Good evening gents,

One quick question regarding Flex/assumed temp takeoffs in medium jet airliner (both Airbus and Boeing).

I know that is legal to flex the engines on takeoff and have the margin (distance left available in the Go or in the Stop case) almost zero.

To avoid in depth analysis of the STOP/GO options in case of stopways and clearways assume a balanced field.

Is there a standard margin that you should aim at (eg 1000m) ? Are there company policies that require a standard value of margin in flex takeoffs? I know that maybe this is company depedent, I am just asking about how much is this margin (any company). I know that this could differ from airline to airline. Also personal habbits about flexing techniques are welcomed. This margin could change with runway conditions I assume.

In my opinion leaving some margin in flex also in relatively short runways is good airmanship, specially in short and wet operations, leaving little extra space above the LOC antenna in the runway end. :)
I also suppose that there are companies out there demanding the use of max flex, leaving the pilot without an option.

Thank you.

Lantirn,

First of all: Safe, Legal, Efficient, in that order. Screw the bean counters with their fancy formulas. I have never been in a situation where procedures would "force" me to use max flex, but also never had written guidelines as to how much margin you keep for yourself.

As for your comment, it would be indeed airmanship and experience that would dictate how much margin you'd bag with performance.

Personally, I would be happy with a flex temperature leaving me around 300 meters margin at the runway end. If I read 260 meters, it's OK too, especially if the runway is dry, as the reversers are not taken into account for the deceleration.
For a wet runway, I may want more like 400 or 500 meters, because most operators (not all though) will take the reversers into account on wet surface, so if you read 10M margin, this is at best what you'll get.

Bear in mind that the weight calculation with standardized weight could be way off the actual, so all these factors thrown in, and a little personal margin would save the day, or prevent you from soiling your trousers! :\

I've flown only for a couple of operators on the 320, so far no mention of a company margin requirement.

If you ever find an airline that squeezes pilots for maximum flex that leaves you only inches of runway left, you can always set TOGA by the time you reach 50 kt! :ok:

Flex

Hi Lantirn,

The stop margin you are looking at, would be available if the real airfield conditions were as hot as your assumed temperature.

Since you'll be using a Flex of say 60 degs C, on a day when it is much cooler, your IAS will increase more quickly than it would in the assumed air density at 60 degs C. Your real stop margin will be much bigger than it appears.

Your engine out performance will also be better than you've calculated provided the real temperature is lower than the assumed.

I'm not bothered about the bean counters - but by using maximum flex, I'll significantly reduce the chance of an engine failure - and still have a bigger margin than I need.

FLEXPWR,

That makes sense. Thanks for your response. :ok:
About your comment take a look here (however that was another error):
Accident: Emirates A345 at Melbourne on Mar 20th 2009, tail strike and overrun on takeoff (http://www.avherald.com/h?article=416c9997/0012&opt=0)
Also some people state that Emirates wants pilots to takeoff on max flex always.
Dont know it thats correct!


Rudderrudderrat,

Thats correct my friend. Taking also in account some other factors, e.g in the stop case: little late reject, little more weight, runway little more wet than damp, little tail, could setup an overrun quickly! In the stop case with short runway and wet, I think that a margin left is a good airmanship!

I dont have a "flex phobia" :} , nor I dislike flex takeoffs, beyond the maintenance savings, engine falure is indeed reduced, however, birdstrike events are more likelly to occur in flex takeoffs as some pilots say. Also some say that you actually burn more fuel as your takeoff is longer and needs more time. I dont know if these are also correct, but these are also opinions.

however, birdstrike events are more likelly to occur in flex takeoffs as some pilots say Do birds know that you are doing Flex Thrust takeoffs? Or could be a case that as the vast majority of takeoffs these days use Flex Thrust, that it appears that crews encounter bird strikes when they are doing Flex.

Mutt

Heheheh :O

I am not an expert on this subject.

I cant evaluate if a longer/shorter takeoff run is better/worse for a birdstrike.

I just say what I hear around.

More time on the roll means more exposure to hitting something on the ground, like the Concord.

Getting off earlier might mean less exposure to bird strike, as most of the birds hang out in the wetlands and such NEXT to the airport, and not usually flying around OVER the runway.

Getting off earlier, then needing to land again, gives you more protection in the airport protected area should you need to maneuver low....on and on...

But all that said, when you use flex, your trading safety for a supposed reduction in engine overhaul costs. At least that's what they tell the airline pilots, who have never managed nor conducted a jet engine overhaul, but none the less feel experienced to come here and wax philosophical on the subject with much enthusiasm.

:ugh:

There is no increased risk of bird strikes with a Flex Takeoff. The data does not support this purported theory in any way.

