Pressure altitude, density altitude and ISA
Navigation · Chapter 52 · DGCA CPL and ATPL
Delhi’s runway at 44 °C performs as if it were at 4 800 ft. Here is the sum.
Remember
- ISA = 15 − 2 per 1 000 ft, to −56.5.
- PA = elevation + (1013 − QNH) × 30.
- DA = PA + 120 × deviation.
- Deviation = actual − ISA.
- SAT = TAT − ram rise.
The standard air
Air is a pile. The bottom is squashed by everything above it, so the air thins as you climb.
Think of a stack of mattresses: the lowest is flat, the top one fluffy.
Everyone’s air differs every day. So engineers agreed on one imaginary average day to compare against: the International Standard Atmosphere.
Going up, it cools by 2 °C per 1 000 ft. Until the tropopause, 36 090 ft, where it stops at −56.5 °C.
ISA = 15 − 2 × thousands of ft
Above the tropopause the rule stops. At FL410 it is −56.5, not −67.
ISA stops falling at 36 090 ft: −56.5 °C.
Real air is warmer or colder than this. The gap has a name.
ISA deviation = actual − ISA
Pressure altitude
Stand on a hilltop. How high are you? It depends where you measure from.
Height above the floor, or above the table. Same head, two answers.
Pressure altitude measures from the level where the pressure is 1013 hPa. Set 1013 on the altimeter and that is what it reads.
On a standard day, that level is the sea. So the two numbers match.
A low-pressure day: QNH 993. The 1013 level has sunk below the sea, so you are further above it. Pressure altitude goes up.
PA = elevation + (1013 − QNH) × 30
A high-pressure day: QNH 1033. The level rises above the sea. Pressure altitude falls below elevation.
QNH below 1013: PA above elevation. QNH above 1013: PA below elevation.
Density altitude
This lesson is part of the Navigation plan. See plans
When only Mach is given
This lesson is part of the Navigation plan. See plans