Purpose of Semi-Fixed De-rates (TO1, To2)
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Purpose of Semi-Fixed De-rates (TO1, To2)
Hi all,
Doesn't it seem a bit redundant that there exist the semi-fixed derates of TO1 and TO2 on airliners, in addition to the option of an assumed temperature derate?
Taking into account the weight of the aircraft, the runway length, altitude, takeoff obstacles, and any other factors which influence the selection of the most appropriate takeoff thrust, aren't I right that it is possible to achieve the most ideal takeoff thrust on any takeoff by ONLY using the assumed temperature method?
Why do the block derates of TO1 and TO2 even exist in the first place?
Doesn't it seem a bit redundant that there exist the semi-fixed derates of TO1 and TO2 on airliners, in addition to the option of an assumed temperature derate?
Taking into account the weight of the aircraft, the runway length, altitude, takeoff obstacles, and any other factors which influence the selection of the most appropriate takeoff thrust, aren't I right that it is possible to achieve the most ideal takeoff thrust on any takeoff by ONLY using the assumed temperature method?
Why do the block derates of TO1 and TO2 even exist in the first place?
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Derate vs flex
Derated thrust is used because it programs the engine in such a way that you achieve the derated amount of thrust when pushing the thrust levers fully forward (when using assumed temperature it is possible to advance thrust beyond flex thrust to TOGA thrust). The derate defines the new TOGA setting. This allows you to decrease Vmca and related speeds, due to the lower yaw tendency at an engine failure which can be quite useful in some scenarios...
At some aircraft even a combination of derate and assumed temperature is possible.
At some aircraft even a combination of derate and assumed temperature is possible.
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Assumed temp is perfectly fine to use, except that its use is prohibited on contaminated runways; whereas derates may be used at any time, and the two may be used together on dry runways.
Use the Pprune search function to find a dozen or more prior discussions on the subject.
Use the Pprune search function to find a dozen or more prior discussions on the subject.
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Derate doesn't necessarily re-program a new max takeoff setting, for example on the 737 it does not do that and advancing the thrust levers if using derate is not recommendet. However as barit said you can use different limiting speeds and that can in fact improve performance during operation on contaminated or degraded braking action.
And of course you can reduce your take off thrust by quite a bit more than the 25% allowed using ATM as the 25% limit is counted from the rated or derated thrust setting.
And of course you can reduce your take off thrust by quite a bit more than the 25% allowed using ATM as the 25% limit is counted from the rated or derated thrust setting.
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25% allowed using ATM
IADFlyer, we can get a higher takeoff weight using fixed derates on contaminated runways due to the lower VMC speeds. They also help in allowing us to reduce takeoff thrust by up to 45% on Boeing aircraft.
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mutt:
The 25% ATM limitation is an FAA one - from the "Reduced TO Thrust" order. It applies to the selected TO rating, be it Full, TO1, TO2, whatever.
Thus - if a 20% derate is selected (i.e. 80% of rated thrust), and a max ATM reduction applied (i.e. 75% of the derated thrust), the actual overall thrust is:
0.80 x 0.75 = 0.60
Or, 40% reduction from nameplate thrust.
Yes, the aircraft mfg must validate all aspects of the protocol, but remember it is within the 25% ATM rule.
Edit: The FAA document is Order 8000.39
The 25% ATM limitation is an FAA one - from the "Reduced TO Thrust" order. It applies to the selected TO rating, be it Full, TO1, TO2, whatever.
Thus - if a 20% derate is selected (i.e. 80% of rated thrust), and a max ATM reduction applied (i.e. 75% of the derated thrust), the actual overall thrust is:
0.80 x 0.75 = 0.60
Or, 40% reduction from nameplate thrust.
Yes, the aircraft mfg must validate all aspects of the protocol, but remember it is within the 25% ATM rule.
Edit: The FAA document is Order 8000.39
Last edited by barit1; 27th Dec 2010 at 14:02.
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Barit1. . . Airbus have approval for 40% ATM on the A32 family as they don't operate with Derates, hence you will see charted temperatures over 80*C compared to less than 70c for Boeing.
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Older types, specifically...TriStar.
No fixed derates for takeoff (at least in our ops), only flex thrust using the assumed temperature method....to a max of 54C.
Climb thrust.
Six individual fixed derates, dependant on ambient temperature/altitude/weight....all selected automatically via the FMC.
When climb rate decreases to 650 ft/min for 25 seconds, the next higher derated thrust is selected, automatically.
OR, if one so desires, any climb derate can be selected manually.
Full time engine thrust management...since 1976, on FMC equipped aircraft.
TriStar set the standard...others followed.
No fixed derates for takeoff (at least in our ops), only flex thrust using the assumed temperature method....to a max of 54C.
Climb thrust.
