Performance /de-rated take-off

[badrace]

De-rated take off and flex take off ,some airlines do them both,for derated take off ,how do aircraft/engine manufacturer have these set reduced power settings meet certain regulations? since it might gets complicated when it comes to Vmcg issue.
I'm always curious about the performance optimization in which airbus seems to have more experience.Anyone can give me some lecture?

[johns7022]

Well I gather the derate numbers are certified for runway and climb gradient requirements...burning up valuable runway to that they can trend monitor an engine out to 25000 hrs before overhaul...

Passenger safety being number one of course....

Around here it looks like the 737s fly it to fence (8000 ft runway) then pull up..

The consensus being that after V1 has passed, your committed, lumbering down the runway waiting for V2, that if you lose an engine rotating at the fence(you planned accel/stop into the overrun) you now have the luxury of bringing up the good engine to max to get over those mountains ahead........lovely....

[Intruder]

De-rated take off and flex take off ,some airlines do them both,for derated take off ,how do aircraft/engine manufacturer have these set reduced power settings meet certain regulations? since it might gets complicated when it comes to Vmcg issue.
Vmcg is taken into consideration in derated/flex TO calculations.

With a fixed derate, you cannot use full thrust on the remaining engines if you lose one. With reduced thrust (assumed temperature), you can use full thrust. I don't know what Airbus uses for flex.

[badrace]

Max thrust should not be used during engine out take offs for de-rate for Vmcg consideration ; .however the airlines now must provide additional take-off performance charts as well as proper training for crew,since it's common sense to firewall the throttle when you feel chanlleged by the remaining runway.
My question again was: how do aircraft/engine manufacturers get these (i.e 6% 10% )prercentage of power certified ,are they just arbitary?do they need to test each power settings?Where do there numbers come from ,which regulation?

[john_tullamarine]

Basic rule is 25 percent max reduction flex for a given rating (although I understand AB has a 40 percent permission as they don't combine the two)

To get more, you need an artificially smaller engine by certifying a derate .. and then you can continue the reduction via flex.

Around here it looks like the 737s fly it to fence (8000 ft runway) then pull up..

That sounds like a significantly unbalanced takeoff with V1 having long gone and use of clearway. The later rotation approaching the end can give the impression of not much runway. Keep in mind that, for an unbalanced takeoff, only half the air distance to screen need be over the runway bits.

you now have the luxury of bringing up the good engine to max to get over those mountains ahead

That should never be necessary although it is optional. However, if you choose so to do .. do it slowly.

[FE Hoppy]

@JT
On out little E-Jets we've got an ATTCS system that will rapidly increase the thrust from a full 25% reduction to about 10% above Rated thrust. I've often wondered how that would look a kt or 2 above VMCG.

[Mad (Flt) Scientist]

@JT
On out little E-Jets we've got an ATTCS system that will rapidly increase the thrust from a full 25% reduction to about 10% above Rated thrust. I've often wondered how that would look a kt or 2 above VMCG.

That would seem to violate part of FAR 25 Appendix I:
I25.4 Thrust Setting.

The initial takeoff thrust or power setting on each engine at the beginning of the takeoff roll may not be less than any of the following:

(a) Ninety (90) percent of the thrust or power set by the ATTCS (the maximum takeoff thrust or power approved for the airplane under existing ambient conditions);

That seems to be saying, at least as I read it, that the ATTCS delta should be 10% above the thrust set for that takeoff, not 10% above the Rated thrust. Whereas what you describe is more like 50% (75% rated to 110% rated).

Our equivalent system works, IIRC, on a delta to the set thrust, not a delta to the Rated thrust.

[Wizofoz]

Basic rule is 25 percent max reduction flex for a given rating (although I understand AB has a 40 percent permission as they don't combine the two)


GE-90s are also rated for 40%, John- we will regularly take off with a 76deg Assumed Temp in the GE powered 777s.

[Wizofoz]

The consensus being that after V1 has passed, your committed,

Not so much the "Consensus" as the bleeding obvious- thats what V1 IS.

lumbering down the runway waiting for V2

No, Vr. V2 come later.

