For most commercial passenger aircraft, the cruise phase 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, staying at the cleared flight level is the most expedient option. On long haul flights, the pilot may climb from one flight level to a higher one periodically, as clearance is given from air traffic control.

Commercial or passenger aircraft are usually designed for optimum performance at their cruise speed or VC. The aircraft is designed for maximum lift to drag (L/D) ratio for that speed. Ideally, designers maximise L/D for a range of speeds, as flights face non-optimum speeds and altitudes for operational reasons. Commercial aircraft manufacturers have to tradeoff aircraft designed with higher levels of L/D over a narrow range of speeds and vice versa.

There is an optimum cruise altitude for a particular aircraft type and conditions including payload weight, center of gravity, air temperature, humidity, and speed. This altitude is usually where the combination of L/D ratio and engine efficiency are maximised.

During long climbs the angle of climb is often reduced. Whilst slowing the speed of ascent, more forward progress is made toward the destination, often saving time when taken over the entire journey. An added benefit is more forward visibility over the nose of the aircraft and an increased flow of cooling air to the engines. If the pilot tries to climb too fast, a stall may occur.

All aircraft, but especially gliders can also ascend by using rising air, (see article on gliding). For lighter aircraft this method can achieve faster rates of climb than by using engine power.

Opposite to the climb is descent.