777 take off power

[geoffb78]

Hi all

I recently read a "pilot profile" article in the BA in-flight magazine. The featured officer flew 777s and stated that engines are engaged to about 50% power on the usual take-off. I find this very difficult to believe and wonder what others think?

Geoff.

 

[markis10]

Most of the times, the airplane may not be at its maximum take off weight and so, on a Boeing 777-200 IGW (Increased Gross Weight), there are six power settings for take offs, depending on its weight and the ambient temperature. The settings are designed for different loads, at maximum gross weight for takeoff the aircraft would use 100%, light loads would result in less power required and therefore reduce stress on the engine although I doubt 50% would be used often.

 

[NM]

Remember that twin-engined aircraft such as the 777 must be able to complete a take-off once the aircraft has reached the V1 speed even if one engine fails. V2 (also calls Talke-off Safety Speed) is calculated for each take-off and is determined by factors including the aircraft's take-off weight, wind speed and direction. V1 (also called Take-off Decision Speed) is the speed below which the take-off roll can be safely aborted and the aircraft come to a complete stop before the end of the runway, and includes factors such as the available runway length.

So, the engines must each have enough reserve thrust individually to be able to continue the take-off after V1 has been exceeded, and for a short runway, that can be quite a bit below V2 and Vr (rotation speed).

The biggest of the 777 engines, the GE90-115 has been tested beyond 115,000lb thrust and is sufficient to maintain cruise of a 747 with just one power plan operating. GE owns a 747 they use for a test bed for their engines and they strapped a 90-115 to this aircraft for their airnborn test program. In this config they ran the other three normal 747 engines at flight idle power and used the 777 engine to maintain flight.

So on a 777, in normal conditions with both engines providing thrust, there is little need to get anywhere near full thrust capability, even for a max weight take-off.

 

[markis10]

So on a 777, in normal conditions with both engines providing thrust, there is little need to get anywhere near full thrust capability, even for a max weight take-off.

Dont forget though that 100% power is not the same as max thrust from the engine, but rather 100% of available derated power.

 

[Kiwi Flyer]

What is derated power?

Slightly OT but in the antarctic overflight (747) when captain showed us the controls we were cruising at 30% power which he said was typical +/-.

 

[markis10]

What is derated power?

Slightly OT but in the antarctic overflight (747) when captain showed us the controls we were cruising at 30% power which he said was typical +/-.

Basically 100% thrust does not mean 100% of available power from the engine, often the engines are downrated in the interests of longer range or reliability, for instance the 772LR is a 772ER, fitted with improvements from the 773 program. It has the downrated engines and wingtips of the -300. It also has a strengthened undercarriage, and increased MTOW!

As for cruising at lower power, the P3 Orions do 14 hour stints on patrol with two engines shut down, still remember reading an incident where a P3 came round the corner at hamilton Island and went underneath the Ansett helo that was operating there at the time prior to the runway being built, the Ansett chopper pilots needed a change of underwear :roll: .