. I had a heavy twin glider on the tow. At about 600 ft AGL, the RPM went down from 5700 to 5200 (Rotax), with corresponding loss of power. The engine kept running, so I just rocked the wings and the glider let go of the tow. I could have cut the line (maybe I should?), but would have if the engine stopped entirely.

The elephant is the circulation

LeSving wrote:

Everyone should definitely watch this video (intermittent power loss):

Certainly should. Fascinating, sobering and humbling… Thanks LeSving.

North Weald, United Kingdom

Yeah, that is a great video. It illustrates emotions going from euphoria to not, I am sure, as the possibility of landing at the airfield comes and goes. How tempting it must be to stretch that glide. Would any of us deal with it differently in terms of ignoring the possibility of landing back at the airport. I can only imagine that almsot every time you are set up for an off field landing and that darn engine comes back to life you think there is a chance of nursing it back.

I have seen partial power loss (as well as complete power loss) as a result of fuel servo icing.

It is probably a lot more common that most pilots know about. Few people will rush to post on a forum that they got a power loss and the engine restarted after some time, at a lower altitude.

If you get it “just right” it can be a partial loss only. You then have to quickly open the alternate air door and hope it was not already frozen in place (because the whole air duct is getting iced up inside). You should probably have been flying with alternate air already (per POH) but alternate air robs the engine of a lot of power so is not good for climbing.

And then when the engine restarts lower down, what are you going to do? Climb back up, into the icing conditions which caused it in the first place? A very difficult decision.

Shoreham EGKA, United Kingdom

The latest issue of the CHIRP General Aviation feedback newsletter (always a good read imho) has a link to an excellent study on partial power loss by the Australian Safety Transport Bureau.

The data shows that during and after takeoff, a partial power loss is three times more likely in today’s light single-engine aircraft than a complete engine failure. Furthermore, there have been nine fatal accidents from 2000 to 2010 as a result of a response to a partial power loss compared with no fatal accidents where the engine failed completely.

The overriding advice therefore is to treat partial loss as a complete engine failure, no matter how tempting to think otherwise.

It’s a comprehensive study with some good stats. Worth a read.

North Weald, United Kingdom

Thanks, that will be useful to people who can’t be bothered to read the opening post ;-)


jgmusic wrote:

The overriding advice therefore is to treat partial loss as a complete engine failure, no matter how tempting to think otherwise.

Well… The advice should be to not lose control of the aircraft because of a power loss, total or partial. Of the mentioned nine fatal accidents, perhaps some were in twins? Pilot reaction to a partial power loss in a twin introduces other factors affecting safe continuation of the flight.

Yes, once you have committed to a forced landing, treat the engine as though it will not produce any power again during that flight. If power increases again during your forced approach, continue to the landing, unless it’s looking like a fatal place to land in the first place. Otherwise, if you have time and altitude, point the plane toward a suitable forced landing area, and evaluate. Is the power being developed steady? The engine may be shaking, though the power steady – a stuck open exhaust valve or fouled plug will do this. The rest of the engine is running fine. If one cylinder of a six cylinder engine is not producing power, five may still be, and will take you onward with adequate power – if you continue to fly the plane safely! As I mentioned elsewhere here, I have knowing flown home a C 150 with one cylinder not developing any power, as maintenance was not possible where it was. No problem, I just took things really easy, and expected 75HP, rather than 100HP. It was a partial power loss from takeoff to landing, and the result was an adequately safe flight. (Don’t worry, the route home was mostly suitable forced landing fields, and I had a friend in formation, in case another cylinder failed).

Yes, I have had friends die, because they reacted too quickly, and ultimately wrongly, to a partial engine failure. Yes, the engine might not have taken them anywhere other than to the forced landing, but the dying part was because they did not continue to control the plane and maintain flying speed, not because the engine was not developing power.

Home runway, in central Ontario, Canada, Canada
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