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

Hi Guys, just been reading our Flight Crew Training Manual (FCTM) and we have a new insertion, stating that;

"The fixed derate is considered a limitation for takeoff. Takeoff speeds consider ground and in-air minimum control speeds (VMCG and VMCA) at the fixed derate level of thrust"

It then goes on to say in the engine out case, you should not increase thrust on the live engine as loss of directional control may occur....

Can anyone give a reasonably easy to understand explanation as to why this may be.

Rgds

JJ757

Back to you SSG Version 1.0-1.9. Inside joke :ugh:

The reason is that those two speeds, Vmcg and Vmca, are determined by thrust. Increasing the thrust on the live engine would increase those numbers and, hence, affect adversely directional control. Further V1, Vr, and V2 are related (Old Smoke, help me here) to Vmcg and Vmca. V1 must be greater than Vmcg for you directional on the runway using aerodynamic controls only; Vr and V2 must also exceed Vmca by certain margins to ensure control airborne. If you increase thrust on that live engine, you can see that the numbers the take-off was predicated on have all increased and IF you followed the planned speeds at increased thrust, it could be bad.

GF

it all makes sense now...thanks

I think that you've argued the case quite well galaxy flyer, no need to call for reinforcements!:ok:

757jetjockey, the author of those words included in your FCTM was a wise person indeed, I've had the pleasure of writing similar prohibitions in our own manuals.

Vmcg and Vmca are indeed predicated upon the Rated Thrust used. Even if a further thrust reduction occurs (as in the use of Assumed Temperature or Flex Thrust), Vmcg and Vmca are still based upon the Rated thrust used. This applies to derates, and similarly (in the reverse sense) to aircraft such as the Jungle Jet with APR OFF, and APR ARMED as Takeoff thrust choices.

Lower Vmcg in many cases leads to lower V1s, and everyone loves lower V1s. Lower V1 becomes particularly attractive when Wet or Contaminated runways are in use, because the appropriately lower Vmcg adds enormously to improved directional control (or JUST meets adequate directional control). Thus, increasing thrust beyond the derated Thrust at V1 may lead to serious directional control problems, or complete loss of directional control. In these circumstances, Reduced Thrust (Assumed Temp / Flex) COULD be increased to the Rated Thrust, but certainly NOT to Full Thrust Rating. My attitude to the latter point is to let "sleeping dogs lie", leave the thrust alone, and enjoy the improved directional control in the knowledge that sufficient performance exists in any case.

Similar arguments apply to V2, with the caveat that not a great number of takeoffs for long haul operations are V2/Vmca limited (because of the higher weights), but short haul operators who frequently use lower V2 speeds (because of the lower weights) governed by Vmca, may more frequently encounter Vmca limited V2 speeds. The problem is, when the engine failure occurs, do you have prior knowledge if your V1 or V2 is Vmc limited, and who does that? I don't, and I'm a performance nerd.:}

Thus, your operator has, in my opinion applied very sound policy.

During my performance nerding at my desk (not in the cockpit), I've found that the aircraft that I primarily handle for performance analysis is frequently V1/Vmcg limited, but the V2 speeds for all thrust ratings is the same. Thus I have incorporated into THAT aircraft's operating manuals, "Following engine failure during Takeoff, thrust must NOT be increased to the full rating until V2 speed is achieved"

EXTREME WARNING - The above-mentioned allowance to increase to Full Thrust after V2 has been reached applies to this particular aircraft only. It may not apply to your aircraft.

In short, using a lower Thrust Rating is primarily beneficial for the continued Takeoff case, particularly on wet or slippery runways. Using the Full thrust rating is primarily beneficial for the Rejected Takeoff case (where Vmcg is not a problem for more than a second or two).

Conversly, for APR equipped aircraft (like the jungle jet), APR OFF is primarily beneficial for the Rejected Takeoff case, whilst APR ARMED is primarily beneficial for the Continued Takeoff case (unless the runway is wet or slippery). (A few other factors come into this argument).