It is also important to remember that by reducing thrust you increase EGT margin and that reduces the wear on the engine which can and does reduce the likelihood of engine failure.

Like any other tool flex thrust must be used prudently.

RainingLogic = SSG........ who has absolutely no concept nor experience about operating airliners or large corporate aircraft...

Mutt

Canuck - There isn't one study that shows pulling the power back reduces engine failure...Flex is for the guys with boroscopes.

Besides...running bearings out to 30000 hours doesn't help your case.

Flex is for the guys with boroscopes. Actually its for the the guys with calculators.

Raininglogic a little research from manufacturers presentations on failure data might be in order........

Reduce temperatures and stress with flex thrust and the probability of failure reduces.

RainingLogic = SSG........ who has absolutely no concept nor experience about operating airliners or large corporate aircraft...
Mutt

please excuse the stupid question , but who or what is ssg? a banned user which claimed something to be who he is or was not ?

My guess, and it's just that, they don't want the pilots getting near max limits. I've seen this in other industries. They pull the numbers back so there isn't an inadvertent over speed.

And over speed on most jets requires some inspection, downtime, money etc. It's just more 'dumbing down' the cockpit mentality.

Actually, I like to "wax philosophical" on the subject! Might it displease some, they can skip the post!
And I don't know nor want to be entertained with a full engine overhaul, same than the Engineers (with great regard) overhauling engines have little interest on sitting in a cockpit for a 10 hour flight. :zzz:

Nevertheless, flex makes sense in many ways, and the simplest I can compare with is your car's engine: anyone revving up the engine to the red line everyday? No? then you are doing a flex engine management... :E

On another note, I would like to expand the discussion to derate as well (if OK with original poster). My knowledge on the derate is very basic, and would welcome views and advice of experienced drivers with margins in this case.

Flex

Hi FLEXPWR,
Personally, I would be happy with a flex temperature leaving me around 300 meters margin at the runway end. If I read 260 meters, it's OK too, especially if the runway is dry,
Using V^2 = U^2 + 2aS where V is max speed achieved, U = 0 (stationary), a will be some acceleration / another deceleration value.
Then S=V^2 * (1/2a).
So the accel/stop distance is proportional to the square of the maximum achieved speed.

If the assumed temperature is about 30° C above ambient, then the assumed TAS will be about 5% greater than actual.
If your calculation showed just 1m margin for your accel/stop available on a 3,000m runway, if you flexed to about 30 degs above ambient, then in real life you would use:
(0.95/1)^2 * 2,999m = 2707m.
So you would have your 293m available, wouldn’t you?

I would like to expand the discussion to derate as well
We derated the thrust on TriStars to lower the VMCG / VMCA so we could lift more weight from short runways. We were not allowed to exceed the lowered "derated" EPR for obvious reasons.
On 747s, we derated the 524-H engines when they were mixed with -Gs (through the FMS), so all were the same rating. Then we applied FLEX.
Similarly, we could operate some aircraft at lower Max TO weight/ Landing weight to save on charges. The engines were operated at some derate 1 or 2 (I seem to remember) to save on maintenance costs, and we still applied FLEX in addition.

My guess, and it's just that, they don't want the pilots getting near max limits. I've seen this in other industries. They pull the numbers back so there isn't an inadvertent over speed.

And over speed on most jets requires some inspection, downtime, money etc. It's just more 'dumbing down' the cockpit mentality.

Well, it's not a very informed guess.

Boeings and Airbusses dating back to the 80s have EECs which provide protection from limit exceedence, and later ones are FADEC- Max Rated Thrust is available simply by fire-walling the TLs without fear of overspeed.

Wiz - It's a fact that operators do all they can to keep the pilots away from max limits. The fact that a Fadec operator chooses to use Flex just goes back to that operator's reasoning...in this case, when the plane is boroscoped, less wear, push the engines farther out.

And yes if the guys with the calculators feel that pushing overhaul times farther reduces operating costs, that's the push...

Although I have always pondered the pilot's philosophy of flying it to fence for a lifetime of white knuckle departures and his inability to consider the safety consequences of his actions.

It's a paycheck right?

lifetime of white knuckle departures and his inability to consider the safety consequences of his actions. Please give us specific airliner performance figures that justify your statement.

Mutt

Mutt - Even If I could touch you on the forehead, instantly giving you the capacity to understand that putting more runway behind you on the roll doesn't not promote a safer takeoff...instantly giving you the moral capacity to actually consider the passengers safety, their families, the industry...to no avail.

