Flex temp requirements

[Speedbird744]

One of the requirements for a flex takeoff, is takeoff thrust must not be less than 25 percent full rated thrust. How can you actually determine this?

Is it built in to the takeoff charts so you know you are guaranteed to not get less than 25 percent?

Secondly, Tmax is the maximum flex temperature...Why is it possible to takeoff with a flex higher than this?

Thanks for your help.

[mutt]

Are you combining Airbus and Boeing terms?

To begin with, Airbus dont combine fixed derates and flex, therefore they are permitted to use flex of 40%, not 25%.

TMAX is the standard temp envelope, you can extend into TMAXFLEX with an increased temp envelope. You can assume a temp hotter than TMAX, but you cant operate if the actual temp is hotter than Tmax.

Mutt

[FlightDetent]

T max is max OAT for given conditions. It is possible to have the reduced thrust setting, indicated with T flex, higher than this.

Setting T flex = 65°C, you are saying to the aircraft:
Hey, now please give me just as much thrust as you would be able to provide if it were this hot outside: 65°C. The maximum setting is determined by the 25 per cent requirement, hence it is not possible to set Tflex=85 because it would represent a larger thrust reduction than is allowed.

A1) The max flex setting is read from RTOW charts or obtained from PC performance calculation.

However, if it really was 65 degrees outside, you may not take-off. it is too hot. Maximum OAT for takeoff is usually lower than that. Tmax(OAT) <> Tmax(flex). The highest values for flex setting are pseudo-temperatures and only serve to set an appropriate level of engine thrust, real OAT for take off is limited by Tmax(OAT).

A2) T(flex) must be less than Tmax(flex) but once this condition is satisfied, T(flex) may be actually more than Tmax(OAT). Takeoff with OAT higher than Tmax(OAT) is prohibited.

FD (the un-real)

[FlightDetent]

To begin with, Airbus dont combine fixed derates and flex, therefore they are permitted to use flex of 40%, not 25%.

Always assumed (now here's the Boeing term!:E) that AST/FLEX is limited to 25% of applicable TOGA, but with derates (no personal experience) you could go lower ... to 40% :confused: Obviously you say otherwise, I promise to do the homework myself but mutt, can you point me in the right direction?

FD.

[ITCZ]

takeoff thrust must not be less than 25 percent full rated thrust. How can you actually determine this?
The FCOM or AFM might specify a minimum N1 or EPR for flex.

N1 or EPR is proportional to Thrust.

[FE Hoppy]

Embraer include max flex temp tables. This is the 25% reduction or just above climb 2, whichever is the lower temp i.e. higher thrust.

[mutt]

FlightDetent,

I would have thought that it was in 25-13, but it isnt! So there must be some obscure amendment! Will try to get the exact reference when i go to school today.

Airbus are using an increased TMAXFLEX of up to ISA+63 on some aircraft, so they must have justification to do so.

Mutt

[barit1]

N1 or EPR is proportional to Thrust.

Errr, not quite, at least not a 1:1 relationship.

Typically a 1% N1 change will give 2.5 or 3% thrust change. Check it out; your mileage may vary.

[Slick]

Dont know if this helps (Boeing) 25% Max reduction in thrust for the T/O thrust rating used for the takeoff.

For example on Ryanairs 800 there are two thrust derates. The engines are 26K and they have 24K and 22K derates line selectable in the FMC. The 25% applies to the derate used. So 22K derate with a 25% thrust reduction would be a lot, maybe as much as 40% of the engines rated 26K thrust. But because you are using 22K derate its legal. One of the many reasons for derates.

As far as knowing where 25% is the FMC protects for that (Smiths). Also in the Boeing performace Inflight section QRH you can see the max temp that would generate a max 25% reduction for 26/24 and 22K takeoffs. The FMC will accept a higher temp for takeoff but the thrust reduction will not exceed 25%

Best Rgds

[mutt]

FlightDetent, Airbus filed a Certification Review Item with JAA/FAA, they were issued with a cover note permitting the use of Flex to 40%. It doesnt apply to all their aircraft nor engine combinations, the particular flight manual will show the appropriate limits.

Mutt

[FlightDirector7]

Speedbird,

You are correct that Tflex cannot be higher by more than 25% of OAT. Basically, the requirements for a flexible take off are :

1. Tflex is > OAT
2. Tflex is > Tref
3. Tflex is < or = Tmaxflex

For A320 only : Tref = ISA+30, Tmax = ISA+40, Tflex = ISA+55 (Tref and Tflex will vary accordingly for A319 and A321).

Simple example : airfield elevation 3000 ft.

--> 2 deg C per 1000 ft lapse rate = 3 x 2 = 6 degrees C
--> 15 degrees standard temp at sea level = 15 - 6 = 9 degrees C
--> Tflex from above = 55 + 9 = 64 degrees C, this means that your flex temp for take off should not exceed 64 degrees (25% of rated thrust or Tref). This is the last row of flexible temp you will see on your RTOW chart corresponding to that particular ATOW.

Tmax for the above example would be = 3 x 2 = 6 degress C

--> 40 + 6 = 46 degrees ( at 46 degrees OAT you cannot take off )

I hope this helps :ok:

Cheers

[FlightDetent]

Thank you, Mutt.

[mcdhu]

On our nice new fleet of minibuses, we are up to a Tmax flex of ISA+60 so a SL T/o at Tmax Flex gives a Flex of 75 degrees and an N1 of circa 78% (FF of appx 2x2800kg/hr). As the ac transitions from the T/o to the Clb Phase, the N1 increases (40% to 25% i guess) to circa 84%. Poetry in motion!!

Cheers
mcdhu

[mutt]

FLightDirector7, your following statement of
You are correct that Tflex cannot be higher by more than 25% of OAT. is incorrect, please read the rest of the thread and you will see that Airbus operate to -40% thrust reduction!

(Tref and Tflex will vary accordingly for A319 and A321).... Ummmmm, are you sure about this?

Mutt

[Down Three Greens]

Mutt's correct. New EEC's standards for the A330 (Trent 772) extend the maximum Flex allowable higher than the standard 25%. Trent EEC 13.0+ if I remember correctly.

[FlightDirector7]

Dear Mutt,

We operate V2725-A5 IAE engines on our aircraft. I would guess that Tmax flex would differ on CFM engines. I must say that I am unaware of the the 40% thrust reduction as we only operate within the range of 25%, hence my previous post.

Secondly, im not sure of the type of engine you guys operate, however I am certain that the Tref on 319 is ISA+30 and on the 321 is ISA+15. Tmaxflex on the 319 is ISA+65 and on the 321 is ISA+45.

I'm quite sure these would vary on the manufacturer.

[ITCZ]

Errr, not quite, at least not a 1:1 relationship.
:hmm: Proportionality (http://en.wikipedia.org/wiki/Proportionality_(mathematics)) is not Equivalence (http://en.wikipedia.org/wiki/Equivalence_relation)

I say again... N1, or EPR, is proportional to Thrust.