737NG "Please carry out full thrust TO"

[Mr.Buzzy]

Hi all,
I have a question relating to 737NG operation, hopefully someone can help.

If we are requested to fly a "full thrust takeoff" by engineering, we perform the takeoff without fixed or ATM derates.

Does the autothrottle system always advance the thrust levers to the FMC calculated N1 takeoff limit or (if the OAT differs from FMC entered data) will the autothrottle advance only to an N1 to give rated thrust? Would you ever expect to see a discrepancy between "needles and bugs" during a full noise takeoff with autothrottle?

ie. Requested to fly "full thrust TO". NO fixed or ATM derates entered into FMC.
An OAT of 12 is entered into FMC but actual temp at time of takeoff is actually say 8 degrees.

Many thanks for your help.

bbbzbzbz

[rubik101]

If you are required to do a Full Power T/O then why would you enter 12C if the actual OAT is 8C?
If you did then you would get a reduced T/O thrust setting.
To get Full power, do as you are asked and enter the actual OAT.

[OPEN DES]

I am no expert on any boeing. But I would suppose that as long as you enter a temperature at or below Tref (flat-rate temp) you should get full rated TO thrust. Tref for a typical CFM engine is ISA+30 (on Airbus at least).
But why would you try to fool the computers if a full rated TO is required?

[Mr.Buzzy]

Not trying to fool anyone.

A situation occured recently where a crew reported to maintenance that the engines "did not make full power" during a rated takeoff. ie. their observation was that the N1 needles did not come up to the N1 bugs during an autothrottle rated takeoff; albeit by about 1.2%.

The question was put to me by the engineer presented with the "problem"

The engineer's suggestion was that if a change in temp occurred between entering FMC temperature and takeoff, without the crew updating (it can change pretty quickly here) then that could have been the cause of the discrepancy.

My question is. Does the autothrottle always set FMC generated figures during a rated thrust takeoff or will it recieve input from perhaps EEC to limit the N1 to a possibly lower value yet still delivering rated thrust?
Has anyone seen this before?

[john_tullamarine]

the engines "did not make full power"

perhaps I'm a tad out of date .. but ... if they observed it ... why didn't they reset to the needles manually ?

[Mr.Buzzy]

Hi John,
not sure what the crew actions were.
Though we are only talking small differences, the question has to be asked whether manually adjusting would then give a figure slightly above rated thrust in this case? I know it matters very little but it's the reasoning behind that interests me now.
:ok:

[Cough]

My 2 penneth worth, albeit from a 737 classic driver...

The engine is flat rated below 30C, but that doesn't mean that the N1 values are constant below this temperature. In my experience the rated N1 value falls as the temperature falls (i.e. less fan rotation for a greater air density..).

Mr Buzzy, my suggestion would be to look at the N1 value for rated thrust at 12C and N1 value at 8C and see whether that corresponds to any shortfall that you have had reported.

[FCS Explorer]

... there are temp sensors IN the engine, so just in case the values the crew entered are wrong the thing doesn't overboost....?

[Rainboe]

Buzzy, for a full thrust take-off, you don't need to enter any temperature into the performance. Leaving it blank is the same as entering current temperature. The engine should deliver full rated thrust. During roll, the A/T goes into 'Throttle Hold'. At this point, if you are not getting max N1, you can advance the throttle up to EGT limit to try and get it. This was a frequent procedure on 747 Classics where very often shortfalls of power occurred, sometimes on all engines, and each engine had to be tweaked up to max EGT to get what power could be achieved. The figures that stick in my mind even today are total EPR .02 per engine, .04 per side, or .06 across all four engines. If you were unable to achieve this power requirement, the F/E would call 'stop!'. It used to happen occasionally- all to do with sticky pressure relief/bleed valves.

In this case, the engineer could be wondering why a shortfall existed if the thrust lever was not advanced up to EGT limit?