Because no matter how much with your new awakened moral and intellectual capacity, would have you fret the numbers before each take off, wringing your hands at the threshold, considering the implications of a greedy corporation putting profits before lives, you would still fly it to the end, pulling the gear up at fence, because in the end you can't find any other place to give you a seat in a plane, and you really want the paycheck.

So you come here and muck up the numbers, trying to convince everyone that flying it to the fence is the right thing to do, with kids flying around on 35000 hour engines thinking they are safe, with blind mechanics behind boroscopes, who just missed the jack screw on purpose, not wanting to say anything about that nick or sulfidation, they don't want to be the one that cost the company money...on and on...it's a culture of sell outs for a paycheck. Pretty disgusting. If the public only knew.

Yep SSG all over again,time for biscuits and tea:E

RL,

Two operators.

One uses TOGA every takeoff.

The other makes sensible use of reduced Thrust TO.

Which will have more engine failures?

Wiz,

I wondering, wouldn't it be more reasonable to ask which one spends more on engine maintenance? Would one really expect more engine failures operating engines nearer to but within their limits with appropriate maintenance?

Ok guys, lets put some facts in.

Copy Paste from a study (Yan Chenghong)

The significance of reduced thrust takeoff.

Prolong the life-circle of an engine by reduction T3.
As we all know,the longer the engine life is,the higher profit and the lower operation cost is.Turbine,as main part of an engine,is easy to be damaged for its bad working condition with high heat load,great centrifugal force and anomalistic pneumatic force. The great centrifugal force will wiredraw the blade. The anomalistic pneumatic force causes vibration and affixation stress to blade,then blade is easy to fatigue and rupture. The turbine inlet gas temperature (T3*) is an important factor of turbine power.Increasing T3* leads to increasing blade load and decreasing metal intensity.It is showed that the metal life has logarithm relation to its temperature. For example,a blade lose 35 percent of its life under the condition with 705 ℃ after operating 3,500 hours but 51 percent under the condition with 870℃ after operating 6.6 hours. The reduced T3* (about 30 to 50 ℃) when taking off with reduced thrust lengthens twice life of the heat section.

-------------------------------------------------------------------------
Ok that was clear enough despite his english.. :ugh:

Laker Airways was the first airline using reduced thrust method in BAC-111's in RR Sprey engines. Rolce Royce had noticed that engines sent for overhaul by Laker airways were the best maintained of any BAC-111 operator.

Dont forget also, that lower takeoff thrust settings could permit a takeoff where with full thrust it wouldnt dispatched, with V1 limited by Vmcg, in short/wet runway. So you can increase your weight or if field limited reduce the ASDA simple by using reduced thrust.

Flex is, a great tool.

I am not into FLEX as a matter of principle.......

In general in corporate it is available, and sometimes used, but the economics in my opinion do not justify it.

I prefer the shorter roll, on the typical corporate airports. I prefer to limit the bumping to the gear, as well as the passengers. I prefer air under the wings creating a margin of safety. I prefer the hills to be below me, and not exactly at my computed level, in that first turn.
I will not do flex, when there is a heavy infront, I want to well inflight before ant residual turbulance.

Would it be fair to say that a derated engine core, benefits less from a FLEX take off, and thus negated the standard arguements.

I did a flex about two years ago, at Stansted......

glf

And, in the corporate world, that may be reasonable.

The savings in maintenance in an Airline, with hundreds of aircraft doing hundreds of takeoffs a year are enormous.

PLUS there is a decrease in the incidence of failures through the use of reduced thrust.

I am not into FLEX as a matter of principle... But yet you teach it! :):):)

We do FLEX as a matter of principle in all our corporate aircraft :)

Mutt

It's been said here, but I think need repeating. There is a built in margin here that makes an assumed temp. t/o safe in most conditions. Let's say it's 15 c outside and we are using an assumed temp of 50 c. With the exact numbers and speeds we are using we could take off at 50 c., but we're using those numbers at 15 c. the huge increase in performance due to the cooler temp. is in our back pocket. We are way exceeding the performance we would achieve if it actually was 50 c.

5LY, this shows exactly what you are talking about, and should also dispel SSG's misguided belief that FLEX thrust means scraping over the airport fence:)

http://farm8.staticflickr.com/7028/6538170219_f7378a25cf_z.jpg ("[url=http://www.flickr.com/photos/pprunemutt/6538170219/)

Mutt

Thanks Mutt.

I was confused by the 2nd segment figures, but have concluded that they're matched to 2.4% where in fact we'd most likely exceed them.

I am also confused with the 2nd segment figures.

Especially with the ROC in 2nd segment.

Anyway thanks Mutt for this.

Look at the difference in TAS.......

