– 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

Using U.S. rules, you use the local altimeter setting up to 18k feet MSL. Above that, you are in Class A airspace where you use the standard altimeter setting of 29.92. Additionally, altitude at this point is no longer feet MSL but Flight Level (FL), roughly corresponding to thousands of feet, so 20k’ feet would be referred to as FL200. The reason for this is that Air Route Traffic Control Centers (ARTCC) control very large regions where there can be large variations in the local altimeter setting. Also, at these high altitudes, obstacle clearance is of secondary concern to traffic avoidance. So, having everyone on the same setting means that you can just assign a flight level and not worry about constant adjustment.

Once you descend out of the Class A airspace obstacles start to become a concern, and their elevations are given in feet MSL. Instead of having everyone do the mental math to convert their FL to actual altitude, we just use the local setting.