Takeoff and Climb thrust w/ flex/derate

[LeCoyote]

Hello,

I am still looking for hands-on experience and information regarding Flex/Assumed-Temp and Derated takeoff scenarii. In particular, I would like to know what happens (or usually happens) when thrust is reduced after takeoff. I am aware that there might be multiple correct answers here, I'll just take them all :-)

So here is the scenario. I am looking and jet (and possibly turboprop) aircraft departing with the following sequence of events:


compute flex/assumed temp and/or derate for current circumstances (normal day, long & dry runway, no obstacles in any segment of the climb)
apply reduced takeoff thrust, roll, rotate, climb at V2+10 (noise abatement)
at 1500ft AMSL, apply climb thrust, still flying V2+10
at 3000ft AMSL, accelerate as planned.

From what I've read, if using a Derated setting, the TO *AND* CLB thrust settings will be decreased from their max rated equivalents. So basically, at 1500ft, we would observe a thrust reduction (please correct me if I'm wrong).

What I don't know is: what happens at 1500ft in a FLEX scenario? What is the desired climb thrust set to? Is it common or uncommon to *increase* the power to use, for example, MCL thrust after a FLEX takeoff?

And what happens later? If climb thrust is lower than MCL/MCT, I believe that thrust will, at some point, be increased to reach that maximum when the air grows thinner at higher altitudes: is that correct? When does that typically happen and how?

Again, I am aware that there might be different scenarii, and therefore answers, out there, and I am OK with that.
Many thanks for your feedback!
Best regards
--
Romain, a very curious ATC ;)

[main_dog]

The answer is complex as it depends on A/C type and engine type as well as operator-specified options.

On our B744s (major carrier that flies into your airport) we have three ratings for takeoff (TO, TO1 and TO2) as well as three climb (CLB) ratings. In addition, each takeoff rating can be further reduced by use of an assumed temperature (flex for Airbus guys).

Typically when programming takeoff thrust, our FMS will automatically select a logical climb thrust (for example, a TO2 takeoff with an assumed temperature would probably result in the FMS "suggesting" a CLB2 rating). This climb rating can be modified by the pilot.

If, say, I plan a D-TO2 53˚ (takeoff thrust derate two with 53 degrees of assumed temperature) and then on the FMS I pre-select CLB1 or even full CLB (because of terrain or other gradient requirements for example), then when climb thrust is selected the thrust will actually increase.

As I recall this was not possible on the A321 instead, where if projected CLB thrust was greater than flex takeoff thrust the takeoff thrust was "bumped up" automatically by the FMGCs to equal climb thrust: in that case when climb thrust was set the thrust didn't actually change.

As far as derated climb thrusts being increased as the climb progresses, again it depends: on some of our PW-engined aircraft the climb derate "washes out" by 15,000', and you have full climb thrust fairly soon. On our other PWs and on our GE engines the derate washes out more progressively between 10,000' and 30,000' or even 35,000'. On our RRs it is different yet again, and on CLB2 might not even wash out entirely.

MD

[LeCoyote]

Thanks for your input main_dog, it sure clears things up a bit.
Of course, it makes sense that the pilot should always be able to decide on a higher CLB setting.

This info about buses' "bumping up" the TO thrust is news to me and that is great info, thanks a bunch.

The washout is also something I had very little information. I found a CFM/Airbus doc where it suggested a washout between M .30 and M.40, which I have trouble understanding. The washout through altitude makes sense.

The bottomline seems to be that unless there are obstacles or specific requirements, there should be no reason to increase thrust from TO setting to CLB setting, and that CLB thrust will only increase much later during the climb (even 10'000ft is "late" compared to, say 3'000ft for example).

Cheers!
--
Romain

[main_dog]

This info about buses' "bumping up" the TO thrust is news to me and that is great info, thanks a bunch.


Take that "info" with a pinch of salt: it was definitely true of my previous airline's CFM-56 powered A319/320/321s, but may be different for other operators/engine types/Airbus models.

As far as climb thrust derate washout, on our B744s it is definitely tied in with altitude, however I could see that it might make sense to tie it in with Mach No. as well. Airliners typically climb at a fixed IAS for the first part of their climb, and as temperature normally drops with altitude this fixed IAS is equivalent to a higher and higher MN as the aircraft climbs: in essence the washout would still be tied in with altitude, albeit indirectly.

