Flex temp
Flex temp is a technique used to reduce engine wear in large airliner
An airliner is a large fixed-wing aircraft for transporting passengers and cargo. Such aircraft are operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an aircraft intended for carrying multiple passengers in commercial...

s by performing take-off
Takeoff is the phase of flight in which an aerospace vehicle goes from the ground to flying in the air.For horizontal takeoff aircraft this usually involves starting with a transition from moving along the ground on a runway. For balloons, helicopters and some specialized fixed-wing aircraft , no...

 at less than full power.

For Airbus
Airbus SAS is an aircraft manufacturing subsidiary of EADS, a European aerospace company. Based in Blagnac, France, surburb of Toulouse, and with significant activity across Europe, the company produces around half of the world's jet airliners....

 and Fokker
Fokker was a Dutch aircraft manufacturer named after its founder, Anthony Fokker. The company operated under several different names, starting out in 1912 in Schwerin, Germany, moving to the Netherlands in 1919....

 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.


For large aircraft, the required runway length is normally calculated before every take-off. Often the aircraft weight and the local temperature would permit the aircraft to take-off on a shorter runway. The crew then calculates a lower-power engine setting where take-off will use most of the runway. Lower-power settings reduce both noise and engine wear and produce considerable savings in maintenance costs.

This technique is based on air temperature, rather than percentage of engine power, because it is easy for the crew to lookup 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
Flight management system
A flight management system is a fundamental part of a modern airliner's avionics. An FMS is a specialized computer system that automates a wide variety of in-flight tasks, reducing the workload on the flight crew to the point that modern aircraft no longer carry flight engineers or navigators. A...

 (FMS) — effectively telling the computer to assume the specified air temperature instead of the actual temperature. When take-off-go-around (TOGA) 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 past TOGA 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
Emirates Flight 407
Emirates Airline Flight 407 is an Emirates flight flying from Melbourne to Dubai using the Airbus A340-500. On 20 March 2009, the flight failed to take off properly at Melbourne Airport, hitting several structures at the end of the runway before eventually climbing enough to return to the airport...

 and Thomas Cook Airlines
Thomas Cook Airlines
Thomas Cook Airlines is a British charter airline based in Manchester, England. It serves main holiday resorts worldwide, from its main bases at Manchester and Gatwick.The airline also operates services from nine other bases in the United Kingdom....

 G-OJMC. Modern procedures are designed to minimize that possibility.

Reduced thrust take-offs are slightly less efficient and consume slightly more fuel than full-thrust take-offs. This is because jet engines are at best efficiency at full power, and because the slower climb to cruising altitudes consumes more fuel.

See also

  • Laker Airways, introduction of reduced thrust take-off technique
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