The well known Derate/Assumed issue
 

Hi everyone! Im new here and yet another newbie asking about Saving Fuel and Engine wear, but this time not to understanding what or how it works but yes to ask you as a community members where to get proper a handy method to obtain Assumed and Derated setting for a Boeing 737-800W, I had take a look at the FCOM but only found Performance charts involving QNH, OAT,Flap setting,Engine Bleed.. but I cant find MTOW relative charts, anyone here can give me a ride with that?

Thanks in advance.

Lets assume aint got on board the wonderful Onboard Performance Computer :=

Runway weight charts?

For ATM we used to use runway weight charts, a long long time ago. Even had a nifty little chart on our laminated checklist to calculate the assumed temperature manually for the -300.

However, i do not think that is practicable for the combination of derate and ATM as there are simply too many possible combinations, especially if you consider the five different take off flap settings. Only did that using software.

MTOW Vs Runway Length

From his location I'd assume the OP is a home desktop sim driver.

PMDG Simulations

B73,


Using reduced thrust for take off does NOT save fuel.


Think about it.

With most operators I've flown for their local XAA allows derates & assumed temp. I then flew under another EU XAA that did not allow this, but allowed only the assumed temp reduction based on full thrust. I thought this very simple. I then wondered why bother with the derate and assumed temp at all. It requires more paper charts for every derate, or more data in the I-pads. It has the possibility of errors in choosing the wrong derate table.
Does anyone have definitive performance answer why using derates & assumed temp is better than just assumed temp from full thrust? I know there are operators with different max thrust, e.g. 22K, 24K, 26K, 27K. It would not matter as max thrust would be the datum. Are there some operators who have mixed max-thrust fleets. If so, would it not be simpler if using ass-temp from max thrust?

Perhaps the reduced thrust puts the engines in lower SFC regime?

ie, a lower ratio of pounds of fuel consumed to pounds of thrust produced?

In that case, would fuel not be saved?

why bother with the derate and assumed temp at all


(a) flex is limited by regulation to a percentage of rated. Ergo, derate plus flex provides for a deeper level of thrust reduction.


(b) derate may provide advantages by reducing Vmc and permit operations from shorter runways by virtue of a lower V1.


(c) especially with bigger (more expensive) engines, maximal use of derate may offer engine maintenance cost advantages to an operator.


I'm sure the experts in the engines arena will have additional reasons to add.

Yup, i guess those are the main reasons. Combining derate and ATM allows reduction of quite a bit more than the usual 30% that is available with ATM alone.

And of course, derate has some merits, especially on contaminated runways where it might allow to carry a higher weight of the field than a full thrust (no ATM allowed) would be able to. The lower VMCG and thus lower V1 make it possible. It has some problems to as it is usually not allowed to go to full unreduced thrust in case of an engine failure as the resulting thrust might lead to loss of control.

Couple of comments -


(a) max flex is 25% - see, for example, AC 25-13.


(b) no particular problems considering reduced Vmcg as the derate AFM data is comprehensive for the derate. In effect, the aircraft has a de facto replacement, lower thrust, engine installed for the takeoff.

Hk37


A reduced thrust take off uses more fuel as that portion of the flight will take longer.


Therefore your overall climb to altitude will be longer burning more fuel.


The shorter time to altitude achieved by using max thrust gets you to cruise level where you burn the least as quickly as possible.

thats not necessairly truth, you can take off assuming and or derating and set the "default" CLB thrust (at acceleration altitude) and achieve cruize level with "normal" N1 and save fuel and wear.

It is necessarily the truth.

A derated takeoff may in fact save fuel - the takeoff takes longer but full rated Takeoff is typically the worst TSFC the engine ever runs at. So with a derated takeoff you're getting more thrust per unit of fuel.

Derated climb is a different story - while I suppose there are some specific circumstances where a derated climb might save a bit of fuel, as a general rule stilton is correct - getting to cruise altitude more quickly saves fuel.

Several years ago, we had a (new) 737-300 operator in Asia who was complaining that their CFM first run engines were running out of EGT margin after only 3,000-4,000 hours (typical was in the 12,000-15,000 hour range). Turns out they were flying short hops but running full rated balls to the wall every single takeoff :rolleyes:

Its all about time to cruise altitude.


Using the maximum power allowed for take off and climb gets you to altitude quicker where you burn less fuel.


You may be pushing more fuel through the engine for the take off and climb but you will more than compensate for the lower fuel burn achieved arriving at cruise level earlier.

That statement wouldnt comply the save engine wear point of ATM

Mr. TDR, could you find some time later perhaps and come back to us on this one? Pretty please, for I've been told otherwise ... on more than one occasion. :sad:

thanks, FD.

Using an assumed temp does cost you more fuel for that flight versus using full thrust and getting to altitude quicker however the engine wear is significantly reduced saving on maintenance.

Over the life of the engine running it with max reduced thrust possible saves fuel as SFC is directly related to EGT which in turn is related to engine wear. The greater the engine wear and stress (high thrust settings) the higher the EGT rises over time and therefore the more fuel the engine will use all the time.

Therefore using reduced thrust where possible does in fact save fuel in the long run.

This is all contained in a CFM guidance document.

Using an assumed temp does cost you more fuel for that flight versus using full thrust and getting to altitude quicker however the engine wear is significantly reduced saving on maintenance.

Why not just use max reduction for takeoff and then full CLB after flaps up, or after flap retraction is initiated? Best of both worlds.