Today at the flying club, I witnessed a Mooney startup. When the pilot primed the engine, there was a fair amount of fuel (I’m assuming) that was drained near the nose gear. What is the reason for this? And how does it work?



1 Answer 1


It appears to be an M-20R which has a Cont IO-550. As a fuel injected engine, you prime it by advancing the mixture momentarily to rich while the fuel boost pump is running. This dumps fuel from the main injectors into the intake ports, rather than have separate primer injectors like on a carbureted engine.

You advance the mixture for a certain amount of time to spray fuel in, say, 3 seconds or some such, and back to idle cutoff. Then crank the engine and move the mixture up when it catches. If you prime too much, raw fuel starts running down the intake runners and into the airbox below the throttle body, that directs either primary or alternate air into the intake. The airbox will have a drain line for this purpose, so that raw fuel is dumped overboard instead of collecting in the airbox and possibly leaking out in the engine bay, creating a fire risk.

Injected engines are quite easy to overprime because it's controlled by counting in your head and you may need more or less depending on how warm or cold it is outside. So it's a bit of an art.

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    $\begingroup$ Just a comment in general, and not about your answer, specifically. In this day and age, where safety of the populace has taken paramount, seems like having something like priming with fuel be 'an art' and not a manufactured-in process, is ... well ... less than safe. I'm sure the number of flight-line fires caused by priming fuel overflow is astronomically low compared to the total number of engine starts, but still. Having even one GA aircraft burn up is too many, especially to that plane's owner. $\endgroup$
    – CGCampbell
    Commented Dec 28, 2023 at 11:22
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    $\begingroup$ Problems is the alternatives, chokes, are not used, mostly because they are a failure risk, so priming is it, and absent electronic fuel injection, it has to be done manually, as nearly all airplanes are carbureted or have mechanical FI. Fuel pissing out the cowl from overpriming is not that common an occurrence, b/c you have to really overdo it, it's still a fire risk, and if an airport manager saw that Mooney dumping asphalt dissolver on the pavement like that, the Mooney driver would know about it. When I say it's an art, it's mostly about learning not to overdo it when an engine is hot. $\endgroup$
    – John K
    Commented Dec 28, 2023 at 13:51

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