Sounds complicated, then again it is Airbus we're talking about... :}

(Airbus guys please don't shoot, I too flew Toulouse products for seven years ;))

[LeCoyote]

I could understand a washout based on MN, but I don't get why it would start at M.3 : that's hardly 200KTAS at SL if I'm not too rusty, so you'd get there pretty quickly
So if the 744 washout is tied to altitude (which makes sense), the Mach-driven washout I read about must be about something else. What I mean is there must be a reason why it is happening so soon, and I'd love to hear about it :-)

Cheers
--
Romain

[main_dog]

You're right, that would washout a bit too soon... sorry can't help you with the Airbus questions!

[TSIO540]

The use of a FLEX thrust setting for takeoff on IAE engines will generally result in a thrust reduction when selecting climb thrust. At max FLEX temp it may be very close to climb thrust, I couldn't say if it can be lower.

With respect to climb thrust, as long as the pilot doesn't modify perameters in the MCDU, climb thrust is determined automatically from entered temp, weight, cost index etc and it is then selected in flight by placing the thrust levers into the climb detent. Pilots don't generally change thrust settings at all after that.. It's all computer driven.

If required it is possible to increase thrust beyond that by selecting MCT or TOGA

[Intruder]

Just ran into this again today leaving HHN:

B744, GE CF6-80, OAT 10C, Assumed Temp 54C. Selected CLB thrust in the FMS (usually use CLB1) due to altitude restriction. At 1500' N1 INCREASED 4% when CLB was activated.

I've run into it before. It just shows that with Assumed Temp thrust settings, CLB thrust is based on full thrust, not the Assumed Temp thrust.

[Denti]

The 737 is pretty similar to the 747 apparently, we have a full thrust rating setting (26k on our 800s, 24k/22k on the 700s) and two fixed derates at 24 and 22k on the 800, 22k/20k and 20k/18k on the 700s, there are three climb thrust settings as well (CLIMB, CLIMB1 and CLIMB2). Usually climb thrust is selected automatically based on fixed derate and assumed temperature (in short based on take off N1) and is user selectable to any other climb thrust setting. Climb 1 usually washes out by around 10.000ft and Climb 2 around 15.000ft.

In my outfit they changed the SOP to always select full climb thrust whenever an unrestricted climb is likely. Wouldn't make sense in the london TMA for example or around paris or moskow, but pretty much elsewhere it does. That way we nearly always have a pretty big increase in climb thrust at the thrust reduction altitude. By the way, full climb thrust in an unrestricted climb saves around 40 to 50kgs of fuel per take off. Doesn't sound much, but in a large fleet it makes a lot of money at the end of the year, and it is one of very few fuel saving measures that is actually fun.

[LeCoyote]

Thank you guys for your feedback.

TSIO540, what you're saying does indeed confirm the logical hypothesis that we had, ie. that the CL detent commands a "FLEX" climb thrust. One question though, if you have that info: what about thrust washout? When does it happen, approximately?

Intruder, indeed, the Boeing method has that difference from the Buses. I'll figure some way of integrating this in out ATC simulator logic. This is what my little inquiry is all about by the way.

Denti, I hadn't thought about the fuel savings of an unrestricted climb, although in hindsight it seems logical indeed. One more question: if you select a 18k derate on a -700, am I right to assume that your climb thrust (eg. CLB2 + perhaps assumed temp) will wash out to whatever is the MCT (max cont) for the 18k derate? Also, in the event of an unrestricted climb, would that mean selectint CLB instead of CLB1 or 2, and therefore using the 18k MCT at thrust "reduction" (heh) altitude?
I know it may sound obvious to all of you, I just need to confirm that I'm getting this right, that the flat derate method implies what I am currently only assuming.

Bonus question: my findings suggest so far that MCT is some figure arout 93% to 95% of max T/O thrust. Does that sound about right? Of course, I now assume that every Boeing derate setting (ie full, 20k, 18k on a -700) has its own MCT, whereas single-aisle Airbuses typically only use the "full" MCT when FLEX thrust washes out.

Also, I've just found out that on the B777, there are two types of washout tapers: a slow one that will last until about FL300, and a fast one that does is quickly by 12,000'. It is apparently not something the pilot can interfere with from the cockpit, but it can be changed. Another interesting bit of info :-)

As always, thanks for your valuable input! I'll go "push some tin" now ;-) Enjoy our weekend!
Cheers,
--
Romain

[Denti]

Well, it washes out to full climb thrust of the full engine rating, in case of a 18k derated 700 to 22k climb thrust. We do not use MCT anywhere except in an OEI case although climb thrust and MCT are pretty similar. The fixed derates are only used for take off until thrust reduction, not during flight. Therefore the climb thrust settings are based on full climb thrust, not on those fixed derates.

