Derated engines
 

Folks,

I understand that derated engines are used to reduce engine wear and for noise abatement.
I read that derated engines reduce the range of the aircraft.

Why is this so ?

Thanks for your time and regards,

Starpanther

On boeings derated takeoff is most of the time combined with reduced climb thrust. Of course cruise thrust and fuel flow is the same, however with reduced take off and climb thrust the airplane takes more time reaching an efficient cruise altitude and therefore uses more fuel.

However the difference is not much, at least on 737s. On our route network which ranges from 40 minute to 6,5 hour sectors it is only an average 40 kg per flight which can be saved if pilots select full climb thrust instead of reduced climb, another 10 to 15 kgs could be saved if using full take off thrust, however the reduction in maintenance cost is a much bigger factor there. Those 40kg per flight are well above a million euro per year though.

:ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh:

You must understand what the terms "derated thrust" and reduced thrust" mean.

Derated thrust means using a lower limit thrust, i.e. not pushing the T/L so far forward and treating the lower T/L angle as a limit. Hence lower Vmcg and Vmca as well as less wear and tear. In the event of any problem, simply pushing the T/L further forward is forbidden because, inter alia, all speed margins are calculated on a lower max thrust and control may be compromised.

Reduced thrust means setting less thrust than the declared limit thrust when a runway is not limiting for the planned TOW on the day. In the event of a problem it is permissible to push the T/L forward to gain more thrust, although this is generally unnecessary and indeed foolhardy because all certified performance parameters can be met at the set thrust, so why destabilise the situation?

Practically speaking, you "derate" thrust to, on Boeings, RTG 1 or RTG 2 depending on your margins available after planned TOW is calculated. After derating often it is possible to reduce thrust further using "assumed temperature" or another approved method of reducing thrust if less thrust will cover your planned TOW. Hence there is no impact on range in the big Boeings, just a way to reduce wear and tear.

Less noise is actually untrue - the aircraft generating said noise is at a lower altitude climbing less rapidly when any less than max thrust is set. Look at noise abatement procedures ex LHR.

based on the fact that MTOW can be achieved with full thrust and MTOW -x% can be achieved with derated thrust, so presuming payload is the same, then fuel capacity is reduced so range is reduced.

Ummm. Correct me if I'm wrong, but surely an aircraft at it's MTOW, subject to Temperature, Airfield Elevation, and Runway length, could easily perform a take roll at MTOW and reduced thrust???? In my days as a load controller we regularly had to remove some of the traffic weight to allow loadsheet / aircraft weights to not be exceeded. Take-off briefs still mentionned reduced thrust take-offs. I may be wrong,but agree with iceman50. (I recall A320s at MTOW and whatsounded to me like incredibly derated take-offs with N1 settings in the mid 80%s at LGW.)
:ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh:

wangus

Be careful to distinguish between an aircraft where MTOW is also the performance-limited weight, and an aircraft where MTOW is not the performance-limited weight, and is simply the structural limit.

At sea level, it's very possible that a full thrust takeoff could be achieved, meeting all the relevant perf requirements, at a weight in excess of the structural MTOW. In that case a derate (or reduced thrust for that matter) takeoff could be used without impacting the TOW, because all your doing is reducing the performance limited TOW, but not below the structural limited MTOW.

The example mutt is giving is for the case where the performance limited weight, at full thrust, is the same as the structural MTOW. In such a case, any reduction in thrust will, in general, result in a lower performance limited weight, and hence a lower allowable TOW. Change the sentence he wrote to say:
based on an example when MTOW can be achieved with full thrust and MTOW -x% can be achieved with derated thrust, so presuming payload is the same, then fuel capacity is reduced so range is reduced.
and this is perhaps closer to what he was trying to say - reduced thrust can mean less range for a case where the reduced thrust means a lower MTOW.

As to the head banging - OBVIOUSLY you woulndt reduce thrust such that your range dropped below the required mission range, as the required tech stop would push up the overall cost of the mission. So you'd only reduce thrust so as to reduce MTOW and hence range for a case where you had excess range at MTOW/max thrust.

Hi mad Scientist,
I'm with you. (I think.) Many thanks, and hopefully no offence caused to prior poster. I enjoy trying to increase my knowledge and understanding, as fATPL theory only skimmed the perimeter.

