Commercial airplanes typically fly at cruising altitudes between 31,000 and 42,000 feet. The exact altitude depends on various factors such as the type of aircraft, the distance it is traveling, the type of engine it has, its weight, and the current strength of the winds. The cruising altitude is the altitude at which air offers the least resistance, allowing planes to travel faster with less fuel. The maximum height that a commercial airplane is allowed to reach when they fly is 42,000 feet, which is known as the "service ceiling". Short-haul flights may cruise at lower altitudes because they do not have enough time to reach higher altitudes before they need to descend again. Smaller aircraft, whether commercially operated or not, will fly at lower altitudes, often below 15,000 feet, due to the type of engine they have which makes them unable to reach the same heights.