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8 Answers

Hot Start Procedure

Asked by: 2858 views , , , ,
Aircraft Systems, General Aviation

There is a hot start procedure at my flight school that requires the throttle to be placed half way before turning on the fuel pump, as this apparently allows air to be pumped into the cylinders to even out the ratio of excess fuel already in the system with more air. I don't see how this affects the priming at all because I understand the throttle to simply control the throttle plate, and moving the throttle back and forth without the pistons running wouldn't do anything, since ambient air would be in the system no matter the position of the throttle. Also, the fuel pump shouldn't affect the the amount of air being sucked in by the engine; it should only affect the fuel. Am I correct in my understanding?

For your reference, here is the actual full hot start priming procedure:

Throttle half way, fuel pump 3-5 seconds (mixture in and out in the last second), pump off, throttle a quarter inch in, mixture a third in, crank


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8 Answers

  1. Sam Dawson on Sep 26, 2014

    Engine type?

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  2. Drew on Sep 26, 2014

    Lycoming IO-360-L2A on a C172S

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  3. Lucas on Sep 26, 2014

    ??? Throttle Plate on a fuel injected engine ???

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  4. Drew on Sep 26, 2014

    ??? Could you provide me with an answer and correct my misunderstanding if I am wrong???

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  5. Mark Kolber on Sep 27, 2014

    Fuel injected aircraft engines is an always fascinating topic. The POH method should be the best since, after all, the manufacturer should be in the best position to know how the airplane works. But through the years, people have found “better” ways for certain aircraft to get a more reliable hot start.

    I understand your frustration, Drew. Sometimes the methods used make no sense. There’s a hot start procedure floating around Mooney pilots in which you put the throttle in the full open position with mixture rich for 10-20 seconds before bringing them back and stating (all with no boost pump). Made no sense to me or anyone discussing it. But after unsuccessfully trying to start using the POH procedure on a warm day after a lunch flight, I figured I had nothing to lose. Started right up. And has done so every time I’ve used it since, even on warmer lays with less time since shutdown. Still don’t know why.

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  6. Kris Kortokrax on Sep 27, 2014


    I see the hot start procedure on page 4-25 of the 172S POH. It looks to me like you crank first, then move the mixture in to the 1/3 position after it fires. I don’t see where the POH shows the procedure you describe.

    You are correct in your assumption that the fuel pump does not affect the amount of air entering the engine.

    Changing the throttle position will affect the amount of air entering the cylinders while cranking, even though the engine is not running.

    Don’t worry about the “throttle plate” terminology. There is indeed a valve which regulates airflow through the fuel/air control unit and into the induction system. In most cases it will be a throttle plate.


    Ever hear of throttle body fuel injection? See this article on fuel injection on the Continental IO-360 engine.


    I’m surprised that you don’t have a video on your website explaining how fuel injection works.

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  7. Lucas on Sep 27, 2014

    We do in the full course Chris!!!

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  8. Mitchell L Williams on Oct 01, 2014

    On some fuel injected engines, in order to prime the engine you have to give it some throttle (and mixture) to get any fuel flow with a boost pump.

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