People can argue whether taking off is the scariest part of a flight or landing? But, if you ask which part is the most relaxing one, then there is surely just one answer; when the captain announces “We’ve reached our cruising altitude of 36,000 ft” and the seat belt signs go off. But, why at 36000 ft only?
Airline dispatchers; people who are appointed by the carriers to determine the ideal height for flight, use one basic rule i.e. how thin the air is? Pilots prefer to cruise over thin air as the aircraft experiences lesser drag force hence it takes up less fuel and flies easily. However, the air should not be too thin as this results in the shortage of oxygen molecules for combustion. Therefore, a compromise is established by setting up the ideal height as 36000 ft for the airplanes to fly at.
Other important aspects that determine the height at which the plane flies are weight of the aircraft, turbulence, direction of the flight and its duration. Planes flying towards Northeast, East, Southeast fly at odd altitudes of 31000 ft, 33000 ft and so on while in all the other directions, the altitude is set on even figures. This is defined as “the cardinal direction altitude rule” by the professionals.
Moreover, the Air Traffic Control dictates the ideal altitude for each aircraft in the cases of turbulence as well as to avoid traffic jams. More than often, 36,0000 feet is the most favourable altitude to fly at even after considering all the variables.
Reference: Thrillist