Flex Takeoffs Margin Question
 

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
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.

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.

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.

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,
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?

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.

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?

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.