Mutt

With respect to the 2nd segment numbers I think the point of Mutt's chart is that it takes a lower rate of climb to acheive the 2.4% gradient. In other words if you are climb limited you have only to make 387 ft/m to meet the 2.4% requirement where if the temp. was actually 40, and your TAS was higher, you'd have to acheive a higher rate of climb.

Look at the difference in TAS.......

Mutt

THIS is the key item in understanding the inherent margins in flex operations.

The aircraft achieves rated-thrust AFM performance at 40C; and using this same thrust on a 15C day, it also makes rated performance but at a TAS (inertial speed) 6 or 7 kt slower. Thus less runway is used, and less ROC is required to make the climb gradient.

As I've said before, I'm perfectly happy with RainingLogic bending his throttles day after day, because that's how my employer makes money on spare parts sales, and that means a better pension and dividends for me. But he can't expect his ideas to gain traction in an industry that makes MANY more takeoffs, and sails or fails on takeoff-related costs. :ok:

Pretty funny..airline pilot logic...you set for 40c, but if it's colder...It's a win!

So if it's 30c, you set Flex to 50c, it's a win!...and if it's 50c, you set to 70c! It's a win!

Hey I have an idea. If you have 50 pax, you can plan for 100 pax..it's a win!

Here's another one..plan to run out of fuel over the ocean but carry enough fuel to make it...it's a win!

Or, throw your charts out the window half way to Hawaii, but have a secret set under the seat, so when you get to Hilo...viola, charts! It's a win!

Raining Logic

I think your game is a rain-out.

GF

As I've said before, I'm perfectly happy with RainingLogic bending his throttles day after day, because that's how my employer makes money on spare parts sales, and that means a better pension and dividends for me

relax relax... since neither raining logic nor the vast majority contributing such heated discussions here are bending any real throttles you will have no luck in a better pension in spare parts sale.

flexing is not an option in real life but company procedure whenever possible with pilots decision not to do when he believes he has reasons for it.

but when you make a full thrust takeoff on a situation when flex takeoff would be every day business you may have to explain why you decided to do this.

best regards

So tell me Raining Logic,

Are you prepared to take off at Max takeoff weight?

Wouldn't it be safer if you just alwys insisted on being 20% under?

Why not always err on the side of saftey?

@<hidden> RainingLogic, ref post 35.

Was I once married to you?

Why not always err on the side of saftey?

Yes - rather than land at the 6000' field close to the boss's meetingplace, you should insist on the 11000' international terminal field 30 miles away. More safety margin, y'know. :cool:

It is a long time since I did a derated thrust take off or even the calculation.
I believe that Air new Zealand was the first to use this system on the DC8-52.
The criteria was that a derate(Wannabe term Flexithrust)was allowed only if there was a runway margine available.
i.e 8000 ft of runway needed for the given weight/ambient/wet,dry, wind performance, but there was 11000 ft of concrete available.
So we could derate to use not more than 80% (8800ft of available runway.
One could not do a balanced field operation ( to cross the far threshhold at 35feet with a critical eng failure AT V1 and maintaining V2)
Having said that, if one was on a derate take off and did loose one after v1; one had to have precalculated the VMCG and VMCA so that normal take off thrust could be applied to the remaining power plants if the vmc/a/g crit was superceeded.
It was also similar in the Bae 11-500, not counting noise abatment climbs.
They could be bloody spookey at places like Luton.

bg,

Thanks a lot for this insight, wouldn't this describe a flexible take-off as opposed to derate?
As I (humbly) understand, a derate cannot be modified once you get airborne, at least not on FADEC equiped engines.

Or were you referring to a flex take-off under derated thrust conditions?

@<hidden>: no, I'm pretty sure it was MY ex...

Flex

FLEXPWR, lots of terms have been used, Derate - Fixed and Flexible were used by Boeing until recently.

You cannot change fixed derate, but Flexible derate is now called Assumed Temperature by Boeing.

Mutt

And interestingly, though it's a customer option, I've never flown a boeing with fixed de-rate.

Max rated has only ever been a push to the firewall away.

Interesting, so you dont use TO/TO-1/TO-2?

Mutt

I believe that Air new Zealand was the first

I don't know who was first but can recall, as an aero eng student in the late 60s/early 70s, having Wal Stack as an Industry lecturer.

At the time Wal was the boss performance man at QF and that operator had been using flex techiques since 707 days. Wal (being an ex-mil pilot, as I recall) had a mixed engineer/pilot view of life and kept a 1000ft ASD pad for the crews' mums and kids.

Lovely fellow and the most wonderfully dry humour.

Interesting, so you dont use TO/TO-1/TO-2?

Mutt

No. Five different operatiors of four doffernt Boeings, and all chose Assumed Temp only for reduced thrust.