Expert: Ray Wilkinson - 6/12/2007

Question
Hi there.My name is Ryan Groenewald& i am a keen aircraft enthusiast.When an aircraft needs to take off,the engine needs cool& hot air to cause combustion& thrust.Is it true that an aircraft uses much more power for takeoff when the outside temperatures are soaring? Why is this? I look forward to your reply.

Regards
Ryan Groenewald.

Answer
Hi Ryan

Gas turbine engines produce thrust in proportion to their mass flow, but the fan and compressor blades move air by volume. As hot air is less dense than cold air, this means that in hot conditions (or high altitudes) the engine can move less air by mass and produces less thrust. there are various means to compensate for this, such as water injection, which cools the air and increases the mass flow rate. However, in hot and high conditions such as soem South American airports, the engines must be derated - it is assumed they produce less thrust than their design, and the aircraft load and/or perfromance adjusted accordingly.

What is also worth mentioning is that the air coming out of the compressor and into the combustion chamber is hot (perhaps 200-300C). This isn't an issue as the engine design takes it into account. However, air is normally bled from the compressor for use in things like cabin air (pressurisation), and to avoid cooking the passengers it needs to be cooled in a device known as the Environmental Conditioning System, or cold-air unit.

I hope this answers your question

Regards

Ray