hkg_clk
Posts: 980
Joined: Sun Jan 07, 2001 2:56 pm

Derated Thrust

I am a bit confused about what is derated thrust. Does it mean that the engine is capable of producing, lets say 90000 pounds of thrust, but when fitted to the aircraft, the computers have made it impossible for the pilots to select that max amount of thurst? Or does it mean that the pilots simply just choose to use less thrust cos the load on the plane does not require the max amount of thrust?
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Guest

RE: Derated Thrust

Take the GE90-90B engine, its capable of producing 90,000 lbs of thrust, when its on the B777 the aircraft has a Max Takeoff Weight of 632,000 lbs.

If the aircraft is taking off at a lighter weight, it obviously doesnt need to use all that power. Using less power increases the engine life and therefore saves money for the airline.

The crew have two ways of reducing takeoff power, one is to tell the computers that the outside temperature is hotter than it actually is, (this is called assumed temperature) The computers will therefore reduce the amount of power available, there is a FAA limit on this power reduction of 25%.

The 2nd method is to Derate the engine, this is pretty much like saying you have a 81,000 lbs (-10%)thrust engine on the wing or even a 72,000 lbs (-20%) thrust engine. These are called Derate 1 and Derate 2, they are selected on the FMC.

You can combine the assumed temperature method with the fixed derate and therefore reduce your 90,000 lbs thrust engine by almost 45%.

Its pretty amazing when you consider that the takeoff is based on an engine failing at a point on the runway.

Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Flat Rated Vs. Derated Thrust

The 2nd method is to Derate the engine, this is pretty much like saying you have a 81,000 lbs (-10%)thrust engine on the wing or even a 72,000 lbs (-20%) thrust engine. These are called Derate 1 and Derate 2, they are selected on the FMC.

I'm afraid you have derated climb thrust mixed up with the "flat rating" of an engine.

Flat Rating an engine does in part what you've said: the manufacturer takes a 90,000 lb. thrust engine, then artificially caps its performance at say 80,000 lbs. This capping is accomplished in the performance and limitation figures, if not physically in the engine installation. For instance, if the engine's normal limiting EPR at ISA sea level was 1.90, then the flat rated max EPR might only be 1.80.

De-rated climb thrust is a feature used to reduce climb thrust in the third, fourth and enroute climb segments. It uses more fuel in climb to altitude, but the engine operates at less EGT's therefore prolonging engine life. Fuel is cheaper than overhaul costs.

Best Regards,

Buff

Guest

RE: Derated Thrust

Hi Buff,

I forgot that we dont have DER1 (-10%) and DER2 (-20%) Takeoff options on the B757, but we most certainly have them on other Boeing aircraft.

We have completely different takeoff charts depending on the takeoff thrust rating.

We also have Derate Climb which cuts in at 1000 feet all engine or at a minimum of 800 feet with an engine out.

(The percentage derates are customer driven.)

Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Derated Thrust

Our RB211-535 E4's are flat rated to 40,000 lbs, I believe. I don't have the data available, and I haven't really dug too deeply into these engines, but I believe they have nominally more power available. Some E4's are flat rated to 38,400 lbs if I recall.

The climb de-rates on our installation are Climb 1 @ 94% and Climb 2 @ 88%. We normally use Climb 2 for takeoff's below 105,000 kg and Climb 1 above 105. The derates are removed progressively as altitude increases, although Climb 2 is usually adequate to take you all the way to optimum cruising altitude on a standard day.

From what I've been told by our Airbus evil-twin brothers, they use no derates on their engines for climb purposes. Perhaps a BusDriver would like to add 2 cents...

Best Regards,

Buff

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