[robert f jones]

I think you need to brush up on your performance study/lessons. Less power used when not needed significently reduces the risk (however small) of engine failure and the problem of reaching Vr / V2 at the end of the take off run and Take off distance available.

[johns7022]

I guess the consensus is that when you make a derate takeoff, there is less of a chance of engine failure then at max takeoff...

Interesting theory...given the whole point of derate take offs is to trend monitor the engines out as far as you can to save money on overhaul cost..

So let's do the math...you run a corporate jet to 3500 hours, and do max take offs every flight if you want..

Or you do some max take offs, some derates and you get to enjoy doing them all the way out to 25000 hrs...

Hmm....let's see...I am at the end of the runway with a couple of 25000 hrs engines that the company mechanic said was OK...and I now need to get over those hills...or I am in a plane that has 3000 hour engines....

How come I feel safer with the 3000 hour engines...is it just me or am I missing something?

[Wizofoz]

...is it just me or am I missing something?

...Decades of data involving millions of takeoffs showing the incredible reliability of large jet engines and a virtual zero incidence of accidents due to engine failure after takeoff?

Yes derate saves money, because it saves the engines.

I am in a plane that has 3000 hour engines

I take it you fly Biz jets...Do you REALLY want to start making comparisons between the saftey records of airliners and biz jets?

[n1_spindown]

much easier on the engines = saves money

[johns7022]

I wouldn't have any problem comparing safety records between biz jet ops and airlines..

If the consensus is that trend monitoring an engine out as far as you can safely do so is ok...then I think it should be ok for biz jet operators to do it as well...fair is fair.

[Wizofoz]

Sure.

Do you have the infrastructure, maintenece systems and large body of data to prove it provides equivelent saftey?

If so, by all means put in the requests for variations of the regs. It should only cost several million dollars.

That's what airlines and airline manufacturers have had to do.

I wouldn't have any problem comparing safety records between biz jet ops and airlines..


EBACE: Business aviation not as safe as it is cracked up to be? (http://www.flightglobal.com/articles/2009/05/11/325757/ebace-business-aviation-not-as-safe-as-it-is-cracked-up-to.html)

This team found that the fatal accident rate for all business jet civil operations is more than eight times that for large Western jets in airline operation, and four times that for large Western-built turboprops flying commercial operations.

[johns7022]

You mean the special interests lobby's, the FAA in your back pocket...to bend and twist the regs to allow planning accel stop into overruns, put 200 hr ab initios into the right seat and have your own mechanics determine with a look see if an engine at 20,000 hours is good enough to go to 25000 hrs..

Nope you got me..corporate aviation doesn't have the where with all to pull that off...

[Denti]

That sounds very familiar, wasn't the same guy around last year spouting the same nonsense?

[Wizofoz]

You mean the special interests lobby's, the FAA in your back pocket

So perdictable that someone of that mindset is also an American Chavenist.

No, those procedures are used world-wide, ratified by a huge number of regulaory agencies, many of whom require their own, entirerly idependent saftey case.

So, Airline do all that, and are STILL EIGHT TIMES safer than Biz Jets?

Do you have trouble getting life insurance?

[johns7022]

'I think what we have here is a failure to communicate'

Let's take a hypothetical....same plane, same 5000ft runway, 1000ft over run, mountains up ahead, have to fly a complicated SID....and your choice is this..

- You can put a 200 hr ab initio pilot right seat or experienced captain.

- You can have 3000 hr engines or 25000 hr engines

- You can run max take off, get off early, or run your accel stop numbers into the over run, and rotate at the fence...

And the justification IS? 'Well gosh, you know...we are trying to save money on engine overhaul costs....yeah yeah I know...we just bought a fleet of all new A300s...yeah yeah, the CEO is making 40 million a year...yeah yeah, I know we chuck money down the toilet all over...but you know, we are really TRYING...and that's what counts.'

lol....always good for a chuckle when I come here....

[Wizofoz]

Certainly is a chuckle when clowns like you appear.

What exactly is your evidence that Airline practices are in any way unsafe?

Are you aware that the accident rate for two Captains flying together is higher than for experienced Captain/ Inexperienced FO?