Six individual fixed derates, dependant on ambient temperature/altitude/weight....all selected automatically via the FMC.
When climb rate decreases to 650 ft/min for 25 seconds, the next higher derated thrust is selected, automatically.
OR, if one so desires, any climb derate can be selected manually.
Full time engine thrust management...since 1976, on FMC equipped aircraft.
TriStar set the standard...others followed.
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mutt:
So - has the FAA order been revised to 40% ATM? I haven't looked at it in a while.
Or perhaps the inner software of FMS works with "phantom" derates not evident to the flight deck, but instead presented as absurdly high assumed temps?
AND how does A32 handle contaminated runway ops, if derates aren't available?
I'm learning something new every day it seems.
edit: The FAA order I cited earlier is now replaced by Advisory Circular 25-13
and I note the "75% of rated thrust" limitation still stands. (para 5.a.4.)
Barit1. . . Airbus have approval for 40% ATM on the A32 family as they don't operate with Derates, hence you will see charted temperatures over 80*C compared to less than 70c for Boeing.
Or perhaps the inner software of FMS works with "phantom" derates not evident to the flight deck, but instead presented as absurdly high assumed temps?
AND how does A32 handle contaminated runway ops, if derates aren't available?
I'm learning something new every day it seems.
edit: The FAA order I cited earlier is now replaced by Advisory Circular 25-13
and I note the "75% of rated thrust" limitation still stands. (para 5.a.4.)
Last edited by barit1; 27th Dec 2010 at 19:30.
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It's still just an Advisory Circular, though, not absolute regulation, so it's possible Airbus and FAA (and indeed EASA/JAA as prime certifying authority) have agreed some other standard of equivalent safety, in which case there would be a type-specific agreement which might or might not be public domain.
It's not unusual for the authorities' documentation to be somewhat behind the state of the art and for the resulting "holes" to be patched until the more formal docuemnts can catch up.
It's not unusual for the authorities' documentation to be somewhat behind the state of the art and for the resulting "holes" to be patched until the more formal docuemnts can catch up.
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It is very simple!
Boeing 757-200
The TO1 and TO2 numbers have nothing to do with derating the takeoff thrust. In both cases the Take off thrust will be full. The numbers relate to the preselection of the climb thrust mode.
Climb 1 represents a reduction of 6% in climb thrust.
Climb 2 represents a reduction of 12% in climb thrust.
In any event the pre-selected reduction (either 1 or 2) will not occur until CLB is selected on the thrust mode selection panel.
Derated Take off thrust is a different function and is obtained by the assumed temperature method and either entering the assumed temperature through the FMC or by manual selection through the EICAS control panel.
If a fixed derated climb (either 1 or 2) is preselected and the take off thrust is derated using one of the above methods then the thrust reference mode will actually show D-TO 1 or D-TO 2 with the assumed temperature selection shown above it on the primary Eicas display.
In summary, the 1 and the 2 are fixed derates of Climb thrust and are simply pre-selections of that mode. The assumed temperature selections are variable selections of Takeoff thrust only.
TO 1 full take off thrust with a fixed climb derate of 6% pre-selected.
TO 2 full take off thrust with a fixed climb derate of 12% pre-selected.
TO full take off thrust with no derated climb pre-selected
D-TO 1 derated take off thrust (as indicated) with climb 1 preselected.
D-TO 2 derated take off thrust (as indicated) with climb 2 preselected.
D-TO derated take off thrust (as indicated) no climb thrust preselected.
Boeing 757-200
The TO1 and TO2 numbers have nothing to do with derating the takeoff thrust. In both cases the Take off thrust will be full. The numbers relate to the preselection of the climb thrust mode.
Climb 1 represents a reduction of 6% in climb thrust.
Climb 2 represents a reduction of 12% in climb thrust.
In any event the pre-selected reduction (either 1 or 2) will not occur until CLB is selected on the thrust mode selection panel.
Derated Take off thrust is a different function and is obtained by the assumed temperature method and either entering the assumed temperature through the FMC or by manual selection through the EICAS control panel.
If a fixed derated climb (either 1 or 2) is preselected and the take off thrust is derated using one of the above methods then the thrust reference mode will actually show D-TO 1 or D-TO 2 with the assumed temperature selection shown above it on the primary Eicas display.
In summary, the 1 and the 2 are fixed derates of Climb thrust and are simply pre-selections of that mode. The assumed temperature selections are variable selections of Takeoff thrust only.
TO 1 full take off thrust with a fixed climb derate of 6% pre-selected.
TO 2 full take off thrust with a fixed climb derate of 12% pre-selected.
TO full take off thrust with no derated climb pre-selected
D-TO 1 derated take off thrust (as indicated) with climb 1 preselected.
D-TO 2 derated take off thrust (as indicated) with climb 2 preselected.
