An aeroplane cruising technique resulting in a net increase in altitude as the aeroplane mass decreases.
Source: ICAO Doc 4444 PANS-ATM
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.
Another issue with clearing an aircraft to perform a cruise climb is that the vertical speed is much lower than the usual 1000-2000 feet per minute. This results in a situation where the aircraft has vacated a level (i.e. is 300 feet away from it) but it is still not available for use by another aircraft. Therefore, ICAO explicitly states in Doc 4444 that if the higher aircraft (A) is performing a cruise climb, the controller cannot clear another aircraft (B) to the level that has just been vacated by A.
The third consideration when clearing a cruise climb is whether the aircraft can reach its final level before the end of the sector. In busier airspace, the time spent in a sector is usually in the range between 5 and 20 minutes. This obviously precludes large vertical movements at a slow rate of climb. If the cruise climb procedure is to be used, its application needs to be specified in local instructions or letters of agreement between neighbouring ATS units.
In practice, cruise climb was used by the Concorde for the transatlantic portion of the flight. This was justified because the aircraft flew above most of the other traffic (the cruise climb was normally between FL 450 and FL 600).
Aircraft authorized to employ cruise climb techniques are cleared to operate between two levels or above a level.
The controller would issue the clearance using the phrases: "CRUISE CLIMB BETWEEN (levels)" or "CRUISE CLIMB ABOVE (level)"
In the flight plan message, the commencement of cruise climb is specified in field 15 (route) by the symbol "C/", location, speed and levels. For example, "C/48N050W/M082F290F350" means "cruise climb starting at 48 degrees North, 50 degrees West, at Mach 0.82 between FL290 and FL350". If the climb is planned to be above a certain level (instead of between two levels), this is specified by using the "PLUS" string instead of the second level.