B757 Assumed temperature T/O thrust time limit?

[boxjockey99]

Hi all

I have a dilemma, I am about to go in the sim for an apparently quite challenging profile and need to find some info on the above. I know that the B757 (RB211-535-E4) has a max TOGA time limit of 5 mins but I hear rumours that this doesn't apply if you have an assumed temperature set. now this to me is common sense as with an assumed temp you will be taking off with less than max thrust BUT where is it written that Assumed temp cancels the TOGA time limit? I'll try to put this in context:

A/c takes off full thrust (1.72 EPR) and loses an engine, the book says that you are limited to 5 mins so in a limiting place such as Chambery or Geneva with a long emergency turn you're going to be in a situation where you need to reduce thrust to MCT earlier than your normal profile would have you do.

so the alternative

A/c takes off at say 39 Degrees Assumed temp and gets lets say 1.68EPR. Same thing happens engine goes bang and you have a long drawn out emergency turn procedure. Notwithstanding knowing all the percentages of the MCT performance bracket of the engine where do I now find myself when I hit the magical 5 mins?:ugh::rolleyes:

I know from experience in the box that often if you have taken of at a high assumed temp (+54) then you often end up increasing power on the live engine hence I would suggest in this case reducing thrust is not really an issue but the point remains where is it written that when using an assumed temp (no matter what that might be) means we can dispense with the 5 minute limit and happily follow the standard EFATO profile no matter how long the emergency turn.

Your help is greatly appreciated:D

[Waldo]

Simple answer is that once your engine goes bang it is normal procedure to apply full power immediately on the remaining engine as this is what the single engine performance is based on. After the 5 mins is up you would need to assess the situation. If critical continue at full power or if not select max cont power.
You would never normally require more than 5 mins at full power. The only place I know of in Europe is Innsbruk. As we operate into this airfield our 757's E4 engines have been upgraded to 10mins at full power.

[boxjockey99]

Waldo

Thanks for your quick response,

This is interesting as our training says that the perf we use is predicated at remaining at assumed temp thrust with the engine out thus you DON'T apply full power or rather you don't HAVE to.

The understanding is that you are taking a reduction in thrust to an assumed temp that will allow the remaining engine to lift the given weight taking into account all the usual perf stuff like obstacles and field length etc. Under your system as you say there is no question as you have full thrust but we remain at the assumed temp thrust and thus the question remains.... when do I reduce if at all?

[Lord Spandex Masher]

Notwithstanding that - if you need it, use it!

[punk3029]

Waldo is not correct. The performance is based on the assumed calculated temp. This sould clear all obstacles. You can however always select full thrust if desired, but not necessary.
i,am not sure for 757 but on (at least our) 737 we have a 10min thrust limit once SE If you use full thrust then it's only alowed for max 10 minutes. so our normal ops is after cleanup to selct Max Cont. Thrust (MCT) to continue the climb.
Only in special cases you could run into the 10min max. When making a SE-go-around with a very high accelaration segment you could run into the time limit.

[boxjockey99]

Lord Spandex Masher

Yes I agree in the real world I would indeed use everything I have and be done with it but in the virtual world a TRE is very likely to simply fail the other engine at the 5 minute point making for a rather unpleasant debrief hence the question so when that point arrives I have something to use in my defense if I was using assumed temp.

Punk

You are talking about the same problem i.e. FULL thrust has a 10 min (737) limit. I am talking about the assumed temperature scenario where you are already derated by virtue of the assumed temp method below MAX TOGA so do you need to worry about the 5 mins? it is a grey area I fear between the TOGA limit and the MCT limit, if you are between the 2 limits when using assumed temp then what is your time limit?

[deltahotel]

You don't have to take full power following an EFATO - see how the ac performs. However, pre briefing (and then doing) 'full power on the live engine please' is one less decision to have to make at a time of great stress!

[punk3029]

Yes, good question

If you are above MCT i would say, safety wise, the time limit for max thrust (5/10min) because you are over your continuous setting.
In practice often when setting MCT after an engine failure in our operation we can ADD thrust, since selected temp often reduced the thrust to below the MCT setting.

[misd-agin]

Never seen a 'assumed/derated' takeoff thrust limit, so it's 5 minutes(or 10 mins), then CONT thrust.

