Planes equipped with jet engines fly at greateraltitudes than propeller-driven aircraft. These include commercial flights, cargo jets and even private passenger jets. The air traffic control tower usually assigns a cruising altitude of up to 39,000 feet, but long flights are typically assigned higher altitudes. By keeping these planes at assigned altitudes, air traffic control creates invisible stacked highways in the sky, keeping enough air space between the flights. Air traffic control may change an aircraft’s altitude assignment as needed. Most general aviation planes use propellers and mustcruise at lower altitudes. For example, a Cessna Skyhawk has a maximum operating altitude of 13,500 feet. Planes that fly at higher altitudes must have pressurized cabins to keep the pilots and passengers safe and comfortable. The pressure and oxygen levels inside the cabin are set to correspond with those experienced at up to 7,000 feet. Also for most commercial passenger aircraft, the cruisephase of flight consumes the majority of fuel. As this lightens the aircraft considerably, higher altitudes are more efficient for additional fuel economy. However, for operational and air traffic control reasons it is necessary to stay at the cleared flight level. On long haul flights, the pilot may climb from one flight level to a higher one as clearance is requested and given from air traffic control. This maneuver is called a step climb.