Where do YOUR FOs gain experience?

If what we do is so heinous, why do you biz jet guys crash so often?

Are you aware you are MORE LIKELY to have an engine failure if you use max thrust instead of flex where available?

Are you aware that it HAS been shown that it is safer to leave a perfectly functional engine alone rather than tear it down every few thousand hours for no good reason?

In other words, each of these has been shown to enhance safety AND cost.

Seems like you like to spend your employers money JUST to make things LESS safe because your ego is to big to actually learn the facts about your own profession.

[Pugilistic Animus]

Nooooo,... again:{:{:{:{

[barit1]

johns7022:

If you look at the engine deterioration rate (EGT, FF, ...) per 1000 flight cycles - I'll stack up a 25000 hr airline engine against a 3000 hr bizjet engine any day.

The big bizjets operate by airline SOP, and a BBJ does just fine that way. Maybe when you get to fly in a pro shop you'll understand.

(BTW: What happened to the old pilot's adage that he'd rather fly a ship in to the shop for heavy maintenance/engine change, than to fly it out after the work's done? Happens eight times as often with 3000 hr TBO!) ;)

[Denti]

Always wonder about this fixation on the 25.000 hour engine. That engine most probably has another 20.000 hours to until it will be taken off the wing anyway, still nearly half its normal life ahead.

But yup, its SSG all over again.

[robert f jones]

Most aircraft, airline or business jet are certified to performance A standard which is predicated on the failure of one power unit at the most crucial point in the take off run. If you reject the take off prior to V1 you will stop within the accelerate/top distance available. If the ASDA includes a declared stopway (which is usual)although the stopway can support the weight of the aircraft it may contain frangible obstacles which could cause minor damage to the underside of the aircraft.
After V1 you will continue to accelerate to Vr and become airborne at the end of the Take Off Run available. If there is a clearway available (again, quite usual) you will be at 35 feet (yes, not a typo, 35 feet) within the runway promulgated Take off distance available and clear of the limiting obstacle. This also applies to the Take off climb surface, still 35 feet clear of the limiting obstace if necessary out to 15 kms.

As a pilot, you not only need to know the airworthiness requirements but need to mentally accept them as a part of the job. Your posts have a rather worrying trend to the contrary.

[Mikehotel152]

Oh goody, a chance to display my ignorance...

At my outfit we use both derate and assumed temperature reduced thrust to reduce engine wear and save on fuel. It's common sense. You don't need 27K to get a lightly loaded 738 off the ground safely on a 3000m runway. There's also the added issue of avoiding rocketing skywards at 5000 fpm on a SID with step climbs into a busy TMA. TCAS RA in London anyone!?

However, for added safety, we also use the performance calculation figures for intersection takeoffs even though we usually use the full runway length - along with everyone else. Happily, there's (almost) always a helluva lot of runway left when we rotate.

Surely it's not an either/or decision. In the real world it's quite possible to neither firewall the engines due to some misplaced fear of mountainous terrain nor risk damage to the frangibles if you have to perform an RTO.

Where performance considerations require it, we use the full shabang. No shame in doing so. And good fun it is too. :p

[john_tullamarine]

If you reject the take off prior to V1 you will stop within the accelerate/top distance available.

a bunch of assumptions is behind that sort of statement. Possibly better to talk about "generally" and, "if everything goes according to Hoyle" rather than making black and white statements.

the stopway .. may contain frangible obstacles which could cause minor damage to the underside of the aircraft.

probably not the case - perhaps you are thinking of clearway ? .. which is not specifically related to ASDA.

After V1 you will continue to accelerate to Vr

as previously observed .. it depends

become airborne at the end of the Take Off Run available.

definitely, not the case. Depending on the certification rules, liftoff is scheduled to achieve either a minimum of a half or a third of the airborne distance to screen over the declared TORA and that only if TORR is the critical takeoff case for the day.

and clear of the limiting obstacle

separate calculation .. while often the TODA is predicated on obstacle clear surfaces, that is not a necessary condition.

still 35 feet clear of the limiting obstace

afraid that is nonsense. The net (straight ahead) surface can be sitting just above (ie 35ft) any critical obstacles .. but the aircraft will be above that surface with the delta increasing as the distance from the runway increases.

if necessary out to 15 kms.

generally out as far as it takes depending on the obstruction profiles. You may be confusing the usual Type A chart survey length ?