[Bealzebub]

Max T/O thrust is limited to 5 minutes, unless an engine fails in which case it is extended to 10 minutes. Normal 2 engine maximum thrust can be extended to 10 minutes by certification. This is often done for operations from airports such as Innsbruck where more than 5 minutes at fully rated thrust may be required (although in practice it usually isn't.) Assumed temperature is used to de-rate the take off thrust, so the point is generally redundant if you are not using maximum rated take off thrust.

Note that N1, N2 and N3 pointer and counter displays are inhibited from turning white to amber, if they enter the (near limit) amber band during take off or go around (TOGA) for 5 minutes after the band is first entered. However if engine failure occurs in that 5 minute period, the inhibit period increases to 10 minutes.

certificate data docs. (http://www.caa.co.uk/docs/1419/SRG_PRO_1044%20Iss24.pdf)

[EW73]

Yes, I agree with 'punk2039' here, any thrust setting in excess of MCT will be subject to the appropriate time limit, it doesn't have to be pushed all the way up to incur the limit time.

EW73

[Denti]

Yup, if you do not know if you are above or below MCT, just set it at the time limit. If you have reduced take off thrust in many cases you will add thrust at that point, not reduce it. Dunno if the 757 has fixed derates used together with ATM, in that case it is actually absolutely normal to increase thrust with setting MCT.

[barit1]

Just a word from an engine guy: I'm fully aware of the 5 min. TO limit, and I also know that an extension to 10 min. (authorized in some jurisdictions) is merely a paperwork exercise (maybe with some shekels crossing the palm) - no spanners ever touch the engine.

And in my career doing performance tests on engines, we started with a 5 min. warmup at MCT, then pushed up to rated TO for 5 min., backed down a few rpm for another 5, then an alternate TO rating for another 5, etc. for a total of 30 to 45 minutes ABOVE MCT. I've done this DOZENS of times, never any hint of an incipient failure. (I shudder to think of the JetA I've burned...)

My message to pilots: Find out what your examiners expect you to know, but when one's churning and one's burning, FLY THE AIRPLANE and don't worry about that other donk! :ok:

[Jonty]

Its a fairly simple answer. If you took off with an assumed temp set (thrust derate) then you are not at TOGA and the TOGA limit does not apply. If you subsequently set TOGA then the clock starts running.

If you need TOGA for longer than the limit, then use it. There is no point in reducing thrust and hitting a mountain. However the time limit should be taken into account by the company when producing the performance profile for the aircraft.

[Checkboard]

If you took off with an assumed temp set (thrust derate) then you are not at TOGA and the TOGA limit does not apply. If you subsequently set TOGA then the clock starts running.
Wrong, I'm afraid. The limit applies if you are above Maximum continuous power. The clue's in the name! If you are running the power continuously (i.e. without a limit) than MCT is the maximum you may set.

At the time limit, simply call "Set MCT" to the other pilot - if they increase the power to do that, fine, if they decrease it - then you have just passed (that portion) of your check ;)

[Jonty]

Er No, its a TOGA limit. The book says "the time limited at TOGA thrust" it doesn't say "the time limited above MCT"

Just to add to this, its actually an EGT limit. 850 deg C for 5 min. SO you could argue if your at 849 deg C the time limit does not apply.

Why I still have the books I don't know, I haven't flown the thing for 5 years!

[rudderrudderrat]

Hi Jonty,

Take Off / GA thrust is what you set to do one of those procedures - it is doing what it says on the tin. You are being checked in the sim to apply the approved procedures within the time limits for your engine settings.

Just to add to this, its actually an EGT limit. 850 deg C for 5 min. SO you could argue if your at 849 deg C the time limit does not apply.
Are you really serious?

[Jonty]

Boeing SOP, Chapter 1 Limitations, 1.7 Engine, 1.7.2 EGT limits.

Have a look.

[rudderrudderrat]

Hi Jonty,

Do you really believe that Chapter 1 Limitations, 1.7 Engine, 1.7.2 EGT limits is saying the material used for the turbine blades is happy for continuous use at 849 degs C?

