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Assum(flex) temp power takeoff on contaimnated runway

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Assum(flex) temp power takeoff on contaimnated runway

Old 2nd Jul 2011, 04:45
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Assum(flex) temp power takeoff on contaimnated runway

Does anybody know why we can not use Assum(flex) temp power for takeoff on contaminated runway. Thanks.
kuobin is offline  
Old 2nd Jul 2011, 07:50
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First thing that comes to my mind is that once a runway is contaminated, there is no garantee as to the uniformity of the contaminant, quality, regularity of depth, etc. So no accurate take off performance would reflect the reality. Therefore MAX Power/TOGA.

But there must some other more pertinent reasons I guess.

Flex
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Old 2nd Jul 2011, 08:17
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In FAA Land, Contaminated Runway information isn't certified, Assumed Temperature information IS certified, the Assumed Temperature Method was approved based on FAA AFM data, and as the AFM didn't have contaminated data there was no way to certify its use with Assumed Temperature.

As technology got better, the basis of contaminated runway data was never changed, its still based on the affects of contaminants during flight testing on a NASA Convair 880 aircraft, and is extrapolated for various aircraft. It is not flight tested for commercial aircraft.

There are limitations on using the data, such as...On an EPR engine, there is no easy way to check the corresponding Assumed Thrust N1, whereas you have charted values for EPR/N1 at actual temperature. On a contaminated runway you can therefore see the Actual EPR vs N1 and guard against wrong readings due to contamination/icing.

And as FLEXPWR has stated, contamination on a runway isnt uniform, so allowing fudge factors makes sense.

Mutt
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Old 2nd Jul 2011, 09:37
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Maximum power for take-off = minimum runway distance covered to get to V1 (Vr) speed = maximum runway left to use in the event of a stop.

The concern is that the stop may be longer than assumed in maximised performance calculations (flex calculations) as it is difficult to accurately determine how much any particular amount of contamination will affect the aircraft.
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Old 2nd Jul 2011, 10:31
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We can on the 747-400.... It is all to do with Vmcg and other complicated stuff that my knackered brain is trying to remember. It is strictly not a "reduced thrust using assumed temperature" but the thrust is reduced...

Roughly, I think, We can lift more off a contaminated runway (sometimes) using reduced power because in the engine failure case Vmcg is artificially higher... because of the reduced thrust.

It is a seriously complicated calculation with many variables of contamination type and depth, runway length and braking action, and one that is best avoided if at all possible.
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Old 2nd Jul 2011, 10:33
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Like Checks inferred above, when the runway's contaminated all
reduced thrust bets are off and therefore its balls to the wall.

The 747 had the option of DERATING the engines which reduced
Vmcg so lowering your V1, and so not having to incur a higher TO
(read payload) penalty.
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