Originally Posted by
Scuffers
Trent is more efficient at 40,000ft than at ground level, they are very much optimised for running at cruise altitude.
Well that depends how you define "efficiency" - if you use SFC as a measure, it's roughly twice as good on the ground at takeoff thrust compared to the value in the cruise. In fact you could argue that the engine is optimised for getting the aircraft off the ground in the first place.
So, A380 at 40,000ft loses an engine, does it fall out the sky or do the other three make up the loss in thrust? (How about if it loses 2 engines?)
No, of course it doesn't fall out of the sky.
A loss of two engines on an A380 or 747 (roughly analogous to losing one engine on a twin) would necessitate the remaining engines producing double their normal cruise thrust if the same airspeed and altitude were to be maintained.
But, as you will have seen from the article you quoted, normal cruise thrust is about a quarter of T/O thrust, so even doubling that will only equate to around 50% of the latter.
What do you think it's max thrust would be at 40,000ft?
In lbf? I don't know. MCT is normally expressed in terms of N1 or EPR, rather than lbf.
But think about it - if an engine was capable of producing SL take-off thrust at 40,000 feet then it would mean that a 747/A380 could maintain height and speed on one engine, which it clearly can't..