At sea level, it's very possible that a full thrust takeoff could be achieved, meeting all the relevant perf requirements, at a weight in excess of the structural MTOW VERBATIM
I assume you are speaking hypothetically, correct? I am slightly confused ny your 2nd paragraph, when you say "not below structural MTOW". Meaning above MTOW? Hypothetically?
(We used to increase taxi fuel, at captains request, sometimes to 1000kgs, to not exceed MTOW on loadsheet. I used to watch take-off rolls from office holding my breath!:\)

He said derated engines not derated take off thrust. Carry on.

Airbus software has an option where it wont show specific runway takeoff weights above structural TOW. This appears to have caused some confusion as some seem to believe that this is the highest weight that you can achieve. If you look at a Boeing chart for say a B772, it has a Structural TOW of 286.8 kgs, but the chart shows weights up to 315.0 kgs. If you plot these weights you can easily see the difference between the takeoff weight achievable with Full Thrust and Derated Thrust, on a specific day/runway, you may achieve structural takeoff weight with both thrust ratings, but there will be a difference between them. (Not including other limits such as tire).

I'm assuming that you do know the difference between Maximum Structural Takeoff Weight and Maximum Ramp Weight?

OK, then answer this, why does Boeing publish takeoff weights above Maximum Structural?

Mutt

Hi Mutt,
(Sorry for digressing a bit.)
Our Loadcontrol computer system allowed us to overwrite weights, as long as you confirmed them. As long as ZFW plus takeoff fuel was less than stated MTOW, if would allow a loadsheet to be produced. This is when we showed 1000kgs taxi fuel to permit calculations. You could enter any weight and index you wanted. And we only had MZFM MTOW and MLAN in our system. No maximum ramp weight on it. We also regularly increased the taxi fuel at captain's request on BAE 146 aircraft as these loadsheets were manual, often with curved drop lines. Manual loadsheets were also used periodically when major weight issues needed overcoming on other types.

OK, then answer this, why does Boeing publish takeoff weights above Maximum Structural?
That I have no idea. I am curious as to why. I didn't know they did that. (Maybe they know how despatchers and flight crew work together to get all traffic on-board through "creative" solutions?)

P.S.
Hi Mutt. Legally is 272 tons the max approved, or could you go for 315 tons on the 772 legally with self loading cargo? I didn't despatch triples so not familiar with weights. (I thought they were heavier than that.)

What's Max Structual for you may not be Max for the next person. Thinking back to the Seven Two there were all sorts of numbers for Max depending on who was the original operator. And the speed adjustments to V1 for a contaminated runway were applied to numbers that the runway would allow that day, not what your actual weight called for. So you would need numbers that were "off the chart" on a normal day.

DE RATED TAKE OFF
 

AIR NEW ZEALAND WAS THE FIRST AIRLINE TO USE DE RATED TAKE OFF IN THE Dc8-52 BACK IN 1967.
IT WAS INTRDUCED BECAUSE IT EXTENDED THE HOT SECTION LIFE OF THE POWER PLANT.
DE RATING WAS ONLY ALLOWED WHERE IT WAS ABLE, AT A BALANCED FIELD CALCULATION.
IT GOT TANGLED UP A BIT WITH PHONEY V1s AND VMCG/VMCA,VLOF WET RUNWAYS AND A MIN HEIGHT OF 35 FT AT FAR END ETC.
iT SAVED DOLLARS!!!
THE REST OF THE WORLD TOOK A LITTLE TIME TO REALIZE THAT TEAL/AIR NEW ZEALAND GOT IT RIGHT(CORRECT)

Perhaps Boeing wants to let the operater be aware of how much of a margin they have. Max TOW is XXX KG as limited by structure but Max TOW as limited by peformance is greater by YYY KG. Now you know if you are close to your performance limit or way below it.

The performance software my outfit is using on the A-320 certainly displays performance-limiting weights and most often they are several tonnes above the structural MTOW...

I have another question on the de-rates. Can anyone explain why Airbus and Boeing have different de-rate philosophies?

AFAIK, Airbus allows fixed de-rates from the A330 onwards (and possibly on the newest 320's as an option). However, they are to be used only to increase MTOW when limited by Vmcg and can't be mixed with FLEX for lower wear&tear...

Boeing, on contrary, happily allows to use both in conjunction without restrictions...

What are the pro's and con's of each approach?

Question for Boeing guys - how do you know when to use higher de-rate with lower Assumed temp, or vice versa?