Unrestricted climbs always save fuel compared to step climbs, however unrestricted climbs at full climb thrust from thrust reduction on (in our case 1000ft) saves even more fuel as the efficient cruising altitude is reached earlier. Besides it provides a smaller noise footprint as well.

[RunSick]

About Airbus it says on the Performance section of the FCOM THRUST OPTIONS - FLEXIBLE TAKEOFF - REQUIREMENTS:

"-Thrust must not be reduced by more than 25% of the full derated takeoff thrust.
- The flexible takeoff N1 cannot be lower than the Max Climb N1 at the same flight condition.
-The flexible takeoff thrust cannot be lower than the Max Continuous thrust used for the final takeoff flight path computation (at ISA +40)"

(you can replace N1 for EPR in other engines)

A bit late but hope this helps.

[barit1]

The flexible takeoff N1 cannot be lower than the Max Climb N1 at the same flight condition.

Other than the fact this push-up for CLB seems (for pilots) counterintuitive, from a performance AND performance retention standpoint, it's nearly ideal. Modern aircraft and airfields usually permit LARGE TO thrust reductions, so no need to thrash the donks at this stage; now the most demanding part of the flight cycle is TOC. Let the reduced TO act as a warmup phase for the climb cycle. :ok:

[LeCoyote]

Good morning,

@Denti: thanks for correcting my assumptions. I think that almost completes the (abridged) picture for Boeing derated TO procedures and that's what I needed. I just have one more question about that washout bit: if you use CLB and not CLB-1 or -2, then there is there any washout at all as you climb? I'm thinking there shouldn't be, just want to make sure.

@RunSick:
Thrust must not be reduced by more than 25% of the full derated takeoff thrust.
The flexible takeoff thrust cannot be lower than the Max Continuous thrust used for the final takeoff flight path computation (at ISA +40)Let's rephrase this: FLEX thrust can be as low as 75% of full thrust, yet it is always more than or equal to Max Cont Thrust at ISA +40? So this MCT at ISA +40 is some sort of lower limit?

@barit1: indeed it sounds counterintuitive, and also to me as an ATC.

Based on all contributions so far, here is how I intend to describe a simplified behaviour for our Tower simulator:
- TO thrust is reduced, with a minimum value defined per aircraft type (we cannot used operator-based values here, but aircraft-based should be good enough). The method used will be assumed temp/FLEX, as I don't think we can incorporate Boeing's Derate + Assumed temp easily in this context.
- Climb thrust should never exceed 95% (to be adjusted) of maximum TO power. This should simulate some form of Max Cont Thrust.
- Whether the aircraft should be allowed to *increase* thrust at thrust "reduction" altitude should be a toggle switch, per-aircraft if possible, globally otherwise.

I'll also try to get in touch with a couple of carriers operating from LFPG and see whether they use increased CLB thrust to save fuel, even though there are usually steps at FLs 100-120 when there is downwind traffic across your path.

One last item I would like your input on is the difference in ROC between an unrestricted climb and a derated one. I guess that if the savings are minute (about 100kg?), the ROC can't be too different, maybe 500fpm max? Any figures to confirm or infirm that reasoning?

Thanks a bunch!
--
Romain

[Intruder]

FLEX thrust can be as low as 75% of full thrust, yet it is always more than or equal to Max Cont Thrust at ISA +40? So this MCT at ISA +40 is some sort of lower limit?
I don't think so, at least for Boeing and the GE CF6...

Since the allowed assumed temperature goes as high as 65C (or thereabouts), I must assume the minimum TO thrust is full TO thrust at 65C or ISA +50. In our operations, the OPS performance computer adjusts to also ensure no more than a 25% reduction.

[Denti]

@LeCoyote, exactly, when we user select full climb thrust there is no washout except the usual thrust variations with altitude.

In the combination of fixed derate and full derate we can reduce our take off thrust by more than 25%. A fixed derate is a new baseline on which the reduction is calculated.

Anyway, we do not fly into LFPG, only LFPO at the moment, although LFPG is usually our alternate.

