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When calculating the pressure altitude for aircraft performance I learned to calculate it the following way:

Pressure Altitude = (29.92" - altimeter setting) * 1000' + elevation

So at an airport at 3000' elevation and an altimeter setting of 29.92" I can expect the aircraft performance to be that for 3000'. I never questioned that but now I wonder why don't we just care about the altimeter setting? If the altimeter says the station pressure is 29.92" sea level, but I am at 3000', then the airplane still thinks it's at sea level when it comes to performance. Is that wrong?

Same for an airport at 3000' elevation and an altimeter setting of 28.42" I can expect the aircraft performance to be that for 4500'. Again, why not 1500'? The airplane should think that it is, regarding pressure, 1500' above sea level.

I think that would make sense for en-route performance. If the station pressure along the way is 29.92" and I fly above at 4000', my pressure altitude will be 4000' (0*1000 + 4000). If my station pressure however is lower, say 28.92" for instance, then the pressure altitude above at 4000' would be 5000' ([29.92"-28.92"]*1000 + 4000).

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3 Answers 3

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QNH is the atmospheric pressure adjusted to sea level.

So, a reported QNH of 29.92 means the pressure is 29.92 inHg at sea level, regardless of the reporting station's elevation.

If you're on ground at 3,000' MSL, you can rotate the knob to display 0' altitude, the pressure reading then on the altimeter window will be QFE (Field Elevation).

enter image description here
(Source) Real pressure would be ~26.81 inHg.

QFE should roughly be equal to the real outside atmospheric pressure.

Related: Why is Cessna 172 POH performance chart based on Pressure Altitude and Temperature instead of Density Altitude?

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If the altimeter says the station pressure is 29.92" sea level, but I am at 3000', then the airplane still thinks it's at sea level when it comes to performance. Is that wrong?

Although the station pressure may be reported as 29.92", that does not mean that a barometer there will show that as the current pressure. The reported value is the actual pressure, plus an extra amount that is calculated would occur if the recording station were lowered down to sea level. Instead of actual pressure, you are setting the QNH pressure.

So sitting at a 3000' airport with the altimeter reading 29.92 does not mean the airplane thinks it's at sea level. Certainly the altimeter doesn't think this, because it's reading 3000'.

You could use the actual pressure outside as an altitude-independent performance gauge, but you don't normally have that value (and ATC isn't going to radio it to you).

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  • $\begingroup$ A manifold pressure gauge will read atmospheric pressure when the engine is turned off. Thats as close as you'll get in the airplane though. $\endgroup$
    – wbeard52
    Commented Mar 17, 2017 at 23:22
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Another way to complement ymb's answer - if you set the altimeter to zero as in the picture on the left, you will get the static pressure at the field - your altimeter will then read AGL and not MSL - and we fly MSL - one way to think of why we put the sea level pressure in the Kollsmann window is that we are making that air inside the aneroid wafer equal to the pressure at sea level - the altimeter should read within 75 ft of field elevation - as the airplane rises, the static pressure decreases, causing the wafers to expand - so the starting point (field elevation) is then adding feet as the aircraft rises.

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