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2 Answers

when we feather propeller, why does manifold pressure gauge rise up?

Asked by: 2739 views Aircraft Systems

I fly BE-76, during run up, I set RPM2200 and brought propeller to full feather. I saw manifold pressure gauge rise up, why was it doing that?

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2 Answers



  1. Warren Webb Jr on Jun 29, 2019

    When the engine is not running (zero rpm) the MP will be equal to the ambient air pressure (i.e. 29.92 Hg). As the rpm gets lower during the prop check (moves toward zero), the MP moves upward toward 29.92.

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  2. KDS on Jun 29, 2019

    You ask an excellent question and we’ll assume a normally aspirated engine, which is what the BE-76 has.

    The manifold pressure is taken at a point between the throttle plate of the carburetor and the cylinders of the engine. It is measuring the vacuum between the two points. When the engine is not running, there isn’t anything sucking air into the intake manifold, so it reads ambient air pressure (on a standard day at sea level is should be 29.92 inches).

    When you do the prop check, you don’t move the throttle, so the throttle plate in the carburetor remains in the same place. However, the engine has a much tougher job turning the feathered prop, so it slows down. The only thing that was creating the vacuum was the engine running and sucking air into the intake manifold. When it is running slower and the throttle plate hasn’t moved, there isn’t as much vacuum.

    At this point I have to digress and tell a story from the past. I was with a young pilot who was showing me how he does a prop check. The first time he pulled the control back and moved it forward he said he was looking for an RPM drop. That did happen. Then on the next pull he said he was looking for a manifold pressure increase. That happened too. On the third pull he said he was looking for an oil pressure drop. That happened as well. I told him that was a good procedure, but in the future he might want to put his hand on the prop control instead of the mixture control.

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