Definition

An aeroplane cruising technique resulting in a net increase in altitude as the aeroplane mass decreases.

Source: ICAO Doc 4444 PANS-ATM

Description

Cruise climb is the most fuel efficient cruising technique. It allows the aircraft to constantly operate at its optimal performance. 

As fuel is burnt, the aircraft gradually becomes lighter. Therefore, less lift is required to balance the weight. This means that either speed will increase, or altitude will increase, or thrust will be reduced. Increasing the speed will also increase drag, hence fuel consumption. Reducing thrust means the engine would run in a sub-optimal mode. Increasing the altitude, on the other hand, will keep the engine setting and reduce drag due to air density reduction.

The downside of cruise climb is that it is often incompatible with ATS procedures and traffic demand. In busier airspaces (e.g. Europe, USA, etc.) traffic levels are such that clearing a flight to perform a cruise climb will deny several others the opportunity to fly at or near their optimal levels.