question about Pressure Altitude

[Speedbird744]

This is regarding working out takeoff ground roll and landing distances from the Performance charts in the Flight Manual.
I understand Pressure Altitude has 1013 related to it somehow, but how would I go about working it out.
If the QNH on the day is 1018, then is it 1013-1018 x 30ft= pressure altitude ?
Can someone clear up what is required here?

[Keef]

Almost. Pressure altitude is what your altimeter would show when set to 1013.

So if you're at sea level, you can use QNH. If you're at an airfield at 600 feet (eg Biggin), then you add those 600 feet or whatever they are for where you are. Inevitably, the places where it matters most are so high that there's no QFE and you have to use QNH. (Big Bear City is one I know well).

In your example, if QNH is 1018 then the sea-level pressure altitude is 1013 minus 1018 times 30 = Minus 150 feet. If the airfield is Biggin, then add the 600 feet to get a pressure altitude of 450 feet.

Or, use QFE. At Biggin, you'd expect the QFE to be 998-ish. So 1013 minus 998 times 30 = 450 feet pressure altitude.

But for takeoff calculations, you need DENSITY altitude, not PRESSURE altitude (unless your takeoff distance table has a temperature line on it to take care of that).

For the density altitude calculation, you need your whizz-wheel or Jeppstar etc. You also need to know temperature where you are - density altitude is pressure altitude adjusted for non-standard temperature.

Any help?

[Genghis the Engineer]

Pressure altitude = altitude as shown on the altimeter.

Standard pressure altitude (what's used for field calcs) = altitude with 1013.25mb set on the altimeter. You can either do the sum as you've shown (but using QFE, not QNH), or simply set 1013 on the subscale and read-off.

G

[Genghis the Engineer]

Brief postscript, some manuals use density altitude, others separately use standard pressure altitude and temperature.

I admit that I prefer density altitude but most POH use the two separately, as does the CAA's standard safety guidance so Keef's advice only works for certain aircraft / manual sets.

G

[Saab Dastard]

250 feet in a twin when an engine fails

Now THATs pressure altitude ;)

SD