When calculating takeoff performance why include airport elevation in pressure altitude? 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).
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).