– If airplanes flies normally with the standard altimeter setting above 10k feet, why ATC gives to the pilots an exact value when they are below 10k ?

Question

Why bother change it when it works perfectly during the flight ?

In: 3

Answer 1

Actually, the crossover is FL180 (Flight Level One Eight Zero, or 18,000 feet).

Altimeter settings are based on ground observations. The higher you are, the greater the potential for error exists between the altimeter and the ground station.

With their altimeters all set to standard pressure, airplanes in the same part of the sky can more assuredly maintain vertical separation.

Above FL180, precisely maintaining a particular true altitude is less important than not hitting other airplanes.

A secondary factor is performance. Calculating performance metrics such as speed or fuel consumption depends on pressure altitude. Performance data assumes a pressure altitude, to which a conversion is applied. If you already know your pressure altitude (because an altimeter set to standard pressure shows pressure altitude) then you can skip the conversion.