A method is developed for determining the optimum cruising altitude to give minimum overall flight time for a given mission. Variation in aircraft performance, atmospheric properties, and winds with altitude are taken into consideration. The solution is performed by digital computer with remote typewriter terminals for input and output. The input is data readily available from the aircraft flight handbook and standard weather service reports. Results show a significant saving in flight time by use of this method over that of cruising at an arbitrary altitude. This result is particularly true in the case of supercharged airplanes operating over stage-lengths of about 100 statute miles.