Usind the Assumed Temp/Thrust Derate - the Engine failure scenario

[john_tullamarine]

Often it's a case of nil prior exposure ...

I recall a similar sim exercise on the 732 wherein I introduced min speed schedules and a significant failure (the operator had a large bird strike modelled from the FDR record of an inflight event) ... min V1 birdstrike opened a few eyes .. a couple of runs and the problem was overcome .. the concern for me was that I didn't think it a good idea for folk to be training at high weight/speed schedules (this operator used high V2 overspeed schedules routinely) .. but then having the occasional min weight/speed schedule positioning flight ...

[Fredairstair]

I have a question..........

Can anyone tell me why in the engine failure case a fixed derate is considered a limitation whereas an assumed temp reduction is not? (Boeing FCTM)

Thanks.

Fred.

[john_tullamarine]

A read through the earlier posts might give you the clue ...

(a) the limitation is whatever rated thrust you are operating ... it is on this rating that the certification is based.

(b) if you are operating at a derate thrust limit, that lesser thrust becomes the limit for the takeoff .. ie one ought not to increase thrust beyond the derate value

(c) if operating flex, the limit is the rating on which the flex is based, whether max rated or derated ...

[Pugilistic Animus]

Ahh----this thread is like the oldern days


Welcome Back Old Smokey!!!


Pugilistic Animus

[Old Smokey]

Hey Pugilistic Animus, great to see your name in the lights again. Stand by for E-mail.

John_T, likewise, pardon my rudeness in not responding to your recent PM, stand by for incoming.

Fredairstair, to add to John_T's wise words, actually your question in many ways summarises what this entire thread is all about.

"Can anyone tell me why in the engine failure case a fixed derate is considered a limitation whereas an assumed temp reduction is not? (Boeing FCTM)"

The Derated Thrust IS a limitation. V1, Vr, and V2 are based upon Vmcg and Vmca (as appropriate). V1 must not be less than Vmcg, Vr must not be less than 1.05 X Vmca, and V2 must not be less than 1.1 X Vmca. Any increase in thrust beyond the Rated Thrust used to calculate the V speeds leads to higher Vmcg / Vmca, and the entire V1/Vr/V2 package is out the window, and the performance is invalidated. Thus, when using V speeds based upon a Reduced Thrust Rating, it is an absolute limit....end of story.

Now we come to Reduced Thrust (Assumed Temp / Flex). Reduced Thrust is calculated in accordance with the Thrust Rating being used, be it Full Thrust, or any of the Derated Thrusts available. Although the actual thrust when using reduced thrust is less than the Rated Thrust being used (the limit thrust for the particular operation), Vmcg and Vmca are still based upon that for the Rated Thrust from which the Reduced Thrust is derived. There is NO CREDIT for reduction in Vmcg / Vmca at the Reduced Thrust, thus, if you wished to, you could push the thrust to the Rated Limit in use with safety. You most definately should not exceed the Rated Thrust in use, as a whole new set of VMCs come into play.

If numbers make a more convincing argument, consider the following V1 speeds (AS LIMITED BY MCG) for the B777-200ER/Trent892 engines. (admittedly these speeds are for low weights, but low weights are the VMC danger zone).

For 30°C/86°F at Sea level, comparing V1 speeds for Full Takeoff Thrust, 8% Derate (TO-1), and 20% Derate (TO-2).

TO : V1(MCG) = 122 Kt
TO-1 : V1(MCG) = 117 Kt
TO-2 : V1(MCG) = 109 Kt

So let's say that I'm doing a low weight Takeoff using TO-2 and the V1(MCG) of 109 Kt. An engine fails right at the 109 Kt V1, and I GO. If I leave the thrust alone at no more than TO-2, I'll have a good day. If I advance the thrust to Full TO at my V1 speed of 109 Kt, I'm a full 13 knots below VMCG, my rudder effectiveness is only 80% of that required, directional control would be totally lost, and I'm going to crash.

Do the numbers and the words require any further development? I don't think so.

A FIXED DERATE IS A LIMITATION.

Regards,

Old Smokey

[Fredairstair]

Old Smokey,

You're a gent, thanks for that.

I'm just off now to cut and paste your reply to my fleet manager.

Best regards,

Fred.

[fleigle]

Just looking at the roll-out of the White Knight 2 and was in wonder at the possible huge thrust asymmetry, that will be an interesting test!!

[john_tullamarine]

couple of comments ..