Best Regards,

Old Smokey

Thanks for the compliment, Old Smoke, but your answer still increased my knowledge by an order of magnitude. As expected :ok:

galaxy flyer,

Loved your little dig at "SSG Version 1.0-1.9". I guess that as SSG doesn't like "taking it to the fence", he/she will be using Full Takeoff thrust in ALL cases.:ugh:

No more taking it to the fence eh SSG, I guess that "taking it to the ditch beside the runway" is a much improved option on a wet and slippery runway.:ok:

BTW ssg, as you state that you see we Airline automatons taking it to the fence ALL THE TIME, you must see an incredible number of engine failures. Those same dumb regulations that allow us to do reduced thrust takeoffs also require a 15% margin on runway length for the all engines takeoff. Thus, on the average 10,000 foot runways used by the airline automatons, we should be at 35 feet by 1,500 feet before the end of the runway. Maybe you have fences across the runway 1,500 feet before the end in your part of the world.:D

Standing by for SSG Version 1.10

Regards,

Old Smokey

Thus I have incorporated into THAT aircraft's operating manuals, "Following engine failure during Takeoff, thrust must NOT be increased to the full rating until V2 speed is achieved"

Interesting comment. While I agree with it, I wonder if many of the 'new generation' pilots understand it. I have not seen it so written in any ops manual of the 10 airlines I've worked for. But, indeed, there was 1 airline where the SOP on V1 cut was to increase to full power. Hm? Another left it to captain's discretion. Hm?

You say a/c type might be significant. B732, 733, 737, 739, B757, B767. I can't remember which of them was go to full power, but I hazard a guess it was a B767-300 operator becasue they didn't undertand the a/c in many other areas either. The Ops manual was written by an old B707 pilot.

.. the next aircraft which ends up on its back in a smoking pile of aluminium, having cartwheeled across the runway head as a result of pushing the levers up after a failure ... won't be the first ...

there was 1 airline where the SOP on V1 cut was to increase to full power

maybe not too much of a problem at high weight ... at low weight/speed schedule .. probably/possibly a death wish.

I always liked the KC-135R (with CFM56s) procedure-idle the opposite engine! It then had about the same thrust as the original with water going!

There appears to be a SSG 1.10 as I.R.PIRATE on the Africa forum-if it isn't him- a good duplicate.

John, I always thought perf planning was hastily covered, after lunch, by the least interesting instructor (not you or OS) and everyone wanted off for the day. Too much stressed drilling thru charts, which bored the crowd, and not enough on the basic physics involved. It is not hard, but does require some thought. The FAA (thru Part 25) dumbs it down more. How many Vmcg threads have there been? And still most people do not understand the concept, the effects of runway condition and crosswinds.

... which is why you, me, OS and mutt like to see them come up from time to time. It's a case of not being significant on every takeoff .. but if the weight is low and the speed Vmc limited .. then a bit of understanding might just avoid Capt Bloggs' getting himself into more strife than he needs.

We Aussies were quite fortunate ... AN, in the olden days, had a pretty good approach to performance for pilots .. the early course notes, put together by folk such as John Walsh, Roger Gabriel, et al (actually, I've never met Al .. but I guess he is a fine chap) covered the certification considerations in considerable detail. Likewise TAA's (subsequently Australian and now Qantas domestic) Pete Turner, Qantas' Wal Stack (Wal's original pilot course - a moderately thick paper - is quite prized by most folk ... I have several copies for auction ...). Certainly, with the operators for whom I did the ops engineering stuff .. the boys got the full going over with certification ins and outs ... and I have no doubt that OS does the same for his current clients. A lot depends on the attitude and desires of the Chief Pilot ...

Unfortunately, the "must know" philosophies have cut the available time down to nothing and, often, the presentations are made by folk whose knowledge is adequate rather than extensive.

C'est la vie, I guess ...

Current VC10 ops direct an increase to max thrust in the event of an engine failure(s) at V1. Engine location vastly different though!