Oh goody, a chance to display my ignorance...

you too ? ... good health, sir.

In the real world it's quite possible to neither firewall the engines due to some misplaced fear of mountainous terrain nor risk damage to the frangibles if you have to perform an RTO.

a practical and pragmatic point of view.

And good fun it is too. http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/tongue.gif

especially empty on a really cold morning .. not often done, mind you, but clears out the mental cobwebs on the odd occasion.

[rsiano]

A critical look at use of reduced thrust takeoffs. Why ever use it?

I have been critical of the use of reduced thrust for takeoffs for more than forty years. The arguments made in favor of using it to save money and engine life were never good enough for me. Especially when you took a good look at the negative side of using it.

When reduced thrust is used for takeoffs the amount of runway required to accelerate to V1 is increased substantially. This leaves you with less runway remaining in the event of encountering a high speed aborted takeoff.

If you do not encounter an aborted takeoff while using reduced thrust, you are still going to pay the piper by extending the time you and your aircraft spend below 1,000 feet above the ground. I believe the time we spend at less than a thousand feet above the ground is more likely to expose us to a greater chance of an accident.

This will happen as a result of the lower rate of climb due to the use of reduced thrust. Maximum thrust will allow you to spend less time below 1,000 feet above the ground and therefore in my opinion, will enhance the over all safety of the flight.

Reduced thrust takeoff power settings were not mandated but instead were to be used subject the captains discretion. He was not permitted to use the reduced power if the runway was not clean and dry or if a tailwind existed or if he suspected wind shear in the takeoff area.

I used to ask my co-pilots how could you not suspect wind shear on takeoff? Is there some magic combination of wind velocity, direction and temperature or other atmospheric aspect that could deliver that information to the pilot? I know of none.

Therefore, I feel the use of reduced thrust takeoffs should never be used on any takeoff. Yet even when I would explain my reasons stated above to other pilots, many would go ahead and use it over and over again.

I guess I was not a very effective communicator. Maybe one of you could offer a plausible explanation why pilots continue to use reduced thrust takeoffs very day. The last time I used it was in 1967 and although I had many company as well as FAA check pilots fly with me through the years, none ever wrote me up for not using it.

What do you think?
Thanks!
Dick

[badrace]

It's always good to get different feed backs ,but still the question is how does the manufacturer come up with these rather Arbitary percentage ?(De-rate take off)

[john_tullamarine]

how does the manufacturer come up with these rather Arbitary percentage

basic limitations are in the design standards otherwise it is the same as any other certification animal - the OEM (applicant) has to show compliance with the design standards - the detail within that is up to the OEM's choice.

[mutt]

rsiano, you have to admit that engine technology has changed since 1967. We operate all of our aircraft with Derate/Assumed and in some cases such as the 787 our engine warranties are based on achieving an average thrust reduction of xx%.
We have never had an incident nor accident attributed to the use of Derate / Assumed thrust.
A pilot who consistently refused to use this procedure would be invited for coffee and dates with the fleet management :)

Mutt

[robert f jones]

My post to johns7022 was an honest attempt to persuade him, not only to understand airworthiness requirements, but to mentally accept them. At the moment his posts indicate otherwise.
I was therefore somewhat dismayed to read your dismissive response, as a moderator you have a resposibility to your pilot readers to have a better grasp of the subject matter. I don't intend a detailed reply but make the following brief comments :

If you stop before V1 you WILL stay within the ASDA. Why are you introducing a doubt in a pilots mind ?

Stopway and clearway are totally different surfaces - one refers to ASDA the other TODA - see CAP 168.

The reference to the take off climb surface.......... sorry, but run out of patience. My other "nonsense" item came out of JAR Ops.

The bottom line is, if you are called on to cope with an engine failure on a performance limited take off (which calls for precise speed control and accurate flying) have faith in the certification rules - they do work.