D-TO derated take off thrust (as indicated) no climb thrust preselected.
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Seems like deja vu.
Derate (on the B737 anyway) is not ATM. Derate is selected separately from the ATM. For instance, on a B738, you can select 26k or 24k or 22k or 27k, depending on the options. You can also select, in the FMC, an assumed temperature. They're not the same thing, they don't provide the same Vspeeds, even with the same N1. You can derate and/or use ATM.
It is also not a physical plug on the motor itself , in the case of the FMC selectable derates.
Derated Take off thrust is a different function and is obtained by the assumed temperature method and either entering the assumed temperature through the FMC or by manual selection through the EICAS control panel.
It is also not a physical plug on the motor itself , in the case of the FMC selectable derates.
Last edited by ImbracableCrunk; 27th Dec 2010 at 21:09.
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Bealzebub
Your explanation of the nomenclatures might be correct for the B757, but it it doesn't apply for other Boeing aircraft such as the B777/B744.
TO 1 derate take off thrust with an airline selected % pre-selected.
TO 2 derate take off thrust with an airline selected % pre-selected.
TO full take off thrust
D-TO 1 derated take off thrust (as indicated) with assumed temperature.
D-TO 2 derated take off thrust (as indicated) with assumed temperature.
D-TO derated take off thrust (as indicated) no assumed temperature.
These aircraft also have CLB1/CLB2 ratings.
Barit1, the FAA Order hasn't changed which is one of the things that makes this interesting, it's also next to impossible to find an Airbus reference for the 40%. But with the clout of a 50 airframe order and the need to certify them with a local authority, they produced an certification authorization based on the fact that Boeing could combine Derate/Assumed to get 40-50% takeoff thrust reductions, and they wanted to level the playing field
Your explanation of the nomenclatures might be correct for the B757, but it it doesn't apply for other Boeing aircraft such as the B777/B744.
TO 1 derate take off thrust with an airline selected % pre-selected.
TO 2 derate take off thrust with an airline selected % pre-selected.
TO full take off thrust
D-TO 1 derated take off thrust (as indicated) with assumed temperature.
D-TO 2 derated take off thrust (as indicated) with assumed temperature.
D-TO derated take off thrust (as indicated) no assumed temperature.
These aircraft also have CLB1/CLB2 ratings.
Barit1, the FAA Order hasn't changed which is one of the things that makes this interesting, it's also next to impossible to find an Airbus reference for the 40%. But with the clout of a 50 airframe order and the need to certify them with a local authority, they produced an certification authorization based on the fact that Boeing could combine Derate/Assumed to get 40-50% takeoff thrust reductions, and they wanted to level the playing field
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Mutt,
Not only for the 757, but also the 767.
It would seem odd for Boeing to use a presentation that means entirely different things on earlier and later model aircraft, but as I have no experience of 737,747 or 777 types I am guided by what you say.
As a matter of interest, what display do you get for a derated (and non-derated) take off with climb 1 or climb 2 preselected, on the 747 or 777?
Not only for the 757, but also the 767.
It would seem odd for Boeing to use a presentation that means entirely different things on earlier and later model aircraft, but as I have no experience of 737,747 or 777 types I am guided by what you say.
As a matter of interest, what display do you get for a derated (and non-derated) take off with climb 1 or climb 2 preselected, on the 747 or 777?
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For the BBJ, the Flight Crew Trainining Manual states the following:
The FMC provides two reduced thrust climb selections on the N1 Limit page”
CLB 1 is approximately a 10% reduction on climb thrust
CLB 2 is approximately a 20% reduction on climb thrust
Reduced thrust climb may also be automatically selected by the FMC depending on the amount of thrust reduction made for takeoff by either the fixed derate or the assumed temperature method. (BBJ Flight Crew Training Manual, Chapter 4, 2003)
In the B777 you will find this…………..
You will note that it doesn’t mention climb. I find the method of use for TO-1 on the B757/767 to be strange, as in newer aircraft such as the 777/744 you will see TO-1 (or other) followed by CLB (1 or 2) after the use of takeoff thrust.
The FMC provides two reduced thrust climb selections on the N1 Limit page”
CLB 1 is approximately a 10% reduction on climb thrust
CLB 2 is approximately a 20% reduction on climb thrust
Reduced thrust climb may also be automatically selected by the FMC depending on the amount of thrust reduction made for takeoff by either the fixed derate or the assumed temperature method. (BBJ Flight Crew Training Manual, Chapter 4, 2003)
In the B777 you will find this…………..
You will note that it doesn’t mention climb. I find the method of use for TO-1 on the B757/767 to be strange, as in newer aircraft such as the 777/744 you will see TO-1 (or other) followed by CLB (1 or 2) after the use of takeoff thrust.
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Thank you Mutt, that is interesting.
