Purpose of Semi-Fixed De-rates (TO1, To2)

[IADflyer]

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?

[FLX/MCT]

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.

[barit1]

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.

[Denti]

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.

[mutt]

25% allowed using ATM This can be increased to 40% if requested by the aircraft manufacturer..


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.

[barit1]

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

[mutt]

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.

[411A]

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.;)

[barit1]

mutt:
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.

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. :ooh:

edit: The FAA order I cited earlier is now replaced by Advisory Circular 25-13 (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2025-13/$FILE/AC25-13.pdf)
and I note the "75% of rated thrust" limitation still stands. (para 5.a.4.)

[Mad (Flt) Scientist]

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.

[Bealzebub]

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.

[ImbracableCrunk]

Seems like deja vu.

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.

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.

[mutt]

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 :)

[Bealzebub]

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?

[mutt]

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…………..
Takeoff Derates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1
TO1 Takeoff Weights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1
TO1 Slush/Standing Water Takeoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2
TO1 Slippery Runway Takeoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3
TO1 Takeoff Speeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4
TO1 Takeoff %N1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.5
TO1 Stab Trim Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.5
TO2 Takeoff Weights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.6
TO2 Slush/Standing Water Takeoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.7
TO2 Slippery Runway Takeoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.8
TO2 Takeoff Speeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.9
TO2 Takeoff %N1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.10
TO2 Stab Trim Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.10

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.

[Bealzebub]

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?

[mutt]

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.

[RAT 5]

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.

[mutt]

I know that the IVW (Dutch CAA) does not allow fixed de-rates Very strange, I wonder if the airlines can statistically show that their engine maintenance costs are higher in the winter season due to the lack of any thrust reduction on wet/contaminated runways..... OR does the IVW allowed the use of Assumed Thrust Reductions?

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 :)

[barit1]

RAT 5:

I know that the IVW (Dutch CAA) does not allow fixed de-rates.

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.

[Mariner]

I know that the IVW (Dutch CAA) does not allow fixed de-rates.

Can't speak for other types, but they (Dutch CAA/IVW) do allow it on the 744.

At my company we use fixed derates TO 1 & 2, and derate those using assumed temp method, if possible.
This gives a lot of flexibility on short or contaminated rwy's, and allows a lot of derate on long dry rwy's.

We also use 744's that don't have the fixed derates, and derate from the max power setting. This reduces the maximum deration considerably.

[RAT 5]

Regarding the Dutchies: it was the RLD in those days, many moons ago. They didn't allow it on our B757/767's and I heard from HV guys that their B733's were all assumed temp of max thrust. I don't know for sure about their B738's, but I think they use assumed off 26K.
I'm not wanting to focus on that point; rather I'd like an answer as to why fixed-derates are a bonus. EU operators I've been associated with have 26K, 24K & 22K ratings. I hear some USA airlines have only 24K or 22K as max. They don't need 26K. If you need 26K on some occasions, fine, but why not just assume temp from 26K? Why have 3 sets of tables to achieve a thrust that matches the performance for the same weight. My performance dept claims to have it from Boeing as a good idea. No further explanation. I can see that lower thrust and lower VMCG might give a higher MTOW from contaminated rwys, but rarely unless very light weights. The reduced CLB could be calculated by the FMC as necessary from the amount of assumed temp. using fixed de-rates and then assumed temp just seems to complicate things and waste paper and printing ink. Save a tree.

[Denti]

Why even use printed stuff? Performance software does a very good job to test all possible combinations in a few seconds, and boeing gives it out on new planes anyway.

Besides, using a derate has its use during winter operation, and of course it allows less thrust used than using only ATM as ATM is restricted currently, whereas the combination of ATM and derate allows to work around that restriction without any special approval.

We used to have 27/24/22 on the 800s and nowadays use 26/24/22 or 22/20/18.5 on the -800 and -700 respectively. Full thrust is only needed for very few occasion, for example out of LPA with full passenger load, full baggage, a ton of cargo and 16 tons of fuel, which requires the SFP version additionally to full thrust sometimes. Most take offs are done using the lowest thrust setting available and depending on load with up to 60° ATM, usually flaps 1 as well.

