Why Boeing can not use assumed on contaminated Runway?
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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.
I think that it has to do with the minimum control speeds which are calculated according to the thrust rating.
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@TowerDog, so is a derated thrust rating, which you can do on a contaminated runway.
I usually went full power on a contaminated runway.
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AC 25-13 - Reduced and Derated Takeoff Thrust (Power) Procedures
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.
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.
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.
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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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.
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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I usually went full power on a contaminated runway.
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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.
I'm only a simple northerner where a spade is a bloody shovel.
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.
I'm only a simple northerner where a spade is a bloody shovel.
SkyGod
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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
I am bowing out of this, retired last year and really don't care anymore :-)
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I am bowing out of this, retired last year and really don't care anymore :-)
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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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.
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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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 25° 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...
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 22:55.
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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.
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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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.
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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.
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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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.
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 it’s 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 can’t quantify contaminated runways to the same level we can TORA, OAT, ATOW, etc.
You might get away with full power in a situation where reduced thrust was not enough, in retrospect. Why risk it? We can’t quantify contaminated runways to the same level we can TORA, OAT, ATOW, etc.
SkyGod
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. Congratulations, i really do envy you with another 20 something years to go...
Fortumalty I invested a few shekels in the up-down-up Florida real estate market and could afford to call it quits after 37 years and more long haul all-nighters than I care to remember.
Carry on Gentlemen and keep the dirty side down.
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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.
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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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..