Just to clear up a bit of confusion. Is there any display of the pre-selected climb thrust fixed rate reduction, prior to take off?
On the 757/767 the number purely relates to the pre-selection of climb thrust.
that is:
TO = Fully rated take off thrust with no fixed rate reduction of climb thrust.
D-TO = Derated take off thrust (with the assumed temperature shown above it.) and no fixed rate reduction of climb thrust.
TO 2 = fully rated take off thrust with climb 2 pre-selected.
D-TO 1 - Derated take off thrust (as above) with climb 1 pre-selected.
Etc.
On the 757 the pre-selected climb thrust will not be activated until the CLB switch is pressed on the TMSP. However on the 767 it will be activated by either this method, or by engaging FLCH / Vnav after departure.
Are you saying that in the example you give, the number relates to a fixed take off thrust setting and not a climb thrust pre-selection? Is it a combination whereby the use of either 1 or 2 will reduce the take off thrust setting by a fixed amount and will give the corresponding climb thust reduction when that mode is activated by whatever method?
Just to clear up a bit of confusion. Is there any display of the pre-selected climb thrust fixed rate reduction, prior to take off?
On the 757/767 the number purely relates to the pre-selection of climb thrust.
that is:
TO = Fully rated take off thrust with no fixed rate reduction of climb thrust.
D-TO = Derated take off thrust (with the assumed temperature shown above it.) and no fixed rate reduction of climb thrust.
TO 2 = fully rated take off thrust with climb 2 pre-selected.
D-TO 1 - Derated take off thrust (as above) with climb 1 pre-selected.
Etc.
On the 757 the pre-selected climb thrust will not be activated until the CLB switch is pressed on the TMSP. However on the 767 it will be activated by either this method, or by engaging FLCH / Vnav after departure.
Are you saying that in the example you give, the number relates to a fixed take off thrust setting and not a climb thrust pre-selection? Is it a combination whereby the use of either 1 or 2 will reduce the take off thrust setting by a fixed amount and will give the corresponding climb thust reduction when that mode is activated by whatever method?
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Where is Old Smokey when I want him to just in to answer B777 specific questions
My understanding for the B777/744 is that the selection of Takeoff Thrust Rating gives the corresponding Climb Rating, but it doesn't work the other way around.
My understanding for the B777/744 is that the selection of Takeoff Thrust Rating gives the corresponding Climb Rating, but it doesn't work the other way around.
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B738 NG. I know that the IVW (Dutch CAA) does not allow fixed de-rates. All takeoff thrusts are made using full power & assumed temps. Why; I do not know? Further, some airlines do not have 26k engines, they have either 24K or 22K max thrust settings. This is because they have done an analysis of their network and decided they do not need/want to pay for the higher thrust. More confusingly I asked my performance dept why we have 26K 24k 22k RTOW tables. I'm assuming the thrust setting is that required for the takeoff weight. Thus, by my simple reasoning there must be a 26K assumed temp = 24K & 22K, and following on a further 26K assumed temp = assumed temps at 24K & 22K; so why, if we need 26K for the network, do we have 24K & 22K tables? Waste of paper etc. etc. I had a reply, which purported to come from a Boeing analysis which was full of techno-bable which I didn't understand. It supported the fixed de-rates, and I gave up asking.
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I know that the IVW (Dutch CAA) does not allow fixed de-rates
We have TO/TO-1/TO-2 for the B744, each of which can be used with ATM on dry runways. As an airline we decided not to use TO-2 because:
1: The engine manufacturer couldn't quantify the savings associated with the deeper derates.
2: TO-2 provided a better contaminated runway takeoff weight only when the runway was a specific length.
So it was decided that the costs associated with using TO-2 weren't justified. Your airline has the same choice with 22k/24k/26k but it needs someone to make the decision
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RAT 5:
That is most interesting, and a distinct reversal from DC-10 days. GE CF6-50A engines were originally supplied at 48K thrust on KSSU aircraft (KLM, SAS, SR, and UTA). These ships were later upgraded to CF6-50C at 50K thrust.
But when the upgrade was completed, the AFM section for -50A thrust was retained (in addition to -50C) providing the crew with the option of operating at either rating. In practice, -50C was only used 5-10% of the time. This was equivalent to using -50A as a derate from -50C.
I suspect the L1011-1 was operated similarly; its original RB211-22C engines were derates of the later -22B model.
I know that the IVW (Dutch CAA) does not allow fixed de-rates.
But when the upgrade was completed, the AFM section for -50A thrust was retained (in addition to -50C) providing the crew with the option of operating at either rating. In practice, -50C was only used 5-10% of the time. This was equivalent to using -50A as a derate from -50C.
I suspect the L1011-1 was operated similarly; its original RB211-22C engines were derates of the later -22B model.