View Full Version : De-rate & assumed Temp takeoff thrust

6th Feb 2004, 00:54
I apologise if this has been thrashed out previously, but I missed it; and having recently changed performance tables a question has been generated, not least by me but some students also.
There are also different rules on their use by different national authorities.

26K engines with fixed de-rates of 24K & 22K.

Not only is a fixed de-rate used, but then an assumed temp from that de-rate.

What is the benefit of this rather than using assumed temp from the top power setting of 26k? There must be an assumed temp from 26K = 24K or 22K. How low do you want to go? Is it that the 25% or CLB rule (ref 26K)will come into play before reaching the lowest possible thrust setting?
I can understand that, on some runways, the a/c performance can be satisfied by the smaller 22K engine, (the larger 26K being there for runway flexibility) and then derated. However, is there an assumed temp from 26K that equals 22k. If so, this would require only one piece of paper per falp setting instead of 3. We have 3 pages for every dry flap setting, then another 3 for wet, and then another 6 if there might be another flap option. 12 pages per runway. Surely 1 page at 26K and using only assumed temp would suffice. In UK the fixed de-rate + assumed temp is allowed, but in some continental countries it is not?? In the latter it is only assumed temp from full power.

The former seems cumbersome, the latter simple and perhaps therefore less likely to error. Certainly saves on paper and space in the books.

Equally puzzling is that under certain conditions reduced thrust is not allowed: i.e. systems failure or contamination, but a fixed de-rate with no assumed temp is allowed. In other words, you've got a 26K engine, you've got some restricting problems, but it's OK to downsize the engine to 22K if RTOW tables allow it. Capt's discretion is still in force.


6th Feb 2004, 02:18
First time that i have heard of someone using all 3 derate options, seems extremely cumbersome!

Remember that the 25% reduction restriction on assumed temperature applies to the rating in use and not the installed engine. This way you can select 22k and then reduce this by another 25%, this will give you approx a 40% thrust reduction. There is a big question about the financial savings associated with deep derates, how much do you actually save by having the 3rd derate option, when you compare the savings between 30-40%, is this worth increasing the crew workload and the margin of error?

One of the benefits of using the derated engine is the lower VMCG, on contaminated runways this might actually increase your takeoff weight. However this only works for a very narrow band of runway lengths where the aircraft is VMCG limited. Therefore the advantages of using the 3rd derate are extremely limited.

Personally i would recommend getting rid of the 22k derate.


Noise Unit
10th Feb 2004, 17:06

Best source of information on this is the Boeing TakeOff Safety Training Aid or Boeing's FlightOps course (I assume Airbus do something similar)

Need to be careful with definitions here. Here are mine (i'm not saying that they are the global definitions or exhausative, just mine) with some thought. Hope this adds to the discussion.

Derate - 'buttons' on flight deck to reduce thrust as required for an individual take-off to reduce maintenance costs, to lower V1 when VMCG limited (low weight or slippery runways). Flight Manual has separate TO charts as they are certificated ratings.

Assumed Temp/Flex TO - flight deck option to reduce TO N1 or EPR to that seen at a higher temperature for an individual take-off. Used to reduce maintenance costs, particularly when Derate not available or was not purchased. No benefit can be taken for reduced V1 as the pilot may opt to go for full TO rating at any point.

Re-rate - Not a flight deck option. Engine is re-rated to a lower thrust on a more permanent basis by changes to the FADEC. Engines may be rerated to a lower thrust setting when engine temperature margins are becoming small at the higher rating to extend on-wing life. It can also further reduce maintenance costs as the maximum thrust available is limited and the derates or ATM have a lower starting point - lower thrust can be used at low weight operations where max derate/ATM is being used with higher rating.

14th Feb 2004, 12:38
rat 5,

we operate our b777 with 3 derates as well TO, TO1 & TO2. TO1 is 10% less than TO and TO2 20% less than TO. It has all to do with economics. as mentioned by mutt, a 25% deduction from TO2 (ie. 20% reduced) is actually 40% reduction from full TO as opposed to only 25% had no derate options were given.

as Vmcg is govern by your derated NOT your reduced thrust, therefore it can be more favourable in a contaminated runway condition as less yaw is produced if an eng fails (unlike a reduced thrust situation still allows one the option to open up the power to full TO if an eng fails thus causing more yaw and increasing Vmcg)


14th Feb 2004, 13:29
Seems a bit pointless (if we are talking about hot end wear and tear) to de-rate then on automatic selection of climb thrust shortly after, the thrust is increased by a few N1. Or am I missing something here?

15th Feb 2004, 04:39
I think you'll find that the climb is also derated to prevent such a scenario.

15th Feb 2004, 08:01
Like SuperRanger we operate our 747-400 Rollers with three derate settings, TO TO1 and TO2 and then use an assumed temperature on top of that. We only changed to using TO2 regularly a couple of years ago. Untiil then we considered it too complex and as one manager said at the time, "What kind of pilot would take off using less than climb thrust?"

