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Why Boeing can not use assumed on contaminated Runway?

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Why Boeing can not use assumed on contaminated Runway?

Old 19th Mar 2016, 15:01
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Why Boeing can not use assumed on contaminated Runway?

Why?? Is there any special reason?
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Old 19th Mar 2016, 15:06
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Uh, assumed would be reduced power, can't do on a contaminated RWY.
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Old 19th Mar 2016, 15:17
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@TowerDog, so is a derated thrust rating, which you can do on a contaminated runway.

I think that it has to do with the minimum control speeds which are calculated according to the thrust rating.
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Old 19th Mar 2016, 16:03
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@TowerDog, so is a derated thrust rating, which you can do on a contaminated runway.
If so, I stand corrected.
I usually went full power on a contaminated runway.
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Old 19th Mar 2016, 16:18
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AC 25-13 - Reduced and Derated Takeoff Thrust (Power) Procedures

5. f. The AFM states, as a limitation, that takeoffs utilizing reduced takeoff
thrust settings:
(1) Are not authorized on runways contaminated with standing water,
snow, slush, or ice, and are not authorized on wet runways unless suitable
performance accountability is made for the increased stopping distance on the
wet surface.
So the AC makes an absolute prohibition against using reduced thrust for contaminated runways. No such limitation applies for derated thrust, which is essentially treated as if the aircraft had been built/certified only with that lower thrust level.

Bear in mind in this context that the FAA's handling of contaminated runways is somewhat less sophisticated than that of some other authorities.
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Old 19th Mar 2016, 17:15
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Assumed temperature frequently will result in a balanced field. The assumed thrust result would allow for a failed engine at V1 and reject or continue. If rejected, stopping on the remaining runway.
Now since the runway is contaminated stopping distance would increase, maybe significantly.
Many aircraft ago, before de-rated thrust, we could calculate assumed temperature if the runway had, as I recall 750 meters beyond balance field.
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Old 19th Mar 2016, 17:18
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I usually went full power on a contaminated runway.
It is somewhat counterintuitive, but using dereate (lower thrust) allows in some cases to take more weight out of a runway than full thrust, since Vmcg is lower and once can use a lower V1.
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Old 19th Mar 2016, 17:32
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Indeed Denti. It takes quite a bit of head scratching for cadets to begin to understand this idea: i.e. less mph = more weight. Back to reduced thrust and contaminated runways: this is another head scratcher. You can use derate 1 or 2 - full, but you can't use full power with assumed temp.
Previously, we had a discussion about the use of derate & assumed temp and I asked why we don't just use assumed temp from full power, as some XAA's insist. The answer was to do with climb thrust v 25% reduction etc. or some such. Thus derate & assumed could be better. BUT, somewhere (B737) 26k assumed temp = derate 1 or 2 full. So the XAA's that insist on 26k only, and then assumed temp, would insist on 26k on contaminated runways, where as another XAA would allow derate 1 or 2 full. They can't both be correct from a safety point of view. To me thrust v weight is a question of physics to satisfy the takeoff performance, both on the ground and in the takeoff segments. Thrust also effects VMCG & VMCA. Thrust is thrust no matter how it is calculated via 26K 24K or 22K.
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Old 19th Mar 2016, 18:29
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. It is somewhat counterintuitive, but using dereate (lower thrust) allows in some cases to take more weight out of a runway than full thrust, since Vmcg is lower and once can use a lower V1
Yeah, I remember some weird results if digging deep enough in the performance manuals. One of them was that 3/4" slush would allow for a higher weight than 1/2". Forgot how but it made sense when the instructor explained it.

I am bowing out of this, retired last year and really don't care anymore :-)
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Old 19th Mar 2016, 19:58
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I am bowing out of this, retired last year and really don't care anymore :-)
Congratulations, i really do envy you with another 20 something years to go...

As far as i understand a user selectable derate is legally the same as using pin programming to derate the engine permanently. So, while you cannot use assumed temperature (see the link to the FAA AC above, but i believe the EASA takes a similar stand) on a contaminated runway, you can use a different engine (rating).

Yes, it does not make sense if you just focus on the reduced thrust, but a derate has some other implications, for example using more thrust might violate the VMcx speeds, so pushing the trust levers up could be a bad idea, especially if taking advantage of the lower possible V1s. I have seen V1s below a 100kts on the -700, which might explain while boeing sticks to the 80kts call instead of the 100kts call airbus uses
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Old 20th Mar 2016, 06:33
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There is no reason apart from some joe bloggs who wrote the rule.

You can use a derate if you have the performance figures for contaminated with derate. This is a cost to the manufacturer to produce the figures.

