Technically speaking, the sentence of my previous post is actually true: if conditions permit an aircraft to take off at MTOW, it's actually taking off at the largest max permissible t/o weight, and hence it does not make sense to use flexible thrust: when the actual T/O weight is equal to the max permissible T/O weight, the assumed temperature coincides with the OAT and therefore there is no thrust reduction.

Wrong again. Yes it always makes sense to derate engines if only conditions allow it: derate = less stress on engines = save $$ on maintenanceInstead of referring to Airbuses terminology and procedures I suggest you visit PMDG forum where there is plenty of discussion about derating 747-400. There are even links to PMDG wiki pages where you can examine for yourself the actual derate tables for Virgin Atlantic's 747-400 prepared for a few airports/runways. And yes, you can derate engines even if you take off at MTOW (Boeing's MTOW = maximum certificated takeoff weight = about 378000 kg for this VA 747).Hang this Mike's sentence above your bed - he is absolutely right:As long as you have enough runway, you can derate the aircraft to your heart's content no matter HOW HEAVY you are.

Actually that is one you'll get engineers arguing over, as far as wear on the engine goes: If you do a derated climb, yes you do have the engine operating at a lower temperature, but your climb is slower so you could theoretically put more wear on the engine with a derate, than if you were to use more power - but for less time - by making a quicker climb, which of course exposes the engine components higher temperatures for a shorter overall time. Most FMC-controlled economy climbs actually up the power setting as the aircraft climbs automatically incidentally. I think U10 on the 737 puts the power up to full at 15,000 feet if I recall correctly, for just that reason, and because the air density is dropping too, obviously.Start that conversation with two engine technicians present, and they will bang on about it for hours LOLOh, and just to clarify, I didn't mean that the problems ingesting hot air can cause are the reason for derated take-offs, just that they are one of the things to consider. More a point of interest than an explanation of it.Al

If you do a derated climb, yes you do have the engine operating at a lower temperature, but your climb is slower so you could theoretically put more wear on the engine with a derate, than if you were to use more power - but for less time - by making a quicker climb.

Al, the economic equation is pretty clear in this case - derating (by how much is another matter) does save airlines money. Derating just by 5% gives you significant reduction in very expensive engine maintenance. I was reading in Boeing's literature that even 2% of thrust reduction on takeoff gives you measurable savings. It is this final 2% of engine power that has such disproportional cost.

Oh I don't disagree with you, just pointing out that you can kick off arguments from engineers on the relative merits of such things.There is one thing that you can do to shut people up on arguments of that nature beyond the relative merits of thermal properties of components, and that is to point out that a derate can also save the airline money when it comes to noise abatement. At my local airport - EGCC - if the noise abatement monitoring stations pick up more than 98PNdB between certain hours, the airline responsible is fined one 'slot' for each infringement. And losing a departure slot could be very expensive indeed, which is another reason to fly those SIDs properly of course!Al

Wrong again. Yes it always makes sense to derate engines if only conditions allow it: derate = less stress on engines = save $$ on maintenanceInstead of referring to Airbuses terminology and procedures I suggest you visit PMDG forum where there is plenty of discussion about derating 747-400. There are even links to PMDG wiki pages where you can examine for yourself the actual derate tables for Virgin Atlantic's 747-400 prepared for a few airports/runways. And yes, you can derate engines even if you take off at MTOW (Boeing's MTOW = maximum certificated takeoff weight = about 378000 kg for this VA 747).Hang this Mike's sentence above your bed - he is absolutely right:As long as you have enough runway, you can derate the aircraft to your heart's content no matter HOW HEAVY you are.

Ok, I think we have established that in the case of Airbus, reduced thrust (flex takeoff thrust) using assumed temperature can only be used when the T/O weight is lower than the max permissible T/O weight.Regarding Boeing, I quote from the "B747-400 FLIGHT PLANNING AND PERFORMANCE MANUAL":"Derated PerformanceTakeoff and landing performance adjustments areprovided for Derate 1 (TO1) and Derate 2 (TO2)operation. Weight adjustments are applied to full ratedtakeoff limit, takeoff climb, takeoff obstacle, takeofftire speed and landing climb limit weights. Each fullrated thrust limiting weight must be adjusted for derateoperation and then the most restrictive weight used forplanning."Marco

"Actually that is one you'll get engineers arguing over, as far as wear on the engine goes: If you do a derated climb, yes you do have the engine operating at a lower temperature, but your climb is slower so you could theoretically put more wear on the engine with a derate, than if you were to use more power:NO! NO! NO! NO! NO! :( Okay look. get "temperature" out of your head. No "Temperature"! Pretend that to derate an engine you put in an certain N1 value. Pretend instead of temperature you enter: 80%. The engines will now only produce 80% of IT'S RATED THRUST....therefore it's "DERATED" At max thrust the engines can only produce 80% of thrust. Why do this? Because 80% is all you need to take off, climb out and transition to climb thrust. You don't need 100%.Now, take that same concept and translate it to temperature. The ONLY reason you are entering a temperature as a value is because it can be translated to performace for a given atmospheric condition, but it DOES NOT CHANGE the temperature of the engine. Your climb is NOT slower because you don't need 100% thrust to climb at 6000 fps! No engineers will argue anything. When you derate an engine, it produces less thrust, therefore works less, therefore, there is less wear and tear, period, end of story. The only engineer to argue this are those in this forum who STILL don't understand the derate concept. There are no nuances, this is an absolute concept used by every single airline in the world.It's really not a hard concept to grasp.

Regarding Boeing, I quote from the "B747-400 FLIGHT PLANNING AND PERFORMANCE MANUAL":

Boeing's derate 1 or 2 is different from the temperature derate. Again, I suggest you study the relevant literature which explains how derate is done and the closely related topic of the balanced field takeoff. If you don't understand the latter you will never understand the key concepts behind derate. I am afraid you pull some quotes from documentation without even understanding what they mean.

Boeing's derate 1 or 2 is different from the temperature derate. Again, I suggest you study the relevant literature which explains how derate is done and the closely related topic of the balanced field takeoff. If you don't understand the latter you will never understand the key concepts behind derate. I am afraid you pull some quotes from documentation without even understanding what they mean.

Hi Michal,I think the problem was caused by the fact that I was looking at it from the Airbus side and not from the Boeing side.So I should rephrase my former sentence this way:"For the Airbuses, the use of assumed temperature and FLEX TAKEOFF THRUST is only allowed if the takeoff weight is less than the maximum permissible takeoff weight at the actual conditions".Marco