Flex Takeoffs Margin Question

[gulfairs]

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.

[FLEXPWR]

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?

@rudderrat: no, I'm pretty sure it was MY ex...

Flex

[mutt]

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

[Wizofoz]

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.

[mutt]

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

Mutt

[john_tullamarine]

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.

[Wizofoz]

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.