There is no physical reason to not reduce with assumed to the equivalent derate thrust and you have the advantage of having more thrust available after the failure. But it's all about the cost of producing the performance data.
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Old 20th Mar 2016, 07:16
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On a flight from KPSM to RAF Mildenhall one cold wintry morning in a KC-135, I was asked to make a water injection takeoff to make sure the system was functional. OAT was somewhere around 25į F and we were light. Runway was covered with packed snow and ice.

A static takeoff was made but when the thrust increased rapidly as the engines took water, the nose rose to such a degree, even with full down elevator, that I had almost no nose wheel steering authority.

Fortunately, I had just enough rudder control to keep us going straight but, in retrospect, I should have immediately pulled back the throttles and aborted the takeoff.

Stupid things you do when you're young and inexperienced...

Last edited by wanabee777; 20th Mar 2016 at 23:55.
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Old 20th Mar 2016, 22:54
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Lower thrust with a derate can also reduce your V1 speed.

Lower thrust with assumed temperature must 'assume' a Vmcg based on full thrust available, thus limiting V1min to a higher speed than is available with a derate.

So, yes, assumed temperatures and fixed derates may result in the same actual thrust energy, but different V1 speeds.
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Old 21st Mar 2016, 01:14
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Yeah, I remember some weird results if digging deep enough in the performance manuals. One of them was that 3/4" slush would allow for a higher weight than 1/2". Forgot how but it made sense when the instructor explained it.
I have noticed the same. Is it the roll resistance from the slush/snow that helps achieving a better deceleration if stopping, hence you can take more weight? That's the only reason I've come up with so far.
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Old 21st Mar 2016, 02:56
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Back to the original question:

I believe the answer is that Boeing doesn't do certification testing in that configuration.

Obviously you could have a situation where a full derate provides the same thrust as the higher rating with assumed temperature, and therefore the takeoff would be safe. However, between Boeing and the FAA in the certification process, this (arbitrary) rule is introduced. In practice, I don't find it limiting in any way.
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Old 21st Mar 2016, 08:54
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You CAN takeoff with assumed temperature thrust reduction from contaminated runways.

All you need to do first (just like with any takeoff) is find performance data which will allow you to do it. Oh yeah there is none.
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Old 21st Mar 2016, 11:31
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Also, it is difficult to determine with much precision what surface conditions actually are. Every runway is different, the distribution of the contaminant, braking action, etc. is varied so itís sensible not to start off with eroded margins.

You might get away with full power in a situation where reduced thrust was not enough, in retrospect. Why risk it? We canít quantify contaminated runways to the same level we can TORA, OAT, ATOW, etc.
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Old 21st Mar 2016, 11:57
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. Congratulations, i really do envy you with another 20 something years to go...
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Old 21st Mar 2016, 14:58
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I have noticed the same. Is it the roll resistance from the slush/snow that helps achieving a better deceleration if stopping, hence you can take more weight? That's the only reason I've come up with so far.
Actually, it's just the opposite. On a contaminated runway, you will most often be limited by the the "stopping case", i.e. you will require more runway to stop for a given V1. To satisfy the requirement to stop after an RTO at V1, you begin to reduce the V1 value with the limitation that V1 must be greater than Vmcg and Vma. With an assumed temperature takeoff, all rated power remains available and thus, Vmcg and Vma are predicated on full thrust. As has been highlighted before, a derated engine is akin to "buying an engine" with lower thrust so the Vmcg and Vma values will be based on that lower thrust and will be lower, allowing a lower V1.

I would be surprised if Airbus didn't have the same restriction on use of assumed temperature on contaminated runways though I suppose it would be possible to reduce the thrust to the point that you still maintain the full rated thrust Vmcg and Vma. If that is so, you would be denying yourself some of the value of the increased payload that you sometimes get when reducing the Vmcg and Vma values.

The really important point that I don't think is emphasized enough is that when using a derate, you risk a loss of control when pushing up the power beyond the derate limit in an asymmetrical situation. When using a derate WITH an assumed temperature, you theoretically have additional power available to you up to the derate limit but unless you actually know what that limit is, you're playing with fire. Most carriers that I'm familiar with do not have procedures for calculating the actual derate limit N1/EPR/TPR, etc..
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Old 21st Mar 2016, 18:04
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The really important point that I don't think is emphasized enough is that when using a derate, you risk a loss of control when pushing up the power beyond the derate limit in an asymmetrical situation. When using a derate WITH an assumed temperature, you theoretically have additional power available to you up to the derate limit but unless you actually know what that limit is, you're playing with fire. Most carriers that I'm familiar with do not have procedures for calculating the actual derate limit N1/EPR/TPR, etc..
Yup, that is why boeing suggests that one does not increase thrust during a derate take off until, in the opinion of the captain, terrain contact is imminent. After all, the take off performance is calculated with the reduced thrust level and the obstacle situation of the runway in question. There is no need to increase thrust if all that fails is just the one engine.
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