Derated engines
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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
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
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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.
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.
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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.
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.
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.
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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.)
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.)
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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.
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.
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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!
)
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!
)
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because all your doing is reducing the performance limited TOW, but not below the structural limited MTOW.
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
I enjoy trying to increase my knowledge and understanding, as fATPL theory only skimmed the perimeter.
Mutt
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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?)
(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?)
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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.)
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.)
OK, then answer this, why does Boeing publish takeoff weights above Maximum Structural?
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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)
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)
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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.
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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?
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?
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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.
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Question for Boeing guys - how do you know when to use higher de-rate with lower Assumed temp, or vice versa?
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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.)
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.)
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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.
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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.