What is the rationale for not including the application of carb heat in the initial multi-engine airplane's engine failure procedure?

I've recently starting flying light multi-engine airplanes again and have been practicing the engine failure procedures.

I noticed in the POH of the aircraft I'm flying that it doesn't mention applying Carb Heat to the dead engine. So I looked at some other multi-engine POH emergency procedures and noticed that Carb Heat is mentioned sometimes, but in the trouble shooting checklist for restarting, after the initial procedure.

Shouldn't carb heat be applied as soon as possible, while manifold is still hot enough to heat the air to melt any potential carb ice? Like immediately after pushing everything forward and getting gear and flaps up?

Alternatively, I can also see an argument that before you identify the dead engine, applying carb heat to both engines would decrease some precious power from the currently functioning engine. Are there other reasons for not using carb heat sooner?

  • $\begingroup$ Once your engine has stopped, carb heat isn't going to do you much good. I assume your "rough running" procedures identify to apply carb heat? Really once the point your engine performance has degraded to "not running", the manifold isn't creating heat (or exhaust flow) anymore and applying it probably won't do anything. $\endgroup$ – Ron Beyer May 7 '18 at 21:30
  • $\begingroup$ @RonBeyer I agree with you. That is actually why I think if you apply carb heat within a few seconds of detecting an engine failure, if it was due to carb ice, there might be enough heat in the manifold get it to sputter back to life and create more heat, etc. The prop is windmilling, it should still suck some hot air through the carb. Also, sometimes when you lose power in one engine, it might not be totally dead, but you're still going to have asymmetrical thrust and run through your checklist. $\endgroup$ – Devil07 May 8 '18 at 2:19

Partly because carb heat is ineffective once the engine is no longer making power, but mostly because the initial engine out procedure isn't intended as remedial action to get the engine running again; the drill is to make sure the airplane is configured to be able to climb (somewhat) in the moments after the failure, so the priority is everything balls to the wall, gear and flaps, identify, confirm, feather with the aim of establishing a climb foremost.

In any case, if you applied carb heat to both engines as part of the drill, the dead engine would be getting probably insufficient heat anyway, and the live engine will enjoy a few percent power reduction from the application of heat. On a piston twin like a Seminole or Duchess at gross on a hot day, that alone may be the difference between barely climbing or descending, so that the remaining engine takes you directly to the scene of the crash as the old joke goes.

  • $\begingroup$ The first part of your answer is probably the most logical reason why its not in the initial checklist. The second part of your answer is the only thing I can think of as to why it wouldn't be prudent to use carb heat until after you "identify" the dead engine. $\endgroup$ – Devil07 May 8 '18 at 2:22
  • $\begingroup$ Perhaps. On a twin engine jet it's the same thing. The only actions taken initially on a failure after V1 is to continue the takeoff ensuring the good engine is at max thrust, raise the gear with positive rate, and fly a modified single engine departure profile with the shutdown procedure done when workload permits on the way up. No remedial actions are tried until all that is done and you are at a safe altitude. $\endgroup$ – John K May 8 '18 at 2:33

You may want to add the aircraft in question as it could be specific to the airframe/engine.

One consideration is that once an engine stops running there is no more airflow through the venturi as no air is being drawn in. In turn there will no longer be a temperature drop in the venturi so carb heat is no longer needed. It may also subsequently work against you during an attempted restart by creating an sudden over rich situation.

  • $\begingroup$ I'm flying a Grumman Cougar. The other aircraft I checked was the Piper Seminole POH which has the Carb heat on the engine restart troubleshooting checklist... which by that time I can't imagine it will do any good. $\endgroup$ – Devil07 May 8 '18 at 2:17

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