I spend about 4 hours on this subject alone in our type rating course. It always surprises me how little is understood (even less after the 4 hours). And that applies equally to ab-initio and long in the tooth pilots.

The whole thing is complicated some what by the use of ATTCS which boosts thrust in the case of an engine failure. But also does so in the case of wind shear. However the pilot can also manually select RSV thrust by pushing the thrust levers forward. Depending on the circumstances the thrust you get when you push to MAX could be Normal TO or TO-RSV or GA-RSV.

I know it saves money but the complication it adds can sometimes overwhelm the pilots.

I have no VC10 knowledge and it may well be that the Type is never Vmc limited nor, indeed, near limited. If that is the case, then the protocol may well be fine, depending on the overall effect of whatever thrust you get .. whether rated or some degree of more than rated thrust.

As I recall, some of the Citations are similar in that the Vmc limits are way below minimum speed schedules.

greetings

Why the assumed Temp is more restricted than derated Thrust, in terms of runway condition, antiskid/one brake inop?:}

Very interesting post by Old Smokey, allways a pleasure to learn something.

Actually we do have the same caution in our manuals for the derate case, however we do have at least a Vmcg table for each thrust setting so we can see if we are Vmcg restricted with our V1, sadly there is no information about Vmca (737-700). We dont use derate on our -300s allthough it is available, no clue why.

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, is it safe to assume that with such high speeds controllability is actually pretty much a non-issue if we get the dreaded engine failure after V1?

Please can I clarify this to my own satisfaction. Airbus person.

My understanding is that this 'gotcha' of inreasing thrust after a V1 cut is only applicable to a derate take off (Boeing) as opposed to a flex thrust (assumed temp) take off (Airbus) - a generality I know. Am I correct?

My concern is operating to increased/improved V2 on a wet r/w when close in obstacle limited as has been debated before on this forum.

Regards
mcdhu

I agree with the argument as far as V MCG is concerned. VMCA is a different issue.

To use more than 5° of bank is no problem as long as you meet the required climb gradient.

So our FCTM only refers to the V MCG even after the amendment.

It applies to all aircraft with "Fixed Derates" and that includes Airbus, (Although i believe that the A320 is generally only sold with the option of Flex thrust).

Mutt

If I may add to Mutt's observation .. include aircraft with pilot limited thrust .. ie those where the pilot can push the levers to command greater than rated thrust. Same concern/result as with derate.

The problem is at the lower weights with low speed schedules. In general, unless there is another problem involved, a simple engine failure ought not require that the levers be moved ....

To use more than 5° of bank is no problem as long as you meet the required climb gradient

some considerations..

(a) 5 deg is the max permitted for certification to keep everyone honest

(b) Vmc speeds are very sensitive to bank

(c) likewise climb performance OEI

(d) given that the optimum bank is somewhere around 2-3 degrees ... a substantial increase puts you in a significant sideslip situation at low speed .. is that a good idea ?

Some years ago I'm in the L1011 sim with the chief pilot, and his idea is to firewall the throttles of the remaining operating engines in the event of the one engine out scenario.
I try to explain that this is not only not a good idea, but downright dangereous.
He says...show me.
Whereupon we 'depart' at quite a low weight with a max derate thrust.
Number three fails just at rotation.
CP applies firewall thrust...and the machine promptly rolls over on its back....or rather is freezes just before doing so.
CP now has eyes as big as dinner plates, and says....'I never thought....'

Yep, he never did.:rolleyes:

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 ...

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.

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 ...

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


Welcome Back Old Smokey!!!


Pugilistic Animus

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:D. 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.:eek:

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

A FIXED DERATE IS A LIMITATION.

Regards,

Old Smokey

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.

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!!

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.

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

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.

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.:ok:

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.:eek:

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!:ok: We learn what NOT to do when operating valuable aircraft with even more valuable human lives on board.:ok:

Best Regards,

Old (and getting older) Smokey

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

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. :ok:

GF

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?

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.

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???

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

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? :confused:

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?:O

Best Regards,

Old Smokey

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.