TOGA and Flex Take off

[George Yogi]

If RTOW from RTOW chart is found out as 80 tons for a specific runway.
MTOW of the Aircraft is 73.5 tons and
ATOW of the Aircraft is 73.5 tons , aren't we supposed to use TOGA
instead of Flex take off ?

I assume Flex take off is possible only when ATOW is less than the MTOW ,
Please confirm

[FlyingStone]

RTOW chart you have required performance at 55°C, but it's only 22°C OAT today. Why would being at MTOM make this any different than if weight was MTOM-1kg, MTOM-5000kg, ....?

[latetonite]

Max structural TO weight has nothing to with performance based max TO weight.

[barit1]

You need to understand the difference between structural limit (a constant) and performance limit weight (depends on runway length, gradient, elevation, OAT, QNH, obtacles, ...)

Elementary essentials for a commercial pilot.

[latetonite]

This kind of questions only come up when one apprehends by rote learning, without understanding the basics.

[compressor stall]

Yes, agreed - smacks of rote learning, and not much of it. It's a sad state of affairs that a company or NAA would consider people lacking such basic knowledge could be put into the front of a passenger jet.

But that's not his fault - at least he has the humility to ask the question(s) than not.

[George Yogi]

I admit the fact that i was a rote learner , if I continued to be the one then i would not have dared to ask such questions in this public forum having so many experienced wings.

Thanks a lot.

[latetonite]

Well, that is a start.

[The African Dude]

Sorry but I think you guys are being too harsh on this chap.
Everybody starts from scratch. I hope George finds his/her understanding.

[latetonite]

Well, not too long ago, I met several captains, taking over a plane with anti ice inop, who applied performance penalties to the max landing weight. Not kidding.

All the result of not being able to think.

[Piltdown Man]

So let's answer. For any given day, your aircraft will normally be able to lift more more than your actual weight ie. you have excess performance. If you have a look at your performance charts (or laptop) you'll often see that it shows takeoff weights that are greater than your MTOW. Also, there's nothing wrong with selecting all the power you have apart from a few things. The biggest is a reduction in engine life due to the higher temperature. Depending on how you lease your engines, you may find cost penalties which reflect thus. Another is that depending on your method of flexing and power selection, a higher thrust setting may give you a higher take off speed, thus a higher possible stop speed. Due to the higher temperature, you may also increase the risk of an engine. So in order to increase engine life you select a lower thrust rating. This is normally done by entering a false (higher) outside air temperature. Also remember, you will actual burn slightly more fuel doing a flex power take off.

But remember there are days when you don't flex. Like taking off when windshear has been reported, or when there are CBs in the area, or the runway is contaminated, or in mountain wave (or when you are really light and you want a giggle) etc. And if you remember your learning, your maximum take-off weight is the lowest of your MTOW, RTOW, MLW plus trip fuel. Flexing is a thing you do when you have spare take-off performance available and full power is not required.

[George Yogi]

Thanks a ton Pilt.
My understanding from your post is as follows :

When ever ATOW < Max permissible TOW or RTOW from Performance chart for a specific r/w and for a specific day (QNH,r/w condition and length, gradient, elevation, OAT, Obstacles) we have excess performance and we are good to use flex take off for improving the life of the engine by preventing the unnecessary over heating of engine turbine blades by using TOGA power , hence MTOW has no significance here (ie If ATOW = MTOW or ATOW < MTOW , We can fool the engine by entering a flex temp corresponding to the ATOW from a performance chart, provided ATOW is less that max permissible TOW or RTOW)

But why Flex take off consumes more fuel than TOGA power ?

[Piltdown Man]

Whenever ATOW < Max permissible TOW or RTOW from Performance chart...

Not quite... MTOW has nothing to do with flexing. It is only actual weight vs performance limited weight (and change 'weight' to 'mass' in EASA land). So change your opening line to: Whenever ATOW < RTOW from Performance chart for a specific... And remove all other references to MTOW. (Remember - You can often buy MTOW increases for your aircraft from your manufacturer).

Regarding fuel burn - assuming standard speeds, the quicker you are at altitude, the lower you overall fuel burn. But I have to say, I do not believe that it is much of a saving.

[George Yogi]

''maximum take-off weight is the lowest of your MZFM+FOB, RTOW, MLW plus trip fuel.'

Is the above rule respected only the because of :-
1. To avoid the over weight landing in-case we have to return to the departure airport immediately after a take off ?
2. To limit over weight landing at destination given by MLW+Trip Fuel

Any other reason why we take the lowest of these ?

[Piltdown Man]

No. Your maximums are maximums, for whatever reason. You may not exceed them. So even if you can lift more weight from a runway (because you gave excess performance) you may be limited by LDW or ZFW limitations. Also, the overweight return does not really have to be considered. This would only be done because you had bigger problems. Furthermore, such a consideration would make long-haul flying virtually impossible.