To be honest? Because the performance software tells us... Back when we used paper charts derates were not used since the amount of manuals would be too big. We use the combined method only since we have an EFB with performance software. The performance software is the big point in the business case for an EFB by the way.

Where I am.. start with highest (lowest?) de-rate first, then assume on top of that if weight allows.

AIR NEW ZEALAND WAS THE FIRST AIRLINE TO USE DE RATED TAKE OFF IN THE Dc8-52 BACK IN 1967

Now, I don't know for sure who started things off, but Wal Stack (boss QF performance man), in the late 60's/70s used to do some time as an Industry Lecturer in AeroEng at Sydney University.

His stories relating to reduced, specifically, and probably derated (too long ago to recall and, in any case, I don't think I understood the difference as a young chap), takeoffs suggested that QF had been doing this sort of thing for quite some time.

(Not an Air Kiwi basher as I have worked for the good folk on occasion with fond memories.)

De-rated and reduced are different. Take the CF6-80C2 engine. Some airlines will run the newer engine on say their 767-300 fleet at nominal 68,000lbs of thrust. After engine performance has deteriorated they will de-rate them to 62,000lbs of thrust then install them on say 747-400 for further usage. De-rating is a mechanical adjustment at the fuel control unit and software change at the TMC.

Actually, no, not necessarily. Boeing allows user selectable derates, for example instead of 26k derates to 24 and 22k. On top of that one can use assumed temperatures for total thrust reductions in excess of 30% on take off.

Typically engines are more fuel efficient when the power is pulled back and you get closer to some efficient L/D....

Most engines are so overbuilt for the job at hand where it will put out X power, but they only give you Y(derated) and the operator only let's you push it to Z (reduced) trying to stretch the life of the engines out indefinately..

Is it possible that some engines are loping along as such reduced power they aren't efficient? Sure....which ones? I don't know...but comes to mind putting a 454 in a Volkswagen Bug is a pretty inefficient use of power, milage sucks, but the engine certainly won't break pushing that little car around...

Personally I would take a lighter engine, less overbuilt, running in a more efficient operating range to save some fuel...if the fuel savings were such it made sense...but not if engines were breaking left and right...

I guess the question is the model of how much over building of engines you need that once powered back you get to some calculation of reliabilty where the engine will last for x amount of hours/years ect....

It allows you to "carry" MEL/CDL penalties right up to MTOW. For example, if you're carrying a slat seal CDL of say 2000kg, if the RTOW was limited to your max structural weight, you'd be penalised by 2000kg. But if the RTOW was the performance limit (quite often higher than max structural) you could possibly takeoff at MTOW even after subtracting the 2000kg.

MTOW: 60t
Max RTOW: 63t
CDL penalty: 2t
Corrected Max: 61t

ie you can still take off at MTOW.

Sillypeoples:You are probably quite right regarding efficiency, but then the issue becomes logistics and commonality.

The CFM56-3 started out at the 3B1 rating, then with some hardware upgrades was "pushed" to 3B2 and 3C1 ratings - all within the same carcass, mountings, accessories, etc. If an operator selected the lower thrust rating for his fleet, it would do him little good to insist on a physically smaller engine, when that would cost more for special tooling, spare parts, etc.

Further, if the operator decided to trade up to a newer fleet some years later, the new buyer of the old planes would have the option of using the higher thrust rating if required on his route system.

Seems to be the standard way where I am as well, BUT.......would it not make more sense to do max assumed thrust first?

We are advised to never go to full thrust(except for emergency) on a derated takeoff due to maximum certified thrust/controllability issues due to VMCG, whereas on an assumed temperature reduced thrust takeoff, there is no such restriction.

Sillypeoples,

"Typically engines are more fuel efficient when the power is pulled back and you get closer to some efficient L/D".

Incorrect. Jet engines are at their most efficient when operating at highest speed/temperature. Selecting a derated climb thrust for example burns more fuel than going for full climb. However the ratio of engine life cost to fuel cost is what a company has to balance. Terms such as Cost index deal with this for climb crusie and descent. Take off is a different matter with the balance of commercial load against engine wear.

