View Full Version : Fixed Derate is a limitation for takeoff but Assumed Temperature is not?

29th Apr 2010, 02:07
In our B747 Flight Crew training manual for it states that the thrust setting parameter is not considred a limitation, takeoff speeds consider ground and in-air minimum control speeds with full takeoff thrust for the actual temperature. Does this mean that the VMCG is always based at actual temperature to provide you with the required protection and if we do happen to operate with a fixed derate that we will not be protected if the thrust is advanced from a Derate-2 Assumed Temp to Full Take Off at actual temperature? Also, what is the exact meaning of the fixed derate being a limitation for takeoff and how is it that Assume Temperature is not a limitation? I just can't seem to figure out the logic behind this!

galaxy flyer
29th Apr 2010, 02:19
A hardy perennial here.

When take-off power is based on reduced power, whatever the method for establishing the reduced power, all limiting speeds are based on full rated power. Vmcg and Vmca are the most limiting speeds.

Fixed derate, however, is like taking one engine off the plane and installing one of greater or lesser thrust rating. Vmcg and Vmca are based on the rated power being used, the full thrust or the fixed de-rate.

Because a reduced power take-off uses the full rated thrust for calculating the limiting speeds, in the event of engine failure, the pilot can advance the power to full rated thrust. In the case of a fixed de-rate, the speeds are based on the de-rated power, so the throttles may only be advanced to the de-rated power.

A search will find some excellent posts by the very knowledgeable Old Smokey and john_tullamarine


29th Apr 2010, 02:31
Shockwave, look at the contaminated runway charts for your aircraft, compare the weights for a Full Rated Takeoff and a Fixed Derated Takeoff, you will see that on certain runway lengths you can achieve a greater takeoff weight with Fixed Derated Thrust (less thrust). This is due to the difference in VMCG, if in this case you are using Derate 2 and you advance the thrust to Full Thrust, you will not be protected.


29th Apr 2010, 03:36
thanks GF and Mutt!!

galaxy flyer
29th Apr 2010, 04:30
And the very knowledgeable mutt, too.


1st May 2010, 18:05

Mutt, it will be nice if you can explain how the difference in the Vmcg can provide a gain in takeoff weight for a fixed derated takeoff compared to full rated takeoff on certain contaminated runways.

Thank you.

FE Hoppy
2nd May 2010, 00:22
How does a lower VMC give a higher TOW?

When examining the Field length we need to consider 3 things:
All engines operating accelerate and go;
One engine inoperative accelerate and stop;
One engine inoperative accelerate and go.

VMCG effects the OEI accel go case as it defines the minimum speed at which we can suffer the critical engine failure and continue the take off. With a very short field length or contamination which effectively shortens the ASDA we many be restricted to operating at low weights and their associated low speeds but when the v1 gets so low that it is restricted by VMCG this will now impose a limit.

To operate in the same low weight low speeds area without the same VMCG restriction we can de-rate. By reducing the Engine thrust we lower the VMCG thereby giving us a choice of lower v1 which in turn allows a slightly higher TOW.

Take a piece of paper and draw a graph with speed along the bottom increasing from left to right and weight up the side increasing from bottom to top.

Draw a diagonal line from bottom left to top right. Label this GO. at all weight and speeds to the right of that line you have enough performance to suffer the failure and go.

Draw a line from top left to bottom right. Label it stop. At all weights and speeds to the left of this line you are able to stop. Where the 2 lines cross defines the Field length limited MTOW and the v1 for that weight.

Now at the left side about a 1/4 way along the bottom draw a straight line up to meet the GO line. Label this VMCG.

You can see that VMCG has limited a certain area of weight and speed combinations. (everything to the left of the line but below the GO line).

Now think of the effect of contamination? It reduces your ability to stop. So think of this as moving the stop line to the left. The more it moves to the left the lower the MTOW becomes and the lower the V1 speed gets until eventually the stop line crosses the VMCG line.