[Wizofoz]

So Jonty, are yours saying that if you set 99 % of TOGAmthere is no time limit?

What relevance do you say MCT has?

[aviatorhi]

From the nice people at the FAA (I'm sure similar language can be found around the world):

"Maximum continuous thrust", with respect to turbojet engines, means the maximum jet thrust that is developed statically or in flight, in standard atmosphere at a specified altitude, within the engine operating limitations established under Part 33, and approved for unrestricted periods of use.

This would mean any thrust higher than MCT has a "restricted period of use". But if you want to trash the engines and then wonder why you have the highest IFSD tally at your company, go on ahead.

[Jonty]

What I am saying is that the Boeing limit is an EGT limit, not a thrust limit. Airlines then convert this limit into an SOP. The airline I work for its the same as most other airlines. TOGA thrust for 5 mins. But that has not come from Boeing, its come from the airlines. The EGT limit for MCT is 790 according to that chapter. I said you COULD argue (not that I WOULD) that you were not at the limit, so a time limit would not apply, but we are now into the realms of legalese.

Back to the OPs question. TOGA is for 5 mins, derate isn't. If you now want to know if the derate thrust setting is above MCT you would have to select CON on the TMSP and see what its giving you, or look it up in FCOM1. You would then have to make a decision. Do we not think this is slightly unfeasible during a V1 cut, maybe in the middle of an emergency turn or dealing with a terrain issue? It seems, having spoken to other 757 operators that Boeing thinks so, and that the SOP in most airlines is that if you select TOGA you have a time limit, if you remain at the derate setting then you do not. If your training department says otherwise that you will have to go with what they say. But I would probably question their priorities.

[Wizofoz]

TOGA is for 5 mins, derate isn't.

So, if Derated TO thrust is more than MCT, how long are you saying you can maintain it for??

[Jonty]

As long as you think its required. Or would you disagree with that?

[Checkboard]

I would disagree with that.

[Jonty]

Really, you might want to have a good long think about that statement.

[misd-agin]

Jonty - so if derated T.O. thrust is above MCT you have no time limit?

wow.

[Wizofoz]

As long as you think its required. Or would you disagree with that?

Yes.

Maximum Continuous is the Maximum you can use Continuously.

A 34 deg Assumed Temp might only be a couple of %N1 less than TOGA- WAY above MCT. And you think there is no time limit on that?

[barit1]

Do you really believe that Chapter 1 Limitations, 1.7 Engine, 1.7.2 EGT limits is saying the material used for the turbine blades is happy for continuous use at 849 degs C?

From a metallurgical point of view, it's TOTAL TIME AT TEMPERATURE that counts. Whether that's 5 minutes today, 5 minutes tomorrow, 5 minutes next Monday - or whether it's a half-hour all at once - makes little difference to the molecules!

(In fact, I'd argue that one cycle of 30 minutes is less fatique than 6 cycles of 5 minutes.)

The 5 (or 10) minute regulatory limit is plainly outdated in light of modern materials and technology. Instead, it serves a convenient CYA function for manufacturers.

[PA38-Pilot]

A 34 deg Assumed Temp might only be a couple of %N1 less than TOGA- WAY above MCT. And you think there is no time limit on that?

Exactly. Anything that is, or could be above MCT, HAS a time limit.

[Jonty]

Do you really believe that Chapter 1 Limitations, 1.7 Engine, 1.7.2 EGT limits is saying the material used for the turbine blades is happy for continuous use at 849 degs C?

I would say that the engine is entirely happy at the TOGA EGT limit for a hell of a lot longer than 5 min.

As for assumed temperature take offs, here's the deal: there is no requirement for you to use TOGA thrust to meet the performance climb gradient. Therefore you do not need to select it. If you do select it there is a time limit on its use, and after 5 or 10 mins you should select MCT (or CON). Even if you are using an assumed temperature for take off you should select CON after 5/10 mins or after the acceleration phase as per your airlines SOP.

HOWEVER, if you get to 5 or 10 mins and you feel you need to use it for a longer period of time then USE IT! Who gives a rats about the time limit. Take heed of this wizofoz, or you may need to reassess your priorities.