Remember the size of the engine and its larger than required thrust on day to day ops is down to the fact that it has to meet the performance requirements when the other engine stops working over a large atmospheric range !

p.s the B777-2 I fly has a MTOW of 347 tons

I am really no expert.. but does it matter how (de-rate or assumed) we reduce the take-off thrust? Any reduction in N1 should save the hot parts, if that's using a de-rate or assumed should be no different?
Regarding VMCG issues I am not sure what to answer. I don't have the RTOW charts here but would let's say a 26K assumed to 55 deg produced the same speeds as a full 22K? (Assuming they give the same RTOW)

Vmcg can only be changed by using a derate. Not by using an assumed temperature. Which gives derate users quite an edge in winder operation as it allows higher take off weights due to lower Vmcg.

But if the higher takeoff weight for a particular flight is not required(as is frequently the case), why not use the assumed temperature method first instead of derated thrust?

Of course, frequently both get used together.

why not use the assumed temperature method first instead of derated thrust?

If you are at a reasonable weight and a speed schedule well above Vmcg/Vmca, no reason why you can't use either approach to achieve the end thrust result.

However, at low weights (typically a ferry or short runway), the decision process involves certification considerations. The AFM pilot requirements are based on the installed rated thrust limits. In turn limiting speeds such as Vmcg and Vmca are based on the limit thrust.

If the pilot elects to depart with a reduced thrust schedule, there is nothing to prevent his/her pushing the power levers up to achieve rated thrust during the takeoff .. say, in the event of a failure. Mind you, if one intends to do that it is appropriate to do it CAREFULLY due to the risk of a thrust exceedance and, at low weights, the very real risk of getting into control problems.

Now, if one uses a DERATED (ie lower certificated) thrust setting, that is the same, conceptually, as unbolting the big engine and replacing it with the smaller one. The pilot cannot command higher than the DERATED thrust setting during the takeoff. The lower Vmcg/Vmca speeds will still apply.

In the real world, of course, we don't change the engines physically but call up a defacto smaller engine either by a fuel control setting preflight (which is not very convenient) or electronic cockpit selection. It is essential, in this case, that the pilot doesn't permit thrust levels to exceed the derate limits otherwise the derate Vmcg/Vmca speeds could catch the pilot out rather badly.

As a for instance, I was involved in the (fatal) accident investigation, quite some years ago, of a twin turboprop which suffered an uncommanded feathering but without an accompanying engine failure.

The crew, apparently, was quite confused by the whole thing and, during the latter stages of the takeoff, increased thrust settings in an attempt to improve the then existing abysmal performance.

End result (with very good spec engines) was that the realworld Vmca went through the roof (VERY rapidly) and it was yaw, roll, and all over, red rover. Fortunately only folks on board were killed but it would have been very much worse had it not been for the existence of the large ditch which took the aircraft out of contention.

While derate/flex wasn't involved, the same over thrust consideration killed them just as dead.

@JammedStab, of course that is sensible and quite often done. Although the Boeing performance software tries to go for lowest engine rating first. Anyway, on contaminated runways and those with a reduced braking action we are not allowed to use the ATM method, but we can of course use derates as derates will allow a better performance in these cases (lower possible V1). One has to be aware of the caveats as pointed out by John tough, it can get nasty very very fast if one advances the thrust at a V1 of 98kts just because one engine quit.

Thanks for the replies.

I would think that if there is an engine failure at V1, the thrust most likely won't get advanced(at least I haven't seen it done in the sim and there are not many real world actual events to analyse).

But there may be other reasons to push the thrust way up. Maybe a feeling at higher speeds that acceleration is not normal, or a object on the runway or the end of the runway itself or windshear. Of course, using your emergency authority to advance the thrust is the obvious thing to do. The chance of having an engine failure at this time is extremely small, especially with EEC's that will prevent an overboost.

It just seems that after reading the warning that you are not to advance the thrust beyond the certified takeoff limit due to potential controllability issues could make some people hesitant to do so even when they should do so and therefore if max assumed temp thrust reduction is used first and most frequently then it could be possible to prevent a potential belief that you must not advance the thrust above the takeoff limit.

With overboost protection and all engines operating, I don't see much adverse risk to firewalling the thrust levers on normally operating engines when an urgent situation requires it.

So I wonder why the Boeing software does derate first. Maybe for displayng maximum payload capability in the quickest manner.

I wouldn't sweat too much on the Vmcg/Vmca consideration. Just be aware of it as part of your pretakeoff planning and general decision processes.