Economics overcame airmanship and now we use TO2 with up to a 61 degree C assumed temperature. We are seeing a lot more of the far end of runways these days! And yes, at climb thrust selection the thrust does increase from around 1.42 to 1.46 EPR.

In the engine failure case it really staggers in the third and fourth segment and it takes ages to get to clean configuration, watching the hills go by...

Pilots don't like it but accountants do so its a done deal. Anyone else use TO2 on a 747?

15th Feb 2004, 08:57
Airbus frequently takeoff below takeoff thrust; it is no big deal. At acceleration altitude, the thrust goes up a little instead of down. It is of no consequence to the pilot.

For the failure case, the pilot always has the option of increasing to rated thrust. In the simulator, I frequently increase the thrust after an engine failure, so it doesn't take all day to climb and accelerate.

The engines fitted to the NG I fly are TO 27K, TO1 26K and TO2 24K. We have used 27K once on a very hot day at very high elevation and MTOW. 95% of our operations use 24K with assumed temperature derate. We have a buffer of 6 degrees on the assumed temp to allow for last minute changes to environmental conditions or passengers.

If we can't get the 6 degree buffer at 24K, we increase to 26K and use a higher assumed temperature. This method maintains the buffer for LMCs, but has the same economic benefit as a 24K takeoff with lower assumed temp.

We are often below the minimum weight for 27K.

17th Feb 2004, 12:25
does anyone know if the thrust increase during thrust transition to climb thrust has any detrimental effect to noise abatement procedures esp. around noise sensitive airfield like AMS, ZRH, etc?


18th Feb 2004, 08:55
I am sure on my little 737-300, we derate...assumed temp etc. for all the same reasons as everyone else. But then my experience differs slightly to other peoples( I think).

On passing 1500' we select climb thrust which then annunciates R-CL on the magic boxes and hey presto we get a REDUCTION in thrust(3% I think). I am sure this to save a few more pennies and for noise abatement requirements. Then at 150 the thrust automatically changes to CL to give you the extra welly at higher levels.

Other responses have been implying that they get an increase in thrust at climb thrust selection..please correct me if I am wrong but I thought we went with a bit more on the ground to get us airbourne and then reduced at a safe height as the extra ooomph(my technical term for that particular stage of flight) was not required.

18th Feb 2004, 22:41
An increase in thrust from takeoff to climb only occurs when at or near maximum derate.

Most of the time, the thrust will reduce when climb thrust kicks in.

19th Feb 2004, 05:29
Many thanks folks; some interesting comments.

True, reduced thrust can give lower VMCG. However, on our engines, if you advance the T/L's after THR HLD you can achieve full 26K even if 22k was enterd via the FMC. Doing this might infringe the V1 v VMCG relationship. Doddgy? but there is no notice not to do so.

About the effect at CLB power. I thought that as the preset thrust is reduced, by whatever means, the CLB is automatically preset to either full, 1 or 2 to match the takeoff thrust setting. Thus at 22k and assumed temp the CLB would likely be CLB 2. Further I thought the FMC would allow the assumed temp to reduce thrust by either 25% or the pre calculated CLB setting. Thus I wonder why at climb power setting there could be a thrust increase. That would suggest that the reduction in takeoff thrust had been too great. It is true that the selected derate and assumed temp are entered first and the selected CLB follows, but surely the FMC would obey the max reduction rules?

What am I missing?

19th Feb 2004, 10:51
RAT 5 I don't think you are missing much at all except that on some aircraft the climb thrust selection is done manually.

From my looong term memory, the B767 requires you to manually select the CLB thrust rating via some buttons on the centre instrument panel. Perhaps there is a company policy that the only choice is CLB or CLB 1. Therefore on the occasion where a large derate/ high assumed temperature is used, where CLB 2 would have given you a thrust reduction, CLB 1 gives you a thrust increase.

The B737 FMC selects the Climb Thrust rating automatically ie CLB, CLB 1, or CLB 2 depending on the T/O thrust setting and is indicated via the CDU. This can be over-ridden if required. In some cases if you think there will be low-level turbulence or whatever and you want to gain altitude ASAP you can select CLB, again via the CDU either on the ground or anytime below 15,000 ft while in flight.

It is also interesting to note that any reduction in T/O or CLB thrust from the full rating actually means you burn more fuel!! Something to keep in mind if you are low on fuel and diverting from a go-around. Obviously the extended life of the engine that results from the lower thrust settings out weighs the extra cost of fuel.

Also with regard to the B737 (and others I suppose), the difference between Max CLB thrust and Rating 1 climb thrust reduces at a gradual rate as you increase altitude, so that at 15,000 ft they are the same. So as you pass through 15,000 ft the "R-CLB" annunciation simply disappears as the difference between them is now zero.


19th Feb 2004, 13:50
RAT 5,

Sure you can push the levers forward on a derate takeoff .. and that would be just as invalidating as doing likewise for a max rated takeoff ... in both cases, you are only permitted to push up to the relevant rated thrust limit.

What the commander might choose to do in an AEO windshear recovery during a derated takeoff, for instance, could be different ....