MP pressure different than QNH



  • In the real airplane when engine is off the MP pressure should be the same as the local pressure (QNH). In the example below QNH = 30 inHg (altitude 2247 Feet) and MP indicates only 27 inHg, actually I tested it for different airports/altitudes and MP always indicate 27 inHg.

    QNH (30 inHg) => airport altitude 2247 Feet
    1d850287-3ca4-4132-bed6-1f120a47b268-image.png

    MP indicated pressure 27 inHg (engine off)
    2331f4e7-d91a-4696-a49a-becdbb640966-image.png



  • Standard ISA pressure at 2247 ft. AMSL is 27.1324 in. HG...

    http://www.calctool.org/CALC/phys/default/pres_at_alt



  • According to https://www.skybrary.aero/index.php/Altimeter_Pressure_Settings the QNH is

    "QNH - The pressure set on the subscale of the altimeter so that the instrument indicates its height above sea level. The altimeter will read runway elevation when the aircraft is on the runway."

    So if the QNH is 30 inHg, then the altimeter would read 2247 ft only if you set the Kollsman scale of the altimeter to 30 inHg. Which is exactly what you did. The actual pressure at 2247 ft would be lower as @RetiredMan93231 already mentioned.

    The manifold pressure gauge in your case seems to indicate less than 27.1 inHg, so that is a bit weird I guess, but it is hard to read off from the picture. For different airports/altitude I would expect different manifold pressures. Perhaps it is a state saving issue?



  • Your MP should match the altimeter setting that gives you an altitude of 0ft on the altimeter. That means that MP equals QFE, not QNH. QNH (which is what you have set in your image, which means the altimeter is indicating your current altitude as referenced to sea level) is the pressure at sea level for your current location, which you could read if you were to dig a well all the way down to sea level.