What I was trying to convey, was that there is nothing written that specifically applies a time limit to any power setting other than TOGA. We all know whats implied by the use of the words Max Continuous Thrust, but by the strict letter of the book it is a time limit of 5 or 10 mins at an EGT of 850 deg C.

If someone can provide a Boeing FCOM reference that says otherwise I'm quite happy to say I'm wrong.

[Jonty]

Just to go back to the OPs original question.

If your airlines has no SOP about using TOGA (or any setting above CON) in the event of a longwinded emergency turn, then I suggest you ask what they want you to do.

My suggestion would be to leave TOGA (or any setting above CON) where it is until A) you are above MSA, or B) terrain separation is assured or C) the emergency turn is completed.

Would anyone disagree with the above? Or would we all like to hit the hill, but have nicely under stressed engines?

Practicalities ladies and gentlemen.

[Checkboard]

there is no requirement for you to use TOGA thrust to meet the performance climb gradient.
No one disagrees with you there.

Even if you are using an assumed temperature for take off you should select CON after 5/10 mins
No one disagrees with you there.

HOWEVER, if you get to 5 or 10 mins and you feel you need to use it for a longer period of time then USE IT!
No one disagrees with you there.

there is nothing written that specifically applies a time limit to any power setting other than TOGA.
Have you looked up the engine section of the FCOM on MCT? It's pretty obvious in any case, isn't it?

We all know whats[sic] implied by the use of the words Max Continuous Thrust,
Do you really? Up to now you seem to be having a great deal of trouble with it.

[exeng]

Can't be bothered with all the details, but just to say if you are above MCT the next limit applies - obvious really.

Having said that I agree with the previous posters that if terrain clearance is in doubt you keep the remaining engine firewalled. In the sim you should be given a realistic scenario, but nevertheless the same rules apply.

[ldo]

From an old Nick Lappos post in Rotorheads:

"Arm,

You ask a good question. Let me try and explain it this way:

Imagine that each minute at TO power extracts one bean from a big jar of beans. One minute, one bean. If you stay at low cruise power, say 65% power, then five hours takes one bean out of the jar.

There is nothing unsafe about running at TO power, it is not "beyond limits". But if there are 10,000 beans in the jar, then you only get so much life from your engine, transmission and drive train.

We manufacturers set a 5 minute limit per takeoff as a way to remind you of that. Inside the factory walls, we imagine that you make about 2 1/2 takeoffs per hour (don't try a 1/2 takeoff, it should only be done by professionals!). We calculate that if one does 2 1/2 TO's per hour, and the rest in cruise then the power systems will last 3707 hours, and you will have performed 9200 takeoffs.
If you make more TO's, plan to overhaul sooner, if you make less, your engine will last a lot longer.

The numbers above are made up, but the idea that a takeoff is several times more damaging than a cruise hour is entirely true.
No thundering limit, where 6 minutes at TO power will end your life. Just a faster draining of those beans from the jar."

http://www.pprune.org/rotorheads/178645-take-off-power-how-often.html#post1942676

[lomapaseo]

I'll have to admit that I got a little lost in all the preceeding posts so I may not fully understand the original question.

given that there is a major annomaly in the operation of the aircraft e.g. an engine out the type certificate I read seems to put the time limit on the temperature limitation at 10 mins.

It defines the limitation based on EGT readings although the thermal damage if any is likely to be in the HPT.

Under most conditions all the thrust and aircraft performance that you need should not even approach the temp limitation. But for those conditions where it does approach or exceed the temp limitation your sense of the onset of serious damage will be a step change increase of EGT above the limit

just food for thought

[Wizofoz]

HOWEVER, if you get to 5 or 10 mins and you feel you need to use it for a longer period of time then USE IT! Who gives a rats about the time limit. Take heed of this wizofoz, or you may need to reassess your priorities.


I take my priorities very seriously thanks.

As such, I work for an airline that has engine out procedures that are designed to have the aircraft clean and clear of terrain inside the (for us 10 minute) limit for operation at greater than MCT.

If circumstances meant needing even TOGA power for longer to avoid terrain, of course I would do it- knowing I was exceeding a limit.

What YOU are suggesting is that the limit doesn't exist.

You're wrong.

[4dogs]

Checkboard (bless his AN socks) and Wizofoz are correct.

