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B757 Assumed temperature T/O thrust time limit?

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B757 Assumed temperature T/O thrust time limit?

Old 16th Feb 2012, 08:55
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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.
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Old 16th Feb 2012, 09:07
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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.

Last edited by Jonty; 16th Feb 2012 at 09:49. Reason: Add type certificate
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Old 16th Feb 2012, 09:45
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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.
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Old 16th Feb 2012, 15:56
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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
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Old 16th Feb 2012, 17:37
  #45 (permalink)  
 
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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.
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Old 16th Feb 2012, 20:53
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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.
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Old 16th Feb 2012, 21:12
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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.
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Old 17th Feb 2012, 17:46
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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.

Last edited by misd-agin; 17th Feb 2012 at 21:57. Reason: spelling
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Old 17th Feb 2012, 19:35
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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...
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