Carb Heat causing Detonation?
Joined: Feb 2006
Aviation Qualifications: LAME
Posts: 1,129
Likes: 168
From: Station 42
A company I used to work for abroad lost a large piston-engined aircraft and its two crew when it suffered an engine malfunction on take off and crashed into an airport building. The failure was attributed to detonation caused by carb heat selection (as well as poor checklist procedure).
Joined: Jan 2004
Posts: 5,658
Likes: 501
From: Canada
A company I used to work for abroad lost a large piston-engined aircraft and its two crew when it suffered an engine malfunction on take off and crashed into an airport building. The failure was attributed to detonation caused by carb heat selection (as well as poor checklist procedure).
In particular the fact that mishandled carb heat can detonate a large radial engine does not automatically mean that the same threat exists for the simple carburated 4/6 cylinder engine found in GA trainers/tourers. I would strongly argue that there is no threat of detonation at any power setting unless carb heat is used at OAT's far far above any that would indicate the possibility of carb ice forming.
As my handle suggests I have considerable experience operating big round engines. I am curious as to what accident you are talking about. My bet is the engine was operated with extreme carb air temps (ie over 40 deg C) for a considerable period ( ie not just a momentary carb temp spike) in order to cause severe enough detonation to fail both engines.
Fleet Manager
Joined: Aug 2006
Aviation Qualifications: CPL
Posts: 7,090
Likes: 2,952
From: Ontario, Canada
Yes, I agree. Of the three engine failures I have had which put me on the ground, my selection of carb heat after takeoff was the direct reason for one. I do have a carb air temperature indicator, and aside from a preflight function check, I never use the carb heat, unless conditions warrant it.
I have noticed that pilots tend to like to move controls, whether the controll really needs moving or not (increases the sense of importance of the pilot role). This can be aggravated by training which teaches to "do because you are told to, not because you necessarily understand why". So carb heat controls get moved a lot, and I bet that 95% of this use is not even effectively preventative, much less restorative.
Do pilots realize that if you really need the carb heat to be effective, it is likely necessary to turn off cabin heat, and to peak lean the engine after carb heat selection?
I have noticed that pilots tend to like to move controls, whether the controll really needs moving or not (increases the sense of importance of the pilot role). This can be aggravated by training which teaches to "do because you are told to, not because you necessarily understand why". So carb heat controls get moved a lot, and I bet that 95% of this use is not even effectively preventative, much less restorative.
Do pilots realize that if you really need the carb heat to be effective, it is likely necessary to turn off cabin heat, and to peak lean the engine after carb heat selection?
Joined: Feb 2006
Aviation Qualifications: LAME
Posts: 1,129
Likes: 168
From: Station 42
Big Pistons Forever:
It was a C47A, the crash happened in Palma de Mallorca in March 1992 and it's not particularly warm there at that time of year, especially early evening when the crash occurred. I'd left the company well before then but the information I received from someone closely involved in the DCA investigation was that as mentioned previously. It was only one engine that failed by the way.
There may have been more to it than that of course, but my source was very experienced in piston-engined operation.
It was a C47A, the crash happened in Palma de Mallorca in March 1992 and it's not particularly warm there at that time of year, especially early evening when the crash occurred. I'd left the company well before then but the information I received from someone closely involved in the DCA investigation was that as mentioned previously. It was only one engine that failed by the way.
There may have been more to it than that of course, but my source was very experienced in piston-engined operation.
Thread Starter
Joined: Jan 2011
Posts: 335
Likes: 0
From: New Zealand
Thanks all, this helps. The question comes from rushing the downwind/final checks and forgeting Carb Heat OFF, did my touch and go with Carb Heat ON and had a bit more of a climb into the air ... figured out why pretty quick (flaps up,.. no thats not it...aha!). Chatting to instructor mates and they both waggled figures and warned of detonation, when i asked them to explain... they couldn't. I guess its one of those "we were told not to do and we'll tell you not to do it"
I don't as a rule run the Carb Heat close to the ground as where i fly can be dusty or grassy so unfiltered air can be full of nasties (i do apply Carb Heat on run up checks of course, and run it longer on cold mornings).
I try to gentle on the engines, no fast throttle movements and i hardly ever lean (don't get high enough for long enough) but i really only know as much about them as i learnt in the Air Tech exams so issues like this can confuse me :-)
Thanks All Again
I don't as a rule run the Carb Heat close to the ground as where i fly can be dusty or grassy so unfiltered air can be full of nasties (i do apply Carb Heat on run up checks of course, and run it longer on cold mornings).
I try to gentle on the engines, no fast throttle movements and i hardly ever lean (don't get high enough for long enough) but i really only know as much about them as i learnt in the Air Tech exams so issues like this can confuse me :-)
Thanks All Again
