Meteorological Navigation: Wind, Temperature and Altimetry
Wind and temperature in the plan
Upper winds, jet streams and mountain wave change groundspeed and drift between the planning stage and the flight. Wind at altitude is stronger than surface wind, and the jet stream core can add or remove 100 kt or more on a long leg. Temperature affects the TAS you actually achieve: TAS rises with height for a given IAS, commonly estimated as about 2 per cent per 1000 ft, and rises further in warmer than ISA air. Cruising at high level in cold air therefore gives less TAS for the same IAS than the rough rule suggests, which feeds directly into groundspeed and fuel.
Altimetry errors
- Pressure error: flying from high pressure to low pressure without resetting the subscale makes the aircraft lower than indicated. High to low, look out below.
- Temperature error: in air colder than ISA the aircraft is lower than indicated; in warmer air it is higher. Hot to cold, look out below.
- True altitude, used for terrain clearance, differs from indicated altitude by both effects; density altitude matters for performance.
- Minimum safe altitudes and step-down fixes assume a subscale setting and, for cold temperatures, a temperature correction.
Worked example
At FL350 in ISA minus 15, IAS 250 kt. The 2 per cent per 1000 ft rule gives about 250 x 1.70 = 425 kt TAS. With a 50 kt headwind the groundspeed is 375 kt, so a 600 nm leg takes 600/375 = 1.6 h, or 1 h 36 min. Add the same leg in the opposite direction and the wind effect reverses.
Traps
Using IAS as TAS in the wind triangle; applying the 2 per cent rule with no temperature correction; forgetting the low-pressure and cold-temperature altimeter errors on procedure minima; and planning with forecast wind but no allowance for a change in the cruise level.