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Should I take the longer runway

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Should I take the longer runway

Old 19th Apr 2016, 21:36
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Should I take the longer runway

You are at an airport with two parallel runways operating a jet airliner. A shorter one and a longer one by perhaps a couple of thousand feet. Both are available for departure. The taxi distance to the shorter one is a couple of minutes shorter.

The aircraft is near max takeoff weight and the takeoff performance calculation shows that at the takeoff weight for the aircraft, the short one is acceptable but it is right at the limiting weight for the runway length using full takeoff thrust.

So it is decided to use the longer runway. The only thing is that with the longer runway a derate and ATM thrust reduction is used for the takeoff along with a different flap setting.

Without including a bunch of variables such as the longer runway covers weather changes, payload adjustments, etc.......How much advantage have you gained by using the longer runway. Is the reduced thrust takeoff negating most or all of the advantage that the longer runway offers?
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Old 19th Apr 2016, 21:54
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Good quesiton, JammedStab. I guess you have gained an advantage of not having to use max thrust for takeoff. Maybe not a big deal on the 744, but on the 742 with the worn out Pratts it was something to avoid, if possible. Since ATM is used on the longer runway instead of a straight derrate (TO 1, TO 2), then you could still push the power up on the rest if you lost an engine. Plus, if you are over guarantee you get more time taxiing to the longer runway. Would management call you on it?
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Old 19th Apr 2016, 21:58
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Thanks. Just a performance question so cost in not a consideration.
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Old 19th Apr 2016, 22:22
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You seem to think that a reduced thrust takeoff is a bad thing?
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Old 19th Apr 2016, 22:48
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The reduced thrust means that all other things being equal, you're probably more controllable in the event of an engine failure. (Although both cases meet minimum certification standards, the manner in which they do so may differ)
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Old 19th Apr 2016, 22:54
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Reducing the take off thrust reduces the 'wear' on an engine, hence cost.
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Old 19th Apr 2016, 23:14
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Assuming that you meet all performance requirements and any additional company requirements like a 15% margin on runway lengths, then either runway is acceptable. That is the point of doing the numbers. As for the economics, they have to be set in context. If you need the extra minutes to get to a closing airfield or an airborne slot, then the shorter runway is obvious choice. But if you are told "de-rate" any any cost, then it's the longer runway only. But if, like most of us here, you are not given the raw numbers you will have to make up your mind. If I'm on time, I would have prepared the numbers for both but I'd flip a coin to decide.


PM
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Old 19th Apr 2016, 23:25
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Is the reduced thrust takeoff negating most or all of the advantage that the longer runway offers?
No.
There is better overall aircraft performance than you may suspect due to the real OAT is colder than the assumed temperature input for the engines. The engines would produce the same thrust if operated at the assumed temperature, but the wing is flying in the denser air at the actual OAT.
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Old 19th Apr 2016, 23:47
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Always work from the hearing backwards...
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Old 20th Apr 2016, 00:17
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If you use a performance computer to calculate takeoff performance, it is VERY likely V1 will be increased significantly on the longer runway, and the stop margin will not increase significantly.

In cases such as you describe, I often run several calculations, with and without derates and ATM reductions. I look for a good balance between thrust required, V1, and stop margin.

Also, I will ask for the longer runway when near the performance limits on the short runway.
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Old 20th Apr 2016, 03:48
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Depends on performance software i guess. The airbus thingy throws out the highest reduction possible, right down to a 1m margin left on the runway. However, it also provides all lower flex temperatures and runway remaining margins for those figures, so take the one with the margin you personally are happy with.

That said, the company wants us to use reduced thrust as much as possible, so if there is no operational downside i would take in that situation the longer runway, reduce thrust a bit and use a larger margin.
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Old 20th Apr 2016, 05:44
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Good answer and a good point by GRivet.
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Old 20th Apr 2016, 14:58
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I don't think you will ever get a clear answer to this question. A longer runway has a higher flex, which lowers maintenance. But if the extra fuel burnt to get to that runway is excessive, than who are we kidding'? If your company's beancounters does not provide specific guidance, then they probably don't care either way.

As far as safety is concerned, as long as the numbers are correct, it should not matter to take the shorter runway. Not even at the hearing afterwards.

Last edited by PENKO; 20th Apr 2016 at 15:12.
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Old 20th Apr 2016, 20:02
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I agree with PENKO.

Many of us operate from a large selection of airfields in a variety of conditions. Unless they are particularly adverse (contamination, etc.) if the numbers work out then the quickest and easiest makes sense.

In some aircraft that use fixed derates as well as ATM I think Iíd rather take the short runway and get some OEI performance!
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Old 20th Apr 2016, 22:13
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I would compare the stop margins and use the plan that gives me the most wiggle room. As one gent said" always work from the hearing backwards".
Could you use the shorter runway? maybe yes, but is it wise, when there is a better solution.
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Old 21st Apr 2016, 01:18
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We often had this at LAX. 24L vs 25R. 95%+ of crews would take 24L if it fit even if pushing right up on the numbers.
 
Old 21st Apr 2016, 04:03
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Originally Posted by Water Wings View Post
We often had this at LAX. 24L vs 25R. 95%+ of crews would take 24L if it fit even if pushing right up on the numbers.
I would take 25R at LAX every time, but then we parked right next to it.
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Old 21st Apr 2016, 06:01
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So, your question is
How much advantage have you gained by using the longer runway.
And then you have said to ignore cost.

So I'm not actually sure what "advantage" you are referring to. If you are referring to performance advantage in the case of an engine failure, the answer is what advantage? Either runway is safe in the case of an engine failure. One will have more thrust, so will be harder to control, but will climb better. The other will be easier to control but not climb as well, but will still clear terrain.

But, are you asking which runway is safer?

I would say the long one. For various reasons, as follows:

1. An engine is less likely to fail at lower thrust. I know many pilots feel better with more thrust, but all they are doing is increasing the likelihood of an engine failure. An engine at lower thrust will also suffer less damage from bird or foreign object ingestion.

2. On the longer runway ASDA for an RTO is less likely to be the limiting factor, so less braking may be required reducing the possibility of a brake/wheel fire or tyre deflation. Again, safety improvement.

3. What may conspire to kill you on the day may not actually be the certified standards of engine failure/stop/go scenarios. In fact, none of these should kill you because the aircraft is certified not to. What may kill you is a multiple failure, an error (eg thrust calculation error), a sticking brake, tyre failure, manipulation or handling error, or other event that is not possible to plan for. In that case, having more runway to sort it out may save you.
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Old 21st Apr 2016, 08:34
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...or you may hit a dog crossing the runway whilst still slowly accelerating towards 155 knots VR when, on the shorter runway you would have been airborne ages ago with a VR of 129 and TOGA thrust.

..or your tire may fail catastrophically at those higher speeds on the longer runway.

..or you might inadvertently cross an active runway whilst taking a detour to the longer runway.

..or that careless buss driver slams into your right wing whilst taking said detour.


It is almost a philosophical question, no clear cut answers
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Old 21st Apr 2016, 08:45
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1. An engine is less likely to fail at lower thrust.
Now, thatís a reason I would have given and it seems obvious but is it actually true of modern (jet) engines? Are there any statistics available to back it up, genuine question? With many piston engines peak mechanical and thermal stress occurs at less than peak power (not that this has anything to do with turbines but it shows that the simple answer is not always the most correct).

2. On the longer runway ASDA for an RTO is less likely to be the limