Pressure Altitude and Density Altitude

[larshgf]

Why do we need both Pressure Altitude and Density Altitude as concepts for assessing an aircraft's performance. Why not just the concept of Density Altitude which corrects for both pressure and temperature?

[Tinstaafl]

Altimeters don't correct for non-standard temps. so data used from an altimeter is Pressure Alt. related.

[larshgf]

So when it is a quetion of evalution of performance we allways use Density Altitude?

[keith williams]

Using a typical performance chart involves putting the the Pressure Altitude and the Ambient temperature, both of which are readily available.

The curves of the chart combine these two variables in such a way as to account for the effecst of Density Altitude, which is not readily available.

If Density Altitude was readily available the performance charts would be desiged to enable us to put in Density Altitude directly. But Density Altitude is not so readily available, so the charts are designed to use the two variables ( Pressure Altitude and OAT) that are readily available.

[larshgf]

Thank you for the answers.

That means that performance (including calculation of KTAS) is allways based on Pressure Altitude and temperature (= density).

And - as far as I understand - Pressure Altitude is used ALONE only for seperating air-traffic.

Please correct me if neccesary!

[Tinstaafl]

Not just traffic separation. Don't forget terrain, including aerodrome & obstacle elevations, and airspace vertical limits.

[larshgf]

You're right!
Pressure Altitude is for "Vertical Awareness"
Density Altitude is for Performance valuation
- I think I grasped it now...