We are now VMCG limited. You can see that if the VMCG line wasn't there we would cross the GO line at a higher weight and lower speed.

In this area if we derate we move the VMCG line to the left, opening an area of weights and speeds that were previously not usable.

That's how a lower VMC can give you a higher TOW. Or at least thats how I tell my kids it works.

2nd May 2010, 00:54
Excellent response FE-H.....

Aerotech, another one for you to ponder, why do we get a greater takeoff weight with a greater depth of contaminant?


Pugilistic Animus
2nd May 2010, 01:23
Wow! that's a tough one Mutt :\

FE Hoppy
2nd May 2010, 01:24
Thanks MUTT.

I was going to put a caveat in part of my response that would have answered your question but decided to leave it open.

2nd May 2010, 03:45
Aerotech, another one for you to ponder, why do we get a greater takeoff weight with a greater depth of contaminant?

Greater Retardation force in the event of an RTO?

2nd May 2010, 19:40

Or at least thats how I tell my kids it works.

Aircraft performance is a new field for me and for some reasons I am interested on it (at least for now).
When I ask questions, I am not challenging, quizzing any person. For me there is always a reason (a good reason) why I am asking questions even if it may seem for some persons as a challenge, a quiz, or my questions (some of my earlier one) seem complicated or even weird.

Since I was member in this forum, almost all of my posts were questions. I always try to post my questions in 2 forums (like pprune.org and airliners.net for the present, and before that it was pprune and air disasters) so I get more feedbacks.
Few months ago I posted some questions, one member in this forum asked to see my own post in airliners.net??? May be this is a good opportunity for me to make some clarifications, so there will be no misunderstanding in the future.
If I know another good website I may try to post there my questions also.

Even tough if a person may think he is knowledgeable (even in his field), in reality he is not unless he keeps learning. Human being mind is limited.
Here is a small example: Some major airlines were “kind of” part of the design team at Boeing for B 777 where their inputs were taken in consideration. B 777 was and still is a successful aircraft. Yet Boeing issued many service bulletins and letters despite that many smart engineers and bright minds designed this aircraft. This is perfectly normal even if we think we are super-smart, more knowledgeable …. and it applies to all aircrafts (all aircraft manufacturers, engine/APU manufacturers,..).

I saw some knowledgeable persons in this forum. They are humble and modest and they insisted on humbleness and modesty on some of their posts.
I guess humbleness and modesty will increase your knowledge because you have the courage to ask questions even if it is related to your field. If you are not humble or modest, you may make think of yourself that you are a “reference” in your field. This may prevent you from acquiring more knowledge :=.

I conclude with this saying:

“You will be a knowledgeable person as long as you keep learning; as soon as you think you are knowledgeable you become ignorant”.

I am grateful to all persons that posted their past posts and the future posts that may post in this forum and other forums. No offence to anyone.

Thank you.

3rd May 2010, 01:36
I think that you might find that PA was mocking me in jest, not you :)...... This forum would be extremely boring if people didn't come and ask "sensible" questions in a polite manner. So please don't stop.

Wiz, you are correct. Its always fun trying to explain to someone that if there is more snow, you can get a greater takeoff weight, its the total reverse of normal logic.....


FE Hoppy
3rd May 2010, 12:47

As you quoted me in your reply I assume it was directed at me.

You may need to turn your sensitivity down a little as the tag line of "thats how I tell my kids it works" is the caveat I have used for the last 20 years as my defence for oversimplification. The oversimplification is due to my limited knowledge.

Sorry if It offended you. Hope the rest of the post was of some use.:ugh:

Pugilistic Animus
3rd May 2010, 20:16
I think that you might find that PA was mocking me in jest, not you

No, no, not at all Mutt

I'm a silly man that's all :)

But how does greater contaminant depth increase MRTOW ---is it the retardation effect of slush? ---PA learns lots from here:ok: