Density altitude
The altitude the aeroplane performs as if it were at, from the elevation, QNH and temperature.
Conditions
Result
The aeroplane performs as if it were several hundred feet higher than the field. Noticeable on a short grass strip near maximum weight.
- Pressure altitude
- 307 ft
- ISA temperature there
- 14.4°C (ISA +5.6°C)
- Air density
- 97% of ISA sea level
- Take-off distance (CAA)
- +10%
- Landing distance (CAA)
- +5%
- Rate of climb (rule of thumb)
- −9%
- Engine power (rule of thumb)
- −3%
Exam method
- Pressure altitude = elevation + 27 ft for each hPa below 1013: 300 ft.
- ISA at that altitude is 14.4°C, so the day is ISA +5.6°C.
- Density altitude = pressure altitude + 120 ft for each °C above ISA: 980 ft.
The exact answer above uses the ISA pressure-height relation and the dry-air density formula. The distances use the CAA Safety Sense Leaflet 09 factors, the same as the take-off and landing tool. Climb and power are Koch chart rules of thumb for a normally aspirated piston: about −10% climb and −3% power for each 1,000 ft of density altitude. The Koch chart gives about +13% take-off distance for each 1,000 ft (+12% today).
Climb and speed today
Typical book figures for the PA-28-161 Warrior II. Check them in your flight manual.
- Rate of climb
- 584 ft/min
- TAS at 79 kt IAS
- 80 kt
A higher TAS also gives a higher groundspeed at lift-off and touchdown. Rule of thumb: +2% TAS for each 1,000 ft (81 kt).
The theory is in ISA and altimetry in the Meteorology guide and performance in Aircraft Technical.