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4 Answers

Constant Speed Propeller indication Question

Asked by: 2649 views Aircraft Systems, Private Pilot, Student Pilot

Hello !!!

I am having a Question about a constant speed propeller indication .


When I am Shutting Down the engine the prop Manifold Air Pressure is indicating 30 

Why is that happening ? i know its not a wrong indication or failure  but i am wondering why ??

Thanks a lot .


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4 Answers

  1. Lucas on May 21, 2012

    Manifold Pressure…
    If the engine is off this pressure gauge does what its supposed to do. It reads pressure, in this case, outside pressure which is usually around standard (29,92) = 30 MP.
    During a hurricane it will read lower than 30, but trust me, you wouldnt want to try that…
    Fly safe and have fun

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  2. Derek Schwalenberg on May 21, 2012

    It is reading the manifold air pressure. It stablizes with outside air pressure when not in operation. The pressure can be more or less when the engine is in operation depending mostly on percent throttle.

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  3. John D. Collins on May 21, 2012

    On a normally aspirated engine (not turbo charged), air enters the manifold thru the induction intake, then thru the air filter, then thru the throttle butterfly valve.  When the engine is running, if the throttle is full open, then the air is relatively unimpeded into the manifold with a small loss around one inch of manifold pressure as it passes thru the air filter.  Air is sucked out of the intake manifold during each of the intake cycles of the cylinders as the piston is drawn towards the bottom of the cylinder. When the throttle is in the full throttle position, the butterfly valve is open and provides very little resistance to the air, so the manifold pressure doesn’t drop very much as a result of the pistons sucking in the air. But as the throttle is retarded, the butterfly valve closes more and more, being mostly closed at idle. The butterfly obstruction prevents induction air from replacing the air sucked out of the manifold, so the pressure drops in the manifold to balance things out.  Since the manifold pressure gage reads the pressure in the manifold, it will be a lower pressure when the engine is running near idle, than at low power, than at medium power, than at full power.  


    As previously stated, there is some loss of pressure across the air filter, so when the engine is running, the manifold pressure at full throttle will be a little below outside air pressure. When the engine isn’t running, the manifold pressure will equalize at outside air pressure which will indicate normal air pressure. So assume the engine is off and the outside air pressure is 29.9, the manifold pressure should indicate close to this value. As the engine starts, the manifold pressure will drop to something close to 10 or so inches, and at full throttle, it will indicate something close to 28.9 inches on a typical system.  As you climb, the outside ambient pressure drops, so the full throttle manifold pressure will drop accordingly, at a rate of approximately 1 inch per 1000 feet of altitude.


    If you have an induction leak, air can enter the manifold past the butterfly valve, usually near a joint in the induction pipe or where the induction system attaches to the cylinder. Since more air is available, you will notice a higher than normal manifold pressure at idle as a tell tale sign.

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  4. Prodromos Galaktidis on May 22, 2012

    Thank You Gentlemen for providing me with some new knowledge about Engines.
    All your answers are useful!
    Thanks Again ,

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