Flex DeRate on Contaminated Rwys

[novicef]

Hi All
Ive tried Google so I thought somebody could come up with reasons.

Why is it, that you can DeRate but not Flex on a contaminated Runway?

Is there a reference some where?

[Denti]

Derate is quite different from flexing, at least in the way boeing uses it on the 737NG, there's no derate i know of on the A320 although the A330 seems and probably other airbus types seem to have some kind of that.

Anyway, derate is the same as using another engine for that take off, it includes different limiting speeds and although you can use full thrust, you shouldn't do as you will go outside the certified envelope for that derate which can result in loss of control. Especially the lower VMCg with a derate can actually improve take off performance on a contaminated runway, as one can use a lower V1.

Flex on the other hand is still using the same limiting speed as the thrust rating used. One can legally advance thrust to the limit of that rating (which might be full thrust if flex and derate are not combined). As far as i know there is no certification point for use of flex on contaminated runway. Therefore it is not allowed by the OEMs as it is outside of what they test and certify for.

But there have been quite a few threads about that issue in the past on here.

[Goldenrivett]

Hi novicef,

Think of a Derated engine + airframe as a different model to the same airframe with with a non derated engine.
see http://www.skybrary.aero/bookshelf/books/2263.pdf 7.2 Derated Takeoff:
“AMJ 25-13 / AC 25-13
(4)(b) Derated takeoff thrust, for an aeroplane, is a takeoff thrust less than the
maximum takeoff thrust, for which exists in the AFM a set of separate and
independent takeoff limitations and performance data that complies with all
requirements of Part 25.”
In this case, “the thrust for takeoff is considered as a normal takeoff operating
limit.”
For a derated takeoff, the limitations, procedures and performance data must be included in the Aircraft Flight Manual (AFM). For each derate level, a specific RTOW chart can be established for a given runway, taking into account such new limitations as the minimum control speeds.

We used to use Flex on top of the derated figures when conditions allowed (i.e. no contamination).

If you are operating from a contaminated runway and are using derated figures or full rated figures then you can't flex.

[742]

Flex would be bad judgment. Given a contaminated runway with unknowns, why give up performance?

DeRate is another thing altogether. As others have pointed out, it is like having different (lower rated) engines on the airplane, and everything that goes with it. And the key is that it includes a lower Vmc, which then leads to lower V speeds. The extra thrust from "bigger engines" is often not worth the effort/time/distance needed to get another 10 knots (+/-) of speed when plowing through slush.

[Gauges and Dials]

Especially the lower VMCg with a derate can actually improve take off performance on a contaminated runway, as one can use a lower V1.


I understand how lower VMCg and lower Vr would improve takeoff performance, but I don't understand how a lower V1 would help?

[midnight cruiser]

Lower V1 = shorter accelerate stop distance, which with a contaminated runway, could be quite limiting.