The well known Derate/Assumed issue
Only half a speed-brake
Well, nick14, that's exactly where I'm coming from. And exactly why I'd love to hear more from tdracer.
RAT5: many a procedure ...
RAT5: many a procedure ...
FD, I wasn't able to find anything meaningful on a quick search (to be fair, I'm on vacation and don't have access to my normal sources), but here is the basic theory:
Nearly all jet turbine engines have what we refer to a "TSFC Bucket" - SFC is high at low thrust, and high at high thrust, but low in the middle - sort of elongated "U" shape (the exact level and shape being a function of altitude and airspeed/Mach). For obvious reasons, the engine cycle is designed so that typical cruise thrust is at or near to the bottom of that TSCF bucket (that's why, even on a quad where this is little drop in cruise altitude/speed with an engine out, engine out range drops so dramatically - you end up operating well up that high side of the 'bucket').
So, pretty much any time you're operating at or near max rated thrust, you're operating well up on the 'high' TSFC side of that U shaped TSFC curve.
Clear???
Nearly all jet turbine engines have what we refer to a "TSFC Bucket" - SFC is high at low thrust, and high at high thrust, but low in the middle - sort of elongated "U" shape (the exact level and shape being a function of altitude and airspeed/Mach). For obvious reasons, the engine cycle is designed so that typical cruise thrust is at or near to the bottom of that TSCF bucket (that's why, even on a quad where this is little drop in cruise altitude/speed with an engine out, engine out range drops so dramatically - you end up operating well up that high side of the 'bucket').
So, pretty much any time you're operating at or near max rated thrust, you're operating well up on the 'high' TSFC side of that U shaped TSFC curve.
Clear???
No question that using full power on take off will be harder on the engine, lead to increased wear and subsequent higher burn long term
I was referring to a single flight.
Very rare to do use full power but, if operating on the edge of the aircraft's range performance I have done this, it has made a difference.
I was referring to a single flight.
Very rare to do use full power but, if operating on the edge of the aircraft's range performance I have done this, it has made a difference.
Only half a speed-brake
tdracer
Understandable.
I assume the U curve of a particular engine curve has different shapes for different air densities and (different EGT values). Also, that you cannot go too far up the side since you'd hit engine rating cutoff. And that engine rating cut-off may be well close to U's bottom, or even before it?. I am thinking CFM 56 wide max. thrust options 33 - 23,37 k.
The reason I poke you so much is the CFM material nick14 mentions. It specifically goes on to say (my recollection) "we need to admit that FULL THRUST tkof will use less fuel per event, yet the associated performance degradation will increase overall consumption. The accumulated effect is more fuel used even before the next shop visit, it makes no sense cash-flow wise to do it"..
Thanks, enjoy your holiday well!
I assume the U curve of a particular engine curve has different shapes for different air densities and (different EGT values). Also, that you cannot go too far up the side since you'd hit engine rating cutoff. And that engine rating cut-off may be well close to U's bottom, or even before it?. I am thinking CFM 56 wide max. thrust options 33 - 23,37 k.
The reason I poke you so much is the CFM material nick14 mentions. It specifically goes on to say (my recollection) "we need to admit that FULL THRUST tkof will use less fuel per event, yet the associated performance degradation will increase overall consumption. The accumulated effect is more fuel used even before the next shop visit, it makes no sense cash-flow wise to do it"..
Thanks, enjoy your holiday well!
Last edited by FlightDetent; 5th Mar 2016 at 13:36. Reason: mixed up, really
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Company policy not to alter climb thrust to reduce risk of level bust, we just input derate and atrt and leave whatever is automatic. We do however delete reduced climb thrust when cleared unrestricted climb.
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Why not just use max reduction for takeoff and then full CLB after flaps up, or after flap retraction is initiated? Best of both worlds.
This consideration (and ramifications) has been around since flex was first introduced.
One of the first operators to play with flex was Qantas, back in Wal Stack's day as Ops Eng boss. I can recall his tale when one of the early flights had the crew coming back with the complaint that they had to push the throttles up to climb power at the end of the takeoff sequence .. he changed the regime then so that flex didn't go lower than climb.
At the end of the day, it is up to the operator, subject to engine contractual requirements, to figure how the engine ought to be operated .. as with tdracer's example, some go all out .. while the majority are a bit more dollar pragmatic.
This consideration (and ramifications) has been around since flex was first introduced.
One of the first operators to play with flex was Qantas, back in Wal Stack's day as Ops Eng boss. I can recall his tale when one of the early flights had the crew coming back with the complaint that they had to push the throttles up to climb power at the end of the takeoff sequence .. he changed the regime then so that flex didn't go lower than climb.
At the end of the day, it is up to the operator, subject to engine contractual requirements, to figure how the engine ought to be operated .. as with tdracer's example, some go all out .. while the majority are a bit more dollar pragmatic.
FD - I've not spent meaningful time working the 737NG, and the last time I did much detailed engine performance work was when we were still delivering JT9D powered aircraft. I do know that on the JT9D-7R4 powered 767 the TSFC curve went up steeply above max climb - not so sure about the CFM56-7 on the NG.
But the 737 has a somewhat unique feature. The 737-300/400/500 all used the same CFM engine but with different ratings - and the NG is the same way. You can't use the max rating on the shorter 737s because the pitchup moment is more than the tail can overcome (on the 737-300/500, there was an actual physical block that prevented full throttle, on the FADEC NG it's simply controlled by the rating plug). So on the lower rated -700 (or -600, although I think that's out of production) it's possible that max TO is low enough that you're not getting into that steeply increasing part of the TSFC curve - the answer might well be different on a 737-900ER.
BTW, I recall we had an issue years ago with one particular 757 operator that was having a rash of compressor related engine failures. Come to find out they were doing derated climbs, which tended to operate the engine in an RPM range that caused stator vane resonance and the vanes would fatigue and break
But the 737 has a somewhat unique feature. The 737-300/400/500 all used the same CFM engine but with different ratings - and the NG is the same way. You can't use the max rating on the shorter 737s because the pitchup moment is more than the tail can overcome (on the 737-300/500, there was an actual physical block that prevented full throttle, on the FADEC NG it's simply controlled by the rating plug). So on the lower rated -700 (or -600, although I think that's out of production) it's possible that max TO is low enough that you're not getting into that steeply increasing part of the TSFC curve - the answer might well be different on a 737-900ER.
BTW, I recall we had an issue years ago with one particular 757 operator that was having a rash of compressor related engine failures. Come to find out they were doing derated climbs, which tended to operate the engine in an RPM range that caused stator vane resonance and the vanes would fatigue and break