[Denti]

And then remember that performance data is only "hard" (whatever that term means) data for wet and dry runway, for everything else it is only advisory information.

Performance data work on a perfect day, with a perfect aircraft and a perfect runway. Introduce rubber deposits on the opposite end, very worn brakes and tires, still warm brakes from a short turnaround, a fatigued crew and overweight passengers and you will be hard pressed to deliver the same results. Better build in some fat, for example calculating an intersection T/O, shortening the TODA in the performance program or add a ton to your weight.

And then you still can derate and assume as well as use improved climb V-speeds which will reduce engine wear a lot.

[john_tullamarine]

RFJ,

somewhat dismayed to read your dismissive response

My response certainly was not intended to be dismissive and, should folk have read such into it, my correction to them and apologies to your good self.

Rather, two main points were pertinent -

(a) certification (and, it follows, "safety" .. however that may be defined) is probabilistic rather than absolute. With the exception of limited circumstances (and even then only somewhat generic) there are no guarantees with aircraft (or machines and life in general - other than death and taxes, I guess) ... only probabilities .. it is the responsibility of crews to do their level best to stack the dice to an appropriate level of risk for each operation - not always nil risk, not even always minimum risk .. but always an appropriate level of risk .. which involves technical and operational knowledge, experience, skill, etc., etc. The level of risk appropriate for one operation may well not be appropriate on another etc.

While I acknowledge the usefulness of SOP, a rigid and blind adherence to procedure or an irrationally misplaced belief in cant has the potential for disaster. A very wise greybeard checkie once observed to me (as a young chap on my first jet during a line check) .. "lad", said he, "lad, the Ops Manual lacks one phrase on page one .. and that is 'to be read with a modicum of commonsense' ".

(b) given that you are styled as a consultant, I hoped (and still do) to generate a useful (and objective) debate on these matters for the educational benefit of the new chums amongst our ranks .. such debates are just about always useful and a major thrust of Tech Log policy.

as a moderator you have a resposibility to your pilot readers to have a better grasp of the subject matter.

I take a slightly contrary view regarding responsibility.

Moderators, in my view, are here to

(a) maintain the basics of site and forum order without imposing idiosyncratic views of the world on discussions

(b) stimulate discussion on occasion

Site policy is for moderators not to stray outside their fields of competence if entering discussion. We all have other usernames to cover those discussions where our competence is no more than routine.

So far as my grasp on the present subject matter is concerned, I have a passing knowledge of things relating to piloting (but, certainly, not to the extent as many others here who are far more experienced) and, more particularly, engineering/maintenance activities .. possibly, on occasion and in respect to the latter, even enough for gov'mint work.

If you stop before V1 you WILL stay within the ASDA. Why are you introducing a doubt in a pilots mind ?

I suggest that the statement would be more reliable if one observed something along the lines of "providing that real world conditions, aircraft capabilities, and crew performance together reasonably replicate certification postulates, then there is a high probability that the operation will match expectations inferred in the certification process and data".

Clearly, history has many examples of a singular lack of success in many areas, including ASDA exceedances. Pilots should always have doubts (perhaps "cautious concern" might be a better term ?) in their minds .. (that's why the Commander gets paid the big bucks) .. so that they plan and act cautiously with appropriate and due consideration of and to all those matters which conspire to subvert the intent of the certification process in the real world.

He/she who blindly believes that the process and rules will look after everything .. is either an incredibly optimistic individual or an accident waiting for the "right" set of circumstances to eventuate. The task for the skilled aviator is to subvert Lady Luck's best endeavours to bring him/her unstuck .. one recalls the old lemma .. "the superior aviator .. uses his(her) superior judgement ... to avoid the need for the display of his(her) superior skills".

Stopway and clearway are totally different surfaces - one refers to ASDA the other TODA - see CAP 168.

Indeed. My concern with your earlier statement related to the apparent inference that stopways might routinely include frangible obstructions .. the stopway can support the weight of the aircraft it may contain frangible obstacles which could cause minor damage to the underside of the aircraft.

