B 737 setting thrust

[Visionman]

Hi guys! Which is the reason for heaving the target N1 set by 60 knots? Why not by 50 or by 70 knots?
Thanks!

[framer]

I'll take a guess.....something to do with keeping the performance data valid.
ie the data assumes a certain thrust, it obviously can't come on instantaneously, but at the same time if you brought it up so slow that you only reached planned thrust at Vr then you would use more runway. My guess is that they calculate the a/c accelerating according to reaching planned thrust at 60kts.
As an aside, sometimes when there is a heap of wind on the nose the auto throttle goes into 'thrust hold' before it gets to the desired thrust setting so you have to keep pushing it up manually. Happpy to be corrected by the 'performance gods'

[Visionman]

Thanks a lot! I was thinking of something like that too and you just confirmed me. Thanks!

[InSoMnIaC]

I thought that performance calculations considered takeoff thrust at Brakes release and not at any particular speed.

i think the reason is more to do with RTO's comment

[framer]

I would be interested to hear from someone who knows how the performance/thrust setting is calculated. If it is as Insomniac says then most F limited take-off V1's would be invalid.

[Old Smokey]

"i think the reason is more to do with RTO's comment"

It's ALL to do with RTO's comment!

Regards,

Old Smokey

[framer]

It's ALL to do with RTO's comment!

Cheers Smokey. If that is the case though, then why do Boeing still say you have to have thrust set by 60kts when the thrust hold kicks in at 84kts? I think there has to be some performance issue in here somewhere....wouldn't be the first time if I was wrong though

[BOAC]

framer - I suspect it came under the heading of 'not important' and there is really no need to change it?

[Nightrider]

framer, according to Mr. Boeing the thrust required is set when inside 1% of the calculated thrust. Due to acceleration and the associated ram effect a "fine tuning" is required. The system gets a chance to do so up to about 84 kn.....

[Centaurus]

I thought that performance calculations considered takeoff thrust at Brakes release and not at any particular speed

From a 737 FCTM: "A rolling take off procedure is recommended for setting take off thrust. It expedites the take off and reduces the risk of foreign object damage or engine surge/stall due to a tailwind or crosswind. Flight test and analysis prove that the change in take off roll distance due to the rolling take off procedure is negligable when ccompared to a standing take off. Brakes are not normally held with thrust above idle unless a static run-up in icing concditions is required".

Full power on the brakes is not used to calculate take off performance in most jet airliners except on some piston engine aircraft.

[300-600]

Not sure if they were all like this but the engines on our old classics were very slow to spool up. On a strong headwind day you would be up to 64 knots CAS in several seconds - well before the automatics could set takeoff thrust.

[captjns]

A throw back to the days of youre before the miracle of the EEC. Primarily for engines that used EPR as the primary thrust setting (ala P&W and Rolls) versus N1 as ala GE engines.