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9 Answers

Engine clearing

Asked by: 9667 views ,
Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Instrument Rating, Light Sport Aircraft, Private Pilot, Student Pilot

What purpose are we trying to achieve by clearing an engine in an idle descent?

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9 Answers



  1. Kris Kortokrax on Dec 24, 2014

    The biggest reason is to assure yourself that the engine is still running.
    If it quit, the prop would still windmill and you would have no indication that it had quit.

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  2. Matthew Waugh on Dec 25, 2014

    More generally I would say it’s to check that the engine will respond. There are a number of situations beyond cut out that will cause an engine not to respond with the appropriate power when requested.

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  3. Kris Kortokrax on Dec 25, 2014

    Matthew,

    Could you name a few?

    “not to respond with the appropriate power” sounds like a euphemism for “it quit”.

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  4. Bob Watson on Dec 26, 2014

    Carburetor ice is the first thing that comes to mind as a way to not respond with appropriate power while not quitting. Having carb heat on while the engine isn’t producing any (or much) exhaust heat might not be enough to keep the carb clear.

    But, peace-of-mind is probably the most compelling reason. After a long descent at idle, I’d want to know I still have the engine there when i need it, just in case we don’t end up at a [long-enough] runway. A long glide to a deadstick landing at an airport is certainly confidence boosting, but there’s a lot that can still go wrong between “engine out” and touchdown.

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  5. Kris Kortokrax on Dec 26, 2014

    Carburetor ice, mixture not set rich, fuel on wrong tank.
    All things which should be considered prior to the extended glide.

    Ultimate result is “it quit”.

    There are others who think that spark plugs will foul. The momentary application of power during clearing would not be enough to remedy fouled spark plugs.

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  6. Matthew Waugh on Jan 01, 2015

    Kris – we’re dancing on the head of a pin. Your initial statement “engine still running” is different from an engine that is running but – when the throttle is advanced either THEN quits or stumbles, burps, bangs and generally does not make “requested power”.
    You can be in descent with an engine that is running that, when requested to make more power does not – but that engine was, is and will continue to run.
    It’s just semantics.

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  7. Gary S. on Jan 03, 2015

    All are good reasons, but another one is also good to be aware of and it’s powerplant temperature. In a prolonged power-off glide such as while performing a steep spiral, it’s good practice to smoothly, but briefly, increase power to cruise power to help keep the powerplant warm and to check it for normal operation. Otherwise, if the engine cools too much one might find that at the bottom of the maneuver that the p’plant will not develop full power. In fact, if the p’plant is too cold it may just quit when all that fuel is suddenly poured into the chambers. In the steep spiral and steep turn maneuvers this is best done when turned directly into the wind so as not to mess with your ground speed and turn radius.

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  8. Kris Kortokrax on Jan 04, 2015

    Matthew

    There is no such expression as “We’re dancing on the head of a pin”. The most commonly seen reference is in the form of a question, “How many angels can dance on the head of a pin?” The actual reference dealt with how many angels could dance (or stand) on the point of a needle. It is most often used to indicate the pointlessness of a discussion. This raises the question: If the discussion is pointless, why would you choose to continue it?

    My 1979 edition of Webster’s New Twentieth Century Unabridged dictionary defines semantics as: “the branch of linguistics concerned with the nature, structure and, especially, the development and change of the meanings of speech forms; semasiology.”

    What do you mean exactly when you state: “It’s just semantics”?

    The meaning of words and phrases is important. For instance, when I hear the term “clear the engine”, it means something very specific to me. I learned to fly in 1970 and when we “cleared the engine”, it meant moving the throttle just off of idle and returning it immediately back to idle. This action confirmed that the engine was still running but gave no information as to whether the engine would produce “appropriate power” (your phrase).

    You would also need to define “appropriate power”. Do you mean maximum rated power? This would imply that by “clearing the engine” you would need to move the throttle to the full open position. You would still not have any assurance that the rated horsepower is available. You could, for instance, have a cracked ring or induction leak and believe that the engine is developing rated horsepower, while it is in fact not producing rated power.

    You spoke of “a number of situations beyond cut out that will cause an engine not to respond with the appropriate power”. I asked you to name a few. You responded with “quits or stumbles, burps, bangs and generally does not make ‘requested power’”. This is a description of the result, not a situation that will cause the engine not to respond (while not having quit).

    It would seem to me that we need to be more concerned with the terms we use in these discussions, rather than less concerned.

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  9. Kris Kortokrax on Jan 04, 2015

    Gary,

    During the 4 or 5 minute glide, the engine will not cool that much. Both the oil temperature and CHT will be within normal operating limits. There is no reason why the engine should not develop rated power due to cooling.

    If I land and taxi back at idle to the threshold for takeoff in zero degree weather, the engine has no problem developing full power for takeoff. It also does not quit when “all that fuel is suddenly poured into the cylinders”.

    By the way, raw fuel is only introduced into the cylinder if the engine is fuel injected.

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