[LeCoyote]

Thank you Denti. This leads me to one (hopefully) last question.
If you are doing a derated/flex takeoff AND climb, assuming A/T is on, what drives the airplane to increase N1/EPR (and therefore thrust) as the air thins out, and finally reach full-rated climb thrust? Is this logic governed by a minimum RoC, a minimum thrust, or something else entirely?
Huh, I just realised I probably used a very long and convoluted sentence for "what is the washout logic" or something. Bah ;-)

Cheers
--
Romain

[Denti]

Actually, good question. It doesn't really say in our manuals. However from observation it is governed by altitude on the 737. CLB-1 washes out at roughly FL100 and CLB-2 at roughly FL150, however any reduced climb thrust is in any case automatically deleted above FL150.

Just a few more tidbits from the manual: CLB-1 reduces climb thrust by 3% N1 or around 10% thrust, CLB-2 is 6% N1 and roughly 20% N1. If a reduced thrust takeoff has been specified on the take off reference page the FMC will select either CLB-1 or 2 so as to avoid a climb N1 greater than reduced take off thrust N1, however as both climb settings are fixed it can't really do it on max reduced take offs (Derate 2 and above roughly 50°C assumed temperature).

[LeCoyote]

Ok, thanks a bunch. I guess the bottomline is, in our environments where an unrestricted climb is inlikely (LFPO and LFPG are fairly similar in that regard), thrust doesn't increase until about FL100. Since my work here is focused on the initial part of the climb, I'll just recommend that washout occurs above FL100 as a basic rule, and that should work in a fairly realistic way.

Thank you all for your input!
Cheers
--
Romain

[TSIO540]

TSIO540, what you're saying does indeed confirm the logical hypothesis that we had, ie. that the CL detent commands a "FLEX" climb thrust. One question though, if you have that info: what about thrust washout? When does it happen, approximately?

When a FLEX temp is entered, placing the thrust levers into the MCT/FLEX detent gives a FLEX Takoff thrust setting (different to CLB THR).

I'm not familiar with "thrust washout" & I haven't seen it mentioned in any Airbus manuals thus far. I can only guess you might be referring to thrust reduction at the end of the takeoff???

At the designated acceleration altitude, if still on both engines, thrust is reduced to the CL detent which sets CLB THR.

CLB THR for the IAE V2527 is usually in the vicinity of 1.400 EPR and rises to around 1.600 EPR in the high thirties. It is determined through the optimization function of the FMGC by referring to entered cost index, temp, level and weight etc. (which selects a thrust setting for the level you're at and then gives an optimum speed to fly at for that given thrust setting - called OPT CLB) It is possible to increase thrust to MCT or TOGA but not desirable from a fuel burn and engine wear perspective.

Let's rephrase this: FLEX thrust can be as low as 75% of full thrust, yet it is always more than or equal to Max Cont Thrust at ISA +40? So this MCT at ISA +40 is some sort of lower limit?

To add detail to Runsick's post from FCOM 2.02.14 P14

- Thrust must not be reduced more than 25% of the full rated takeoff thrust (TOGA)
- The flexible takeoff EPR cannot be lower than the max climb EPR for the same conditions. The FADEC takes the above two constraints into account to determine the flexible EPR
- The flexible takeoff thrust cannot be lower than the max continuous thrust used for the final takeoff flight path computation (@ ISA +40) This constraint limits the the maximum flexible temperature at ISA +42 (57 degrees at sea level)
- the flexible temperature cannot be lower than the flat rating temperature, TREF, or the actual OAT

[mutt]

I'm not familiar with "thrust washout" & I haven't seen it mentioned in any Airbus manuals thus far. I can only guess you might be referring to thrust reduction at the end of the takeoff??? if you takeoff with reduced climb, as the rate of climb decreases the thrust will increase until you have actually reached max climb thrust, so the reduced thrust was washed out. As an airline you can also set the altitude that you want reduced climb thrust to be washed out.

Mutt

[LeCoyote]

Yep, I was referring to the process where derated TO thrust washes out to max climb thrust, thank you mutt.

I now realize that the logic behind this can be quite complex, considering the CI and other factors, so we will have to figure something out to simplify that behaviour, and ignore the fact that it can be aircraft- or airline- specific.

Cheers
--
Romain

[TSIO540]

if you takeoff with reduced climb, as the rate of climb decreases the thrust will increase until you have actually reached max climb thrust, so the reduced thrust was washed out. As an airline you can also set the altitude that you want reduced climb thrust to be washed out.

Thanks for the explanation mutt :ok:

I've not read about anything like this in any of the Airbus manuals. Could it be just a Boeing thing? Or have I just missed it?