For Airbus and Fokker aircraft the technique is known as flex temp or just flex. Other aircraft may use the terms Assumed temperature thrust reduction, Reduced take-off thrust or Factored take-off thrust.
The runway length required for an aircraft to take off is calculated for each flight. Depending on the aircraft weight, the air temperature and the wind speed, this length may be shorter than the available runway length. The crew can then calculate a lower-power engine setting where take-off will use a larger portion of the runway. Lower power settings reduce noise, engine wear, and maintenance costs.
This technique is based on air temperature, rather than percentage of engine power, because it is easy for the crew to look up or calculate the highest air temperature at which any particular take-off can be performed. That temperature is the highest flex temp that can be used.
Jet engines produce reduced thrust as the ambient air temperature increases. The flex temp is the highest air temperature at which the engines would produce the required thrust. The crew finds that temperature and enters it into the flight management system (FMS) — effectively telling the computer to assume the specified air temperature instead of the actual temperature. When flex (FLX) thrust is selected during take off, the engine controller produces maximum thrust for the assumed (flex) temperature. If necessary, the crew can push the throttles into the Take Off/Go Around (TOGA) detent and request full power.
A number of aircraft incidents and accidents have occurred when the flex temp was incorrectly calculated or entered — for example, those involving Emirates Flight 407, US Airways flight 1702 and Thomas Cook Airlines G-OJMC. Modern procedures are designed to minimize that possibility.