Take off thrust settings for Windshear
 

Reading the FCOM SP section regarding Windshear and it states,

"Takeoff with full rated takeoff thrust is recommended, unless the use of a fixed
derate is required to meet a dispatch performance requirement."

I have two questions:

(a) When would we be required to meet a dispatch performance requirement thus reducing the thrust setting to a fixed derate?

(b) If your aircraft has an extremely light takeoff weight - in this case, would you consider using 'airmanship' (common sense) and do a derated takeoff instead of a full rated takeoff with an 'insane' vertical rate? Taking this scenario even further, what if there was a low level off altitude (e.g. 3000 ft)?

a) With contaminated runway where you are V1 limited (lower thrust setting - lower VMCG).

b) Yes, it's only recommended - and you can use other techniques in that scenario - like using improved climb or increasing VR (field length permitting).

and you can use other techniques in that scenario - like using improved climb or increasing VR (field length permitting).

If you don't need the derate for field length, surely you will then just go to full rated ?

The part b), was in case the aircraft was very light. Might be overkill with full thrust, in my opinion. Also may increase risk of tail strike, depending on aircraft type.

(Of course that is a subjective question: How light is "light"?)

Thanks for your response, Cosmo and John.

So I am understanding the correct application of thrust setting selection:

Here's a scenario - We are departing off a contaminated runway where the contaminant is standing water and it's reported on the ATIS as a wet runway, there's been windshear reports (PIREPS) and the winds are gusting, say 15G25 or stronger.

What takes priority now in deciding what thrust setting is to be determined? The PIREPS or the runway status?

I can't find a reference that recommends or suggests something like, "If you have contamination then start with TO2, if the figures show you can get airborne stay with this" or "Use TO MAX for assurance of a,b,c,d"

The part b) answer makes perfect sense now - I'm on a B77W. Despite it has tail strike protection capability, I'm "old school" and prefer not to "test" Boeing's assurance.

Well, if your take off performance doesn't allow you to take off with full thrust, that kind of settles it?

The scenario is a short, contaminated runway. With full thrust you will have a high VMCG. Hence, a high V1 and performance-wise it won't give you enough space to stop from the high V1.

With reduced thrust, your VMCG gets lower and subsequently the V1 can be lowered too, allowing more space to stop.


Combined with wind sheer it all get's a bit theoretical! Perhaps wait for the runway to be cleared? :)

With reduced thrust

I have no doubt that Cosmo knows the drill but, for any newchums reading, he meant derated thrust rather than reduced thrust. Speed schedules are based on the relevant (de-)rated thrust .. subsequent reduced thrust takeoffs don't alter the basic considerations.

Much appreciated, Gents. :ok: