B737NG Thrust Rating Question

[jetblues]

So you are sitting at the end of the runway in your shiny new NG with an assumed temperature of 38 degrees and derated 20k (22k) available.

You stand the thrust levers up and set 40 % N1, then you hit TOGA and start rolling down the runway.

At what point do you have 22k thrust delivered to the engines again ? Is it when you pass 15000ft on climb, is it on approach ? Is it never unless selected ?

[Shaka Zulu]

On some older -300's we have the possibility to select manually on the FMC/CDU to either go with 20 or 22k thrust in case of performance problems. but I don't think you can reselect it in flight.
On our NG's the Engine itself is rated for 27k thrust but by putting some chips in the engine, the beast assumes just 20k thrust, hence all perf manuals are all for 20k thrust aswell.
this to increase engine life cycle and most of the time the 73 climbs like a rocket anyway on 2 x 20k.

pressing TOGA just sets the Assumed Temp Thrust over a 20k derated engine

[Herc Jerk]

The answer to the original question is at lift off.

At weight off wheels you'll notice the take off ref page swap to the climb page. At this point there is no further reference to derate (ie your 20k derated engines are 22k again).The thrust levers are in THR HLD (ARM at 800'), then the next setting the A/T sets is N1 at 1500' which will be based on your chosen climb(climb,-1,-2) setting. This may even mean increasing thrust if a high assumed temp was used for TOFF. It is this reduced climb which has the upper limit of 15,000'.

Hope this helps

HJ

[IBTheseus]

On lift off, you go from TO to climb, and power is now dependent on the climb thrust you have set on the N1 LIMIT page. The actual automatic setting of climb power is dependent on when you select climb power to take effect on the TAKE OFF REF p2. This is not less than 800ft, (Auto throttle goes to ARM) and not more than (from memory) 9999ft

Setting CLB, instead of CLB-1, or CLB-2 would now give you max rated climb power, in this case 22K. Full rated climb power is indicated by CLB N1.

[Shaka Zulu]

If the engines are derated permanently to improve engine life cycle then any FMC mode is based on the 20ks, climb 1 & 2 are then reductions of climb power over 20k.No change to the original de rate of the engine itself!

[DDG]

If your engine has been de-rated to 20k it will never deliver more than 20k as the EEC will limit N1 to provide 20K.
The only way to get more out of the engine is to up-rate it.
To change the thrust rating of an engine the EEC/HMU/FUEL PUMP/ENGINE ID PLUG/LOADABLE SOFTWARE all need to modified and or changed depending on mod status and part number to change the thrust output of the engine.

[Shaka Zulu]

That's indeed what I've been trying to say :)
The original question is not quite right though, depends what he is actually asking for the right answer.
But indeed a DeRated engine cannot be changed into a Full Rated engine in flight. (not to my knowledge anyway)

[capt.topgun]

Hi guys,

While we are at it, u might as well clarify 1 thing. We ( our airline )have two different types of engines installed on the NGs(-700). One with 22K eng. and FMC derates available( TYPE 1) and the other which have been permanently de-rated to 20K with no FMC derates available( TYPE 2). Now, assumed temp. does reduce the T/O thrust on the TYPE 1 on 22K if no FMC derate is selected, right ? If an FMC derate is also selcted to say 20K on TYPE 1, when is 22K available ? TYPE 2 is pretty simple with only Assumed Temp available for Red. Th. T/O. :ok:

Thanks

[john_tullamarine]

One must keep in mind that derate and flex should not be confused.

(a) FMC selected derate, in effect, is the same as hanging a different engine on the wing for the particular flight. The derate thrust level becomes the maximum thrust level available for scheduling during that flight

(b) flex thrust (typically using the assumed temperature technique) is a means of limiting thrust for a particular takeoff, but with the option of increasing thrust (if the crew so desire) up to, but not exceeding, the present rated thrust (which may be a derate rating)

(c) overboosting beyond the takeoff rating (whether such rating be the maximum or a derate setting) can compromise things such as speed schedules (Vmca and Vmcg considerations)

[jetblues]

Appreciate all the responses.

Lets clarify a little. I am referring to say a B738 with 26k engines however you select 24k derate manually on the ground, and ALSO select an assumed temp of say 36 deg.

Whilst I accept the responses, I NEED technical references to back up your claims. Do you have a Vol 2 or FCTM reference ?

[Dehavillanddriver]

FMC selected De-rate is called T/O de-rate - this applies ONLY for the takeoff phase of the flight ie up to 1500' at which point CLB thrust takes over - assuming all engines operating etc. (Having read other responses about the change over at lift off - I agree, however the actual thrust setting doesn't change until 1500 when N1 annunciates...if you get my drift!)

At 1500 ft when N1 annunciates on the FMA the takeoff de-rate is effectively removed and the aircraft sets the climb thrust selected - if you are using a de-rate you would normally get CLB-1 or CLB-2 so that you don't get a thrust increase when it goes to N1 mode.

Regardless of the thrust rating indicated on the FMC ident page if you "firewall' thr throttles you will get 24k for a 700 and 27k for an -800 (VOL 2 7.20.4)

T/O de-rate is considered a limitation for that TAKEOFF - ie you shouldn't push the levers forward because of the reasons stated by John Tullamarine.

[IBTheseus]

With my Co's Vol 2, 7.20.4, dated March 29, 2004, it does not specifically mention that for the 800, you will get 27K.

Are you referring to the section titled Structural Limit Protection, where the EEC provides N1 and N2 redline over speed protection?

The inferrence here is that these limits are the max N limiting power the engine could produce which is 27K (assuming not EGT limited).

If the airline uses an engine limit of 26K, then could maintenance allow the EGT margin to deteriorate further to a 26K engine, and not a more restrictive 27K engine?

[Dehavillanddriver]

The bit I talk about is in EEC Normal Mode

It says that full rated thrust is achieved at a thrust lever position less than the forward stop - that would be the thrust rating as indicated on the FMC ident page.

If you jam the levers full forward you get a standard reserve thrust rating

for the 700 CFM56-7B24 rating

for the 800 CFM 56-7B27 rating