Specific standards detail will vary slightly with jurisdiction. You have cited CAP168 .. that document, while permitting a restricted stopway use of frangible obstructions in very limited circumstances restricts maximum height to a value which doesn't sit all that well with your more general statement. Hence my observation that the statement may have been more appropriate to clearway design standards.

However, I'm not an airports engineering expert by any means .. I'll leave the expert comments to our good colleague OverRun should he choose to play in this thread.

The reference to the take off climb surface.......... sorry, but run out of patience.

Might I invite you to cite an authoritative source which supports your contention that ... this also applies to the Take off climb surface, still 35 feet clear of the limiting obstace if necessary out to 15 km .. in which case, I shall both enhance my existing knowledge base and apologise most obsequiously.

The net surface may involve such a clearance from the bumpy bits but the real world obstacle to aircraft clearance (which will approximate something between net and gross but probably reasonably close to gross) .. will be greater and, indeed, increase with distance from the runway. I read into your comment that you intended the latter meaning ? If the former, then I recant.

have faith in the certification rules - they do work.

I've done sufficient test and performance scheduling work ... far too many associated sums .. and been frightened witless by aeroplanes far too many times .. to have the same level of faith as you obviously do.

I prefer to see certification as my starting point ... and a starting point which defines the routine limit of prudence.

The very fact that the certification rulebook is an ever evolving animal suggests that one should have a degree of circumspect regard for such things.


Let the detailed discussions continue ... as for me .. so far I prefer Denti's more pragmatic views on life, death and the universe ...

[johns7022]

To be true to the OPs question: Reduced thrust numbers are certified through the flight testing process..

[robert f jones]

What happened to my post criticizing your somewhat vague and negative reply to mine. Seems have disappeared. I'm not what sure what sort of qualifications you have to countermand well established airworthiness rules re stopping before V1. Your comment, "it depends.... " doesn't seem to appear in any of my manuals.

[john_tullamarine]

Robert,

Appears I have offended you. Certainly, that was not my intent and, for that, my apologies. Indeed, presuming that you are the competent chap of vast experience as is suggested by your profile, my desire was, and remains, to entice you into a useful debate on aspects of certification .. for the benefit of the younger folk.

What happened to my post criticizing your somewhat vague and negative reply to mine. Seems have disappeared.

I'm a tad confused here ? The post has neither been edited nor removed.

[I]I'm not what sure what sort of qualifications ..

Afraid I don't get into willy waving competitions these days .. I left that sort of thing behind me far ago in my youthful, adrenaline and testosterone drenched past. In any case, such competitions generally only serve to deflate one's over-assessment of one's personal endowments ....

Suffice it to note, for your interest, that I am a greybeard consulting engineer, originally qualifying in aeronautics, with the usual, and relevant, Industry accreditations and Regulatory delegations (although the latter are no longer current due to my not being in those lines of work these days).

Does that mean I know heaps about everything ? Of course not... the areas in which I am abysmally ignorant are legion. However, I do have a passing knowledge of a small number of subject areas.

you have to countermand well established airworthiness rules re stopping before V1.

Not trying to do that at all.

The rules (usually, but not always) are based on declared and specific boundary conditions.

If Industry folk are aware of the background then they should be well-placed to interpret the application of such rules in the real world. Unfortunately, the majority most certainly are not. Therefore, we expend a lot of effort in Tech Log trying to throw such things into the discussions with the explicit intention of encouraging folk (especially the younger folk) to incorporate such things into their approach to the aviation task.

While most of us older folk have excelled in making mistakes which we probably would prefer not to have made, there is the possibility that such discussions just might help the odd new chum to avoid making a foolish mistake needlessly .. especially when one observes that some mistakes are terminal.

Your comment, "it depends.... " doesn't seem to appear in any of my manuals.

Of course not .. the manuals are predicated on a presumption of knowledge competence regarding the rule book boundary conditions etc. However, when the crew is doing its best to apply those rules in the real world, there is a real need to have some understanding of the delta between certification and rule book speak and real world speak.

I ask you to embrace the discussion and throw in the sum of your Industry knowledge and experience for the benefit of the readership.