The limits on the engine are not managed by the engine certification data - they are managed via the Type Certificate Data - and there are occasionally differences.

Not all engines allow 5 minute AEO TO thrust to be used for 10 minutes OEI.

If the take-off is a derate - no thrust increase is permitted as the speeds are predicated on lower minimum control speeds

If the take-off is an assumed temperature flex takeoff, thrust may be increased to actual max TO thrust if required - noting that it is not necessary because terrain clearance should be assured if the RTOWs are correctly designed.

The sim scenario would seem to be designed to ensure that you understand the engine limits and the various thrust options for takeoff and continued flight. I'm thinking you need to start a clock at TO thrust application in any event - if you subsequently suffer a failure, any increase in thrust will be "money in the bank" for terrain clearance and MCT should normally be selected at 'aircraft clean' and inside the relevant time limit (unless you are conducting an approved procedure that permits you to accelerate at MCT with slats/flaps still deployed).

As an aside, the range of responses suggest that there are some Heads of Training and Checking who need to do their jobs better!!! :mad: :mad: :mad:

Stay Alive,

[Jonty]

I will come at this another way to try and explain my point.

The engine has a limit of 5 mins at TOGA (850 deg C), at the 5 mins point I select CON (750 dec C) how long is it before I can reselect TOGA, and what is the time limit when I do? I could also ask how many times am I aloud to re-select TOGA on a single rotation. Is the 5 minutes a cumulative total or a total for a single application? None of this is mandated anywhere, we have to use our common sense.

According to the letter of the book, it would be perfectly acceptable to use TOGA for 5 mins then select CON for 30 seconds and then re-select TOGA for a further 5 min. However, we don't live in that world and in practise most of us would find this unacceptable abuse of the engine.

So what exactly is my point? My point is that there are very few absolutes in aviation. And with a little extra knowledge we can make educated decisions about such things. Hopefully in the full knowledge of what the limits are, and the likely consequences of operating beyond them. Its why we get paid the big bucks.

[londonmet]

Jonty,

Our airline (mixed fleet) contacted the various engine manufacturers about this. the 5/10 minute rule is a THRUST limit and not a time limit. So regardless if the EGT achieved is less than the maximum you MUST select a lower THRUST setting - unless terrain is a factor then all bets are off.

[aviatorhi]

Jonty Says:

The engine has a limit of 5 mins at TOGA (850 deg C)

Well, in effect you're saying that when it's -40 in FAI I can push the engine to 120ish% N1 (red line 105%) and still not be at the engine limit... I don't think so.

FAA Says:

within the engine operating limitations established under Part 33

FAR 33.7(c) applies, and I'm sure similar certification criteria can be found around the world.

Takeoff thrust is limited to 5/10 minutes, as appropriate;

When commanding takeoff thrust you cannot cross the red line on any limitation (ie. EGT), though, on very cold days, at sea level, etc. some (possibly all) of the red line limits may not be reached when takeoff thrust is achieved. Ever heard the phrases "temperature limited"/"field limited"/"climb limited"/"altitude limited" etc.

When commanding climb thrust you have a set of limits, which again must be followed, though climb thrust may occur before reaching those limits.

Same goes for MCT and GA, etc. etc. etc.

That being said, every manufacturer has built a bit of fudge factor into the engines. I prefer not to test it with all the fans spinning. When pucker factor becomes an issue... I'll start testing.

[Jonty]

Well, in effect you're saying that when it's -40 in FAI I can push the engine to 120ish% N1 (red line 105%) and still not be at the engine limit... I don't think so.


I would like to see you try! These things have EECs to ensure you don't.

But maybe what I am saying is that if you select TOGA thrust and your EGT is below 750 deg C then the time limit would not apply.

I have yet to find where is says the EPR time limit (or any time limit on any EPR setting). Maybe someone can point me in the right direction?

Here is the JAR type certificate for the B757-200:

http://www.caa.co.uk/docs/1419/srg_acp_fa28-05.pdf

Maybe you could point to where the EPR time limits are established? The rotational limits are there but not the EPR (thrust) limits. Surprisingly the EGT limits specified are above those in the Boeing FCOM.

[aviatorhi]

These things have EECs to ensure you don't.