[schlener6]

Some airlines had manufacturer's warranties that rewarded the customer for using a "smaller" engine for takeoff for a certain number of years. The more you could safely use TO1 or TO2, the happier the manufacturer was, and the better "deal" on coverage. The TO, TO1 and TO2 settings essentially provided 3 different certificated engines: big, medium and small.....as they were "fixed" derates. Each one was considered "max" for that "size" of engine. The AT derates provided even further cuts....up to 40% total from TO. On a fully AT-derated TO2 takeoff (obviously light weight with friendly runway/obstacles), we called them "Idle Power Takeoffs."

[HazelNuts39]

I wonder if the airlines can statistically show that their engine maintenance costs are higher in the winter seasonThe maintenance benefit is obtained by reducing engine hot section temperatures when conditions permit. At wintertime ambient temperatures the engine operates anyway at temperatures well below the limiting gas temperature. Therefore the benefit of FLEX/ATM/derate is in the area of 'diminishing returns' and possibly almost nil..

EDIT:: Derating beyond 25% has no benefit and is just silly, IMO.

[Avenger]

IAD Flyer,

I'm not going to cut and paste, however, if you refer to Boeing Systems Manual, FMC, " Thrust Management", the whole subject is covered and the question you originally asked is answered.

The subject of Fixed Derate, Assumed and a combination of both is clearly explained.

[Brenoch]

Bealzebub;

There seems to be discrepancy in our manuals. This is from my systems description manual B757-200.

Displayed (green) – selected FMS thrust reference mode:
Takeoff:
• TO – maximum rated takeoff thrust
• TO 1 – takeoff thrust one, climb one preselected
• TO 2 – takeoff thrust two, climb two preselected
Assumed Temperature Takeoff:
• D–TO – assumed temperature derated takeoff thrust
• D–TO 1 – assumed temperature derated takeoff thrust one, climb one
preselected
• D–TO 2 – assumed temperature derated takeoff thrust two, climb two
preselected

(Edited to say sorry for drifting)

[Brenoch]

And further, the Thrust Mode Select Panel;

Thrust Reference Mode Select Switches
PUSH –
• manually selects desired thrust reference mode
• selected thrust reference mode and reference EPR are displayed
TO/GA –
• selects TO mode on the ground or GA mode inflight
• cancels preselected climb one or two
• cancels selected assumed temperature
• selecting 1 or 2 with TO or D-TO reference mode displayed:
• preselects CLB 1 or CLB 2 on the ground if autothrottles not engaged
• subsequent push cancels any preselected 1 or 2
• cancels selected assumed temperature
• selects TO 1 or TO 2

[AtoBsafely]

"Derating as much as 40% is just silly, IMO."

It makes a lot of sense to me when TOW can vary by more than 40%. Just saying...

Joe

[8che]

Sorry Beazlebub its not that simple,

You clearly have not flown a 757 or 767 with boeing optional company fixed derates installed. Thomson guys among others know all about it. If you try and preselect climb 1 on the ground you actually select the fixed derate and then subsequent assumed temp reduction gives you what we call a double de-rate. (ie you derate the fixed derate) The trick is in the eicas. Selecting 1 or 2 on your machine gives a WHITE 1 or 2. On fixed rate machines the fixed rate shows as an all green TO-1/2 etc when 1 or 2 are pressed. Boeing have a huge number of optional extra's available. Your aircraft are just one of several different fits

The 777 EICAS thrust headings are exactly the same using fixed derates then assumed temp. (climb thrust selection is much cleverer though). All selections through the FMS of course.

Why wouldnt you want to use as little thrust as possible and still meet all the performance criteria both from a commercial and risk reduction point of view

[theficklefinger]

I would be curious to know the real overhaul times......certainly more then a few pilots are pushing balanced field into the weeds to get points, when maybe pushing it 'that far' is not necessary to get the extended overhaul advantage.

[mutt]

Pushing it into the weeds? You obviously have little or no understanding of the dynamics of assumed derate thrust. . Are you and incarnation of SSG?

mutt

[Pugilistic Animus]

SSG

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