(a) you could push the thrust to the Rated Limit in use with safety

but do it reasonably slowly to avoid any dynamic effects which might surprise the chaps up front

(b) the certification Vmc speeds are constrained for a variety of variables and the today's failure real world figures generally will be lower ... the problem is that the pilot doesn't know where the real world figure is .. so one is better off treading warily and conservatively. CG, especially, is a significant driver.

[alf5071h]

For those who would like to read the certification basis for the discussion on Detrate / Reduced and why derate has a take off limit see CS 25 Large Aircraft Amendment 4. AMC Appendices, AMC 25.13 (about page 699).

A few extracts:-

Reduced take-off thrust, is a take-off thrust less than the take-off (or derated take-off) thrust. The aeroplane take-off performance and thrust setting are established by approved simple methods, such as adjustments, or by corrections to the take-off or derated take-off thrust setting and performance. When operating with a reduced take-off thrust, the thrust setting parameter, which establishes thrust for take-off, is not considered a take-off operating limit.

Derated take-off thrust, is a take-off thrust less than the maximum take-off thrust, for which exists in the AFM a set of separate and independent, or clearly distinguishable, take-off limitations and performance data that complies with all the take-off requirements of CS-25. When operating with a derated take-off thrust, the value of the thrust setting parameter, which establishes thrust for take-off, is presented in the AFM and is considered a normal take-off operating limit.

And an interesting snippet re differences between take off and in-flight; which presumably also applies to max thrust or ‘uprates’ used in windshear escape.

The AFM limitations section should indicate that when operating with derated thrust, the thrust setting parameter should be considered a take-off operating limit. However, in-flight take-off thrust (based on the maximum take-off thrust specified in the basic AFM) may be used in showing compliance with the landing and approach climb requirements of, provided that the availability of take-off thrust upon demand is confirmed by using the thrust-verification checks.

Previous/similar discussion here: http://www.pprune.org/forums/flight-...e-failure.html

[galaxy flyer]

OS:

You bring up an interesting point, while Vr and V2 are limited at 5% and 10% above the configured Vmca; V1 is only limited by Vmcg with no "pad" to cover the pilot. And, as several knowledgeable types have noted, Vmcg demonstrations for the regulatory authorities are pretty much a circus act. The pilot knows it is coming, there is no allowance for reaction time and all things are favorable. The real world pilot, on his 14th hour of duty doing a take-off on a 25m wide runway (I've been there) is really "on the edge" if one donk quits at V1 where Vmcg was limiting.

GF

Oh, and there is a 10 knot crosswind on the failed engine side, which isn't computed for in any civil charts.

[Old Smokey]

A few interesting comments arise in the last 3 posts, starting with the venerable John_T

My comment "you could push the thrust to the Rated Limit in use with safety", lead to John's comments of "hmmm, OK, but do it carefully" (or words to that effect)

I agree entirely, from the certification and legal point of view, you could push up the thrust in a Reduced Thrust takeoff to the Rated Thrust in use, but.....Vmcg IS a risky area not to be fiddled with. As I alluded to in an earlier post on this forum, I prefer (and so does my company) to leave the reduced thrust as it is, enjoy the improved controllability, in the acceptance that, from a performance point of view, aircraft performance without increasing thrust should be quite satisfactory.

alf5071h, in his usual "spot on" style has quoted wise regulatory words. Readers beware that the first quote relates to Reduced Thrust (not De-Rated Thrust) and is NOT a limit. Alf's third paragraph then goes on to say that De-Rated Thrust IS a limit. Spot on Alf, but readers beware of the oft confused difference between Reduced Thrust and De-Rated Thrust.

galaxy flyer, I'm totally in agreement, the "padding" that we enjoy in so many other performance areas is simply not there with Vmcg. True, Nosewheel steering inop is considered for the Vmcg certification, and you MIGHT have NWS available to you to significantly assist with directional control problems. I say MIGHT because on many aircraft types, NWS is programmed to become progressively reduced in authority as speed increases, up to the point with some aircraft where it is TOTALLY unavailable at V1 speeds. That "padding" is now gone, there is an unfavourable cross-wind not accounted for in certification, and you're in big trouble.

One of the frequent contributors on this forum (I dare not name him as he values his anonymity as I do mine) has had significant exposure to Vmcg testing on a certain airline aircraft. Try to get him wound up if you can, his tales of imminent aircraft losses in Vmcg tests are truly frightening. As galaxy flyer points out, the test pilots knew it was coming, the shagged out pilot at his duty limits may not cope so well.

