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3 Answers

primer not locked in

Asked by: 7581 views Aircraft Systems

How does a primer operate mechanically? Why is a primer in the unlocked position bad for flight?

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3 Answers



  1. QOL on Feb 16, 2017

    Also, will the primer work when the throttle is all the way out or when the mixture is in the idle cut-off position?

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  2. Best Answer


    Skyfox on Feb 22, 2017

    The primer is a little mechanical pump kind of like a syringe that sucks in raw fuel when you pull it out, and then injects that directly into the cylinders or intake manifold when you push it back in. As far as I know it works independently from the throttle and mixture. However, the fuel shutoff valve would cut fuel to it when in the off position. It’s supposed to be in and locked during flight because in the unlocked position, it could potentially vibrate its way out and fill with fuel, and then if the pilot intentionally or accidentally pushes it back in that could cause the engine to flood out and stall, or foul the plugs.

    If you’re flying and find the primer has worked its way out, you’d want to throttle back a bit and very slowly push the primer in so the engine can burn up the excess fuel without flooding. Once it’s in and locked, put the throttle back to where it was.

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  3. John Scarry on Mar 12, 2017

    If the primer is not locked, it allows fuel into the cylinders by the vacuum sucking fuel through the primer. It can cause the engine to run richer than you intend or to not shut off when the mixture is pulled.

    The engine on my Cherokee is hard to stop by leaning it out. At the last annual we decided to fix it. The IA noticed that the the primer was not sealing. He explained that it could be letting fuel into the engine when when the mixture was off. Upon further investigation, two primer injection lines were plugged and one was broken, so that wasn’t the problem. It also explained why we couldn’t start it on a cold day by priming.

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