pressure altitude

[bellend]

Hi

Anybods care to help me on this, I sort of get the calculation to find the pressure altitude

Pressure altitude = Aerodrome Elevation + (Standard 1013 - QNH)x30

If Qnh is low say 990 that puts 1013 below station elevation ?

23mb diff *30 =690 feet below 250ft

+250 - 690= -440ft ?

so does Pressure Alt= sea level or -440

I ask this as the performance graphs for the PA28 only show Pressure altitude as sea level and above upto 6000ft

cheers

[Jumbo744]

Hello,

when I get negative numbers, I consider it Sea Level, I think you don't have any other choice anyways, but I may be wrong ?

[tbavprof]

PA value is negative. Aircraft performance responds as if it's negative. POH performance charts only deal with sea level values and above. POH wins, treat it as sea level for performance calcs.

[bellend]

Ok guys thought as much, thanks for the clarification

[Islander2]

Worth doing a 'sense check' when you make these sort of calculations, bellend. If you stop and think about it, would you really conclude that 990 QNH is a pressure altitude at or below sea level, with all that means for enhanced performance? No, I thought not! Perhaps you should check your calculation ... it's wrong.

[Flying Farmer]

If you wind your altimeter from 990 up to 1013 that will tell you your Pressure Altitude if you were sat in the aircraft. 940 ft give or take if the airfields at 250 ft.

The easy way to think about it is that low pressures, those under 1013, give lower performance i.e. higher pressure altitudes.

[bellend]

HI Islander

could you expend on this a bit, I am getting confused on this subject as i have been told by an instructor that my calc was correct?

what have i done wrong?

thanks

[bellend]

Flying farmer cheers, makes more sense now

990 QNH puts me higher than 1013 ISA to start off with

I was adjusting the ISA datum to work the figures

cheers