Temperature, Pressure, Density and Their Effects on Aircraft Performance
What the exam tests
This domain tests how temperature, pressure and density change aircraft performance, and how to use pressure altitude, density altitude and ISA deviation.
Core rules
- Pressure altitude is altitude with 1013.25 hPa set; density altitude is pressure altitude corrected for temperature and, in precise work, humidity.
- High density altitude reduces lift, propeller/thrust and engine power, lengthens take-off and landing runs and reduces climb rate.
- QNH gives elevation when set on the subscale; QFE gives height above the reference datum. QNE is the standard setting used for flight levels.
- A common approximation is density altitude ≈ pressure altitude + 120 ft per °C of ISA deviation.
Worked example
Aerodrome elevation is 2,000 ft, QNH 1013 hPa and OAT 30°C. ISA temperature at 2,000 ft = 15 - (2 × 1.98) = 11.0°C, so the deviation is +19°C. Density altitude ≈ 2,000 + (19 × 120) = 4,280 ft. Expect noticeably degraded take-off and climb performance compared with a 2,000 ft ISA day.
Traps
Using OAT instead of ISA deviation; applying the 120 ft/°C rule without first correcting pressure altitude for temperature; forgetting that humidity raises density altitude; assuming QFE and QNH are interchangeable.