Density altitude
The altitude the aeroplane thinks it is at. Enter the field elevation, QNH and temperature (or pull them from a METAR) to get pressure altitude, ISA deviation and density altitude, with the exam rule of thumb alongside the exact answer.
Conditions
Density altitude
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 rule of thumb 27 ft/hPa: 300 ft |
| ISA temperature there | 14.4°C so the day is ISA +5.6°C |
| Density altitude | 964 ft rule of thumb (PA + 120 ft per °C above ISA): 980 ft |
| Air density | 97% of ISA sea level station pressure 1002 hPa |
What it means
Koch chart rules of thumb for a normally aspirated piston: about +13% take-off distance, −10% rate of climb and −3% power per 1,000 ft of density altitude, against the sea-level ISA figures.
Expect roughly 590 ft/min today. At 75 kt indicated, TAS is about 76 kt (rule of thumb +2% per 1,000 ft: 76 kt), so the groundspeed at lift-off and touchdown is higher too.
Pressure altitude from the ISA pressure-height relation; density altitude by the exact dry-air formula (as E6B apps and the NWS calculator use), with an optional moist-air correction. The exam method is the rule of thumb shown alongside. The theory lives in ISA and altimetry in the Meteorology guide and performance in Aircraft Technical.