To end with a personal anecdote, by coincidence I operated a short sector yesterday on a low weight B777 using TO-1 and Assumed Temperature 60°C, a VERY low thrust setting. The F/O flew the sector, and, with this forum's wise words still in my mind, added to the briefing "If an engine fails at V1 don't even think about increasing the thrust". A very "switched on" guy he was, fully accepting and understanding the rationale behind my briefing. Maybe he's a Prooner too?

How many lives / incidents / accidents has PPrune saved? I suspect many, PPrune in my opinion has significantly contributed to aviation safety, and continues to do so. And Prooners, take it easy on the occasional total jerk who disturbs rational comment on these forums with "way-out" and crazy ideas. We learn something from them too! We learn what NOT to do when operating valuable aircraft with even more valuable human lives on board.

Best Regards,

Old (and getting older) Smokey

[mcdhu]

Old Smokey - look on the bright side, you may be getting older, but you're getting even wiser at the same time!!!

Thanks to the usual gang of contributers for such well informed debate.

mcdhu

[galaxy flyer]

If you cannot serve as a good example, be a horrible warning! Or the old one about a learned man passing on is like a library burning down. I, for one, have learned much here, thanks to likes of OS, mutt, J_T and others.

GF

[FE Hoppy]

Well if we are back slapping let me join in too!
I've really benefitted from some of the wise old (and young) sages to be found on these pages!

did you see what I did there?

[Centaurus]

I dont know if its true, but using derate, assument temperature and improved climb can lead to pretty high V-speeds (V1 past 150 and Vr well past 160) on long runways

The tyres, mate- wot about the bloody tyres....?
From what I have read in quite a few accident reports involving high speed rejected take offs, the cause has been tyre failures not necessarily engine failures. I wonder if the powers-that-be who decide on take off configurations involving de-rates and assumed temps etc that necessarily require higher V speeds, ever consider the statistics of engine failures near V1 versus tyre failures near V1.

[Pugilistic Animus]

Centaurus--that's a good thought---in terms of certification they only consider the certified Tire Speed Limits in terms of limiting V speeds---at Vlof actually--but it does raise interesting questions--let's just hope those tires ain't filled with Oxygen


perhaps it all balances statistically--in the end anyhow???

[Pugilistic Animus]

EPR SET

---A reduced thrust procedure is when you tell the engines to produce the thrust[[that would limit that TO on a hot day at a particular weight] that they would produce anyways---- IF it were a hotter day--except the day is just fine---but you've limited it anyways---so yes you put less fuel through the engines--but it mainly to be easy on those expensive babies though--as a slower climb has it's own associated penalties

PA

[kijangnim]

Greetings

Mainly saving on Hot parts maintenance parts, 5% for assumed/derated Takeoff much less for derated climb.
derated climb is at the expenses of climb performance time and fuel so is it still worth it to derate climb?

[Old Smokey]

kijangnim,

If we were to operate our flights with maximum fuel saving in mind, we would conduct all climbs as "Cruise Climbs".

The Optimum Cruise Climb is to fly the aircraft at the Maximum Range Cruise Speed (MRC), with the thrust set at the optimum speed for Thrust Specific Fuel Consumption (TSFC).

Typically (but PLEASE check the characteristics of your own aircraft), Normal full Climb Thrust (MCL) is at an engine speed somewhat above optimum TSFC speed. Optimum TSFC is more likely to be found closer to MCR. Thus, Climb at a derated Climb Thrust is more than likely to be closer to optimum TSFC engine speed. Of course, for maximum efficiency, the speed flown should be as close as possible to MRC.

At lower altitudes this does make some sense, but, as we are within 10,000 feet or so of Optimum Cruise Level, the Rate of Climb drops to operationally "annoying" levels. Thus, as a good compromise between optimum fuel savings, and practical aircraft operations, Climb at De-Rated Thrust makes good sense (for fuel economy) at lower levels, and at full MCL at higher levels to provide a reasonable level of aircraft performance.

It's not my idea, Boeing and Airbus beat me to it. De-Rated Climb thrust automatically reverts to full climb thrust at a pre-set altitude for later generation aircraft. For earlier generation aircraft such as the A300-B4, Airbus recommended climb at MCR until the Rate of Climb reduced to 1000 fpm, whereafter they recommended increasing to full MCL.

Nothing's new is it?

Best Regards,

Old Smokey

[groundfloor]

Hi Old Smokey, can you direct me to any flight test reports relating to VMCG handling tests... You know where the test pilot writes how he waited one or two seconds then took action, shortly followed by visit aft to remove the adrenaline.

Adrenaline in my experience being brown!

Airbus specific reports would be cool.