737-800 reduce thrust take-off???
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
Quoth checkers:
Very true.
However - the greatest economic benefit (in parts life) comes from the first few percent (say, 10% thrust, or 4% N1 IIRC). Merely going for the greatest reduction - the highest FLEX temp in other words - may turn out to be false economy because of longer TO times, and higher total fuel burn. Not sure if anyone's really pursued in-depth analysis of this issue; a good subject for someone's thesis perchance.
There is a the consideration that, with at least the FAA, the maximum flex/thrust reduction you can have from full-rated is 25% (thrust, not N1).
So, if you compute your takeoff performance with a 22K full-rated takeoff and then apply a maximum 25% reduction, then you can have a deeper effective thrust reduction than if you used a 26K rating.
So, if you compute your takeoff performance with a 22K full-rated takeoff and then apply a maximum 25% reduction, then you can have a deeper effective thrust reduction than if you used a 26K rating.
However - the greatest economic benefit (in parts life) comes from the first few percent (say, 10% thrust, or 4% N1 IIRC). Merely going for the greatest reduction - the highest FLEX temp in other words - may turn out to be false economy because of longer TO times, and higher total fuel burn. Not sure if anyone's really pursued in-depth analysis of this issue; a good subject for someone's thesis perchance.
Yes they have!
Pratt and Whitney did a study years ago when they intoduced that greatest peice of junk known as the JT9. (as opposed the the fantastic JT8)
Although an individual takeoff at full thrust will save fuel on that takeoff, the fuel burn will be higher over an average life as the deterioration in engine performance increases.
Cannot remember the numbers exactly but .14% fuel reduction per 1000 cycles was the number mentioned.
Although an individual takeoff at full thrust will save fuel on that takeoff, the fuel burn will be higher over an average life as the deterioration in engine performance increases.
Cannot remember the numbers exactly but .14% fuel reduction per 1000 cycles was the number mentioned.
Join Date: Sep 2003
Location: here and there but mostly lgw
Posts: 166
Likes: 0
Received 0 Likes
on
0 Posts
I may be wrong, its a while since I flew the 800. But surely a second push on the toga during the takeoff ( 80 kts to 800')does sweet fa to the thrust. ground logic, you are in takeoff , not go around. It may adjust the carrot or give you the fd's back after 90 kts but Im sure thats about it. The thrust levers are in hold at 80 knots until 800 feet , then arm I believe.
After 800 feet and arm, i dont know . depends if take off mode (vis vis thrust) is a speed/pitch mode or an n1 mode or not. Cant remember. That is to say it will stay armed (pitch/speed) until another mode (n1/a/p/lvlchange/alt cap etc) is chosen or happens. pretty common boeing thing. 75/76 has no thrust mode until chosen after arm fma ie alt cap does not default to an epr/n1 mode as i beleive it does on the 800 , hence the low level early cap issues and runaway speed due to no thrust mode control.
oh I dont know, back to the wine.
After 800 feet and arm, i dont know . depends if take off mode (vis vis thrust) is a speed/pitch mode or an n1 mode or not. Cant remember. That is to say it will stay armed (pitch/speed) until another mode (n1/a/p/lvlchange/alt cap etc) is chosen or happens. pretty common boeing thing. 75/76 has no thrust mode until chosen after arm fma ie alt cap does not default to an epr/n1 mode as i beleive it does on the 800 , hence the low level early cap issues and runaway speed due to no thrust mode control.
oh I dont know, back to the wine.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
Although an individual takeoff at full thrust will save fuel on that takeoff, the fuel burn will be higher over an average life as the deterioration in engine performance increases.
The scenario I proposed is somewhat different: I'm comparing 10% thrust reduction to 20 or 25% reduction. The deterioration effects are quite non-linear; it's not evident to me that the additional performance retention of a higher flex temp will pay for the added fuel burn due to slower climbout.