These (my) things have a cable linkage connected to an FCU with no "protection" other than the firewall.

[boxjockey99]

Wow,

I didn't think this subject would stir up anything like 3 pages worth of discussion. I have spoken at length to various peeps in my company and also studied my sim prep and it would appear that in this case the question is moot as the profile itself prohibits assumed temp takeoff as the runway is contaminated but the discussion has been very useful to see various sides of the argument.

The simplest mental picture I have should the same profile be put but at an assumed temp is that I do what the book says and simply adhere to the 5 min limit then set MCT. In many cases this gets me more thrust but in the odd occasion with low assumed temps then it might well mean a reduction. As at this point we are down to one engine I would suggest that not kicking the wotsits out of the other one unnecessarily would be preferred to creating a 100Tonne glider.

I started this thread to see if anyone knew of any actual paper document that says where assumed temp is used then the limit is out but this fabled document seems not to exist so I will personally be going for MCT at 5 mins and passing my sim check (hopefully)

May thanks for all the contributions

[aviatorhi]

boxjockey...

The maximum takeoff thrust limit is 5/10 minutes, using anything at or less than MCT (as it is the first setting at or below which a time limit does not exist) you are not limited/required to stay at that setting for 5/10 minutes.

The "paper" you're looking for is the definition of the term. "Maximum takeoff thrust" does not necessarily equal "reduced takeoff thrust", "assumed temperature" and so on, unless you have chosen a non-derated/non-reduced takeoff.

[AeroTech]

Hi,

Just a word from an engine guy: I'm fully aware of the 5 min. TO limit, and I also know that an extension to 10 min. (authorized in some jurisdictions) is merely a paperwork exercise (maybe with some shekels crossing the palm) - no spanners ever touch the engine.

And in my career doing performance tests on engines, we started with a 5 min. warmup at MCT, then pushed up to rated TO for 5 min., backed down a few rpm for another 5, then an alternate TO rating for another 5, etc. for a total of 30 to 45 minutes ABOVE MCT. I've done this DOZENS of times, never any hint of an incipient failure. (I shudder to think of the JetA I've burned...)

My message to pilots: Find out what your examiners expect you to know, but when one's churning and one's burning, FLY THE AIRPLANE and don't worry about that other donk!

Barit, are you talking about installed thrust or uninstalled thrust?
I think you are talking about engine on a stand.

During takeoff, pneumatic/electric/hydraulic power are extracted from engine. In addition there is engine air inlet fitted in the engine. These factors affect the engine.

Takeoff thrust may be used only for a maximum of five minutes, with one exception: the time limit is extended to ten minutes for airlines that have purchased a special Airplane Flight Manual appendix called the “ten-minute appendix”. The Certificate Limitations section of that appendix states specifically:
“The time limit on the use of takeoff thrust is increased to 10 minutes provided this use is limited to situations where an engine failure actually occurs and there is an obstacle in the takeoff flight path.

I think this quote is from Boeing document.

[barit1]

Barit, are you talking about installed thrust or uninstalled thrust?
I think you are talking about engine on a stand.


Fair question, and you are correct about my example being on a test stand. But as long as temp values (and rpm, oil pressure etc.) are within redline, then my point is there's nothing magic about the time limit on the metallurgy. In fact, typically the engine may run a few degrees cooler after a minute or two at TO, as clearances stabilize.

The exception may be when climbing through a temperature inversion (i.e. temp profile is less than the adiabatic lapse rate). If the engine control doesn't accommodate this, then turbine temp may rise from when on the runway.

[misd-agin]

boxjockey - 3 pages? If you remove the posts by several posters stating that Jonti is wrong, and his posts restating his position, it would probably still be on the first page.

[Pugilistic Animus]

I definitely think the time limit above MCT is important and that exceeding MCT after the the relevant time limit, even if only by a small margin is not allowed.

I doubt that the performance experts would leave one so badly placed on the OEI escape route that one would have to exceed MCT. However one can end up badly placed in the route for other reasons such as a load sheet error...therefore, if the need truly arises to use more thrust, up to full thrust I would definitely not worry about the time-limit...I don't think the engine will simply disintegrate because one has exceeded the time limit...:)