B738 full takeoff power retrieve

[kuobin]

B738 pilot please,
If you use 24k derate for takeoff, during take off you notice strong tailwind en-counted ,can you get your full power(26K) back by place throttle to forward stop?
:confused:

[MTBUR]

Not a driver, but doing my TR:

Yah, autothrottles in the THR HLD mode can be changed by flight crew to whatever value they want. i.e. if the servos cut out and leave the levers at whatever is required for the derate, you can advance them further if need by in the THR HLD mode for takeoff.

[punk666]

The take off perf also considers tail wind if you used that table that is.
If your using a runway that has a headwind then all of a sudden it changes to a strong tail wind on take off I personally would consider that as windshear.

Boeing state that adjustments to get the target N1 should be made before 60kts. If you do decide to advance the throttles forward to get 26k your Vspeeds will be invalid.

[EW73]

Of course, this is exactly the difference between a derated takeoff and an assumed temperature reduced power takeoff, not the Vspeeds but the VMCG.

Boeing states quite clearly that a derated takeoff power is to be considered the maximum power for that takeoff, unlike the assumed temperature reduced power setting. But then again the assumed temperature method employs the maximum power figures for applicable VMCG calculations, whereas the derate setting uses the actual forecast derated power setting for VMCG calculations.

All this of course goes out the window if windshear is encountered, which sounds like what you are suggesting.

Cheers
EW73 :)

[delsur]

NEGATIVE!

Airplane configuration comes set up from boieng, if its 24k, thats the max you ll get out of it.

The only reduccion you can do, is by the assumed temp, which in that case you ll have less than 24k, and in this case you can changed to full power being 24kmax.

[STBYRUD]

True, firewalling will only give you the maximum rated thrust.

[blablabla]

The question he is asking though is if you have a 26k rated Aircraft but you select a derate of 24k in the fmc prior to take-off what power will you get if you firewall the thrust ?? Will you get full 24k as selected in the fmc or will you get full rated 26k ?

[ImbracableCrunk]

This question makes its rounds every once in a while.

One of my manuals states:

The maximum rated thrust is available at the forward stop. The EEC limits the maximum thrust according to the aircraft model. The 737-800 is designated as a CFM56-7B27 rating.

The full rated takeoff thrust for the installed engine is available at a thrust
lever position less than the forward stop. Fixed or assumed temperature
derated takeoff thrust ratings are set at thrust lever positions less than full
rated takeoff.

From reading this, 27k is available at the stops. A derate (eg. 24k) will be at a position at less than the stops.

If the plane came from Boeing with 24k engines, 24k will be all you can get at the stops. If you select a 22k derate, you can still get the 24k at the stops, but you might VMCA yourself into the ground.

Any other ideas?

[STBYRUD]

Yes, whatever is set in hardware at the pins will be the maximum you can get, if the FMC allows you to derate further this will not matter for the 'firewall thrust' available.

[Denti]

If the plane came from Boeing with 24k engines, 24k will be all you can get at the stops. If you select a 22k derate, you can still get the 24k at the stops, but you might VMCA yourself into the ground.


Indeed, and that is the reason why boeing advises against advancing the thrust levers if you have used derated thrust.

The only reduccion you can do, is by the assumed temp, which in that case you ll have less than 24k, and in this case you can changed to full power being 24kmax.

Not true. Usually you have three thrust settings available, full thrust, derate 1 and 2, in the case of the 24k thrust rated engines that is 24, 22, and 20k. If you use those you have a different set of performance data including different Vmca and Vmcg for each thrust settings. Additionally you can use assumed temperature on each of the reduced thrust settings as well.

[Capt Chambo]

Not true. Usually you have three thrust settings available, full thrust, derate 1 and 2,

Careful in dealing with absolutes. The -NGs I fly now do not have fixed derates, ones I have flown for other operators did. It's down to what the customer orders.

[RRAAMJET]

Ok...bits n pieces of the picture on this thread

As stated previously:
Let's assume you're in the USA on a -800:
You're going to have 27k bump engines for SNA ops
Warranty invalid at that rating for any other occasion.
"THR HLD" .... 84 kts :
Push thrust all the way fwd EEC's will control engines to first limiting parameter. You may not get 27k, may be EGT limited.

[Avenger]

If you advance the thrust levers to the forward stops you should get full power, forget warranties, vmcg etc, we are discussing power.
Now, if the derate was in place this would be considered a limitation but the EEC would not stop the full power as you are manually pushing the thrust levers.But.. added for info:

" The EGT limit must be obesrved by the crew because the EEC does not provide EGT redline exceedance protection"

[Denti]

@Capt Chambo, thats why i put that "usually" in there. Having the derate thingy is the standard boeing configuration, you have to specially select an option not to have it. Same as you have to specially select an option not to have the aux battery or the lower low fuel warning threshold.

By the way, the 27k thrust rating used to be just our normal full thrust rating, not even considered a bump option. However we choose to rate all engines and planes down to 26k since we never really had to use 27k and it is a hell of a lot cheaper in maintenance that way. 98% of all take offs on the -800s are done at 22k anyway, on the -700 its like 99% at 20k (max rate 24k).

[ImbracableCrunk]

If you advance the thrust levers to the forward stops you should get full power, forget warranties, vmcg etc, we are discussing power.
Now, if the derate was in place this would be considered a limitation but the EEC would not stop the full power as you are manually pushing the thrust levers.I thought if the EEC is in normal mode, then you are stuck with whatever derate you selected in the FMC.

Hence, you don't use ATM or derate if you are taking of in possible windshear.

Are you saying otherwise?

EDIT: Yep, I just contradicted myself. That's what I get for writing before my coffee. You get what Boeing gave you at the firewall.

[Avenger]

Yes I am saying otherwise.

ATM thrust reduction if windshear is forecast is another topic.

EEC normal and alternate modes are not applicable either as we are not using the A/T system , thrust levers manually held in fwd stop, in the situation described.

See Systems manual, Chap 12 7-20.