One needs to be low weight and using a minimum speed schedule to be exposed. That is, the weight range where you see the V speeds run constant with reducing weight.

However, be very wary, also, of crosswind operation at low weight. The declared V speeds will be based on nil wind (7kt for older UK certifications). In the real world, the V speeds will increase with crosswind from the failed side. Think of increases in the order of 0.5kt/kt for twins and in excess of 1.0kt/kt for quads ... if you have a high crosswind limit and you are at low weight with a min speed schedule .. the potential is there for a BIG surprise.

One area which you can control is that of empty ferries .. if the runway isn't short, why go off at the minimum speed schedule ? Maybe use a higher speed schedule appropriate to a higher weight ?

You may also achieve a deeper reduction using the highest thrust as the start point if your aircraft automatically boost thrust to Rated or ever Higher (ATTCS/reserve/APR) as the thrust level post Vef is higher than if you had used a de-rate.

We operate the largest in the sky, the ge90-115b derated to 110k lbs. an engine like the PW4062 is commonly derated to 60k. This is usually a program plug installed on the EEC.

8che -

First off, 30 seconds at reduced thrust on a departure so the mechanic can walk out there and keep signing off blades that 'don't look that bad' is the reason why you put thousands of feet of runway behind you and climb out over the bushes every day is hardly a sales point for 'safety first'...

That said, as far as 'efficiency goes'...just because an engine can burn hot doesn't mean the fuel flow is going to make it to Hawaii.....

Banging full thrust on the next takeoff (which is coincidentally ferry flight) just after the same engineer signed off the same blades "that don't look that bad" is much more likely to have an engine failure than by using appropriately reduced takeoff thrust.

You will not find companies in this world (and especially in this economy) operating large numbers of medium or large engines (let's say 20K+ thrust) doing continuously only full rated thrust takeoff and then replacing engines at the first small sign of problem.

Sillypeople

Firstly you dont de-rate to leave thousands of feet behind. The longer the runway used the more de-rate you can take. Yes ? Exactly what is unsafe about meeting the performance requirements while causing the least stress to the engine ?

Maybe the entire industry is wrong and you are right ?

Secondly we are talking about engine efficiency here not airspeed. Why do you think Jet engines are more economical at high altitude ?

Wow, there are still some that answer to ssg posts?

8che -

Bottom line you can't fly all problems into the air at V1 like they would have you believe...so it's nice to have some runway ahead in the event you don't have a flyable plane.

If you don't get this then the chief pilot did his job...getting everyone to not think, a GO mentality, standardization is key, no lose cannons, no independent thinkers, and enough insurance to cover the Concords and GOAs that should have been stopped on the runway...but hey....that would have meant hiring pilots that could make a decision...nope we can't have that....better to crash a few planes that couldn't fly then hire guys that might stop a plane and put it into the weeds once in a while....or whatever they are thinking.....

Funny, when did max take off power become dangerous.?

The manufacturer has derated the engines so far anyway for normal ops to pretty much guarantee that they will go double typical standard overhaul times....

Anyway...watch what happens when the FAA stops 'on condition' determination of engine wear....all of a sudden you guys will flip flop around to 'better to get it off early'....just like you flip flopped on CRM, or hiring kids instead of pilots....

Always fun to come here and talk to guys that don't think....

So let me get this straight. Doing an assumed temperature takeoff with TOM x and PLTOM y, which gives you the same performance as the aircraft would have if it's TOM was y is BAD, even if you can push TO/GA once more (Boeing) and get full rated take-off thrust.

But, being employed by beancounters who will sell the aircraft before engines have to go to overhaull and they regularly fill the aircraft so that TOM = PLTOM is the best job ever? In theory, if the s**t hits the fan at V1 (above Vef, just to be clear), in both cases you end up past the runway if you abort and the reversers don't deploy - the only difference is that with assumed temperature you can still get that extra performance you would normally have, should you decide to continue the takeoff.

Given that I myself fall into the group of people mistakenly thinking assumed temperature is a relatively safe and proven method of prolonging engine life, I have a suggestion. ALL airports involving operations of aircraft, certified under Part-25 (FAR/JAR/CS) shall be at -2000ft MSL, Siberian-like climate (max. -30°C in the summer), QNH shall never be below 1060 hPa and minimum runway length is 20 kilometers, so you can do a few touch and goes when you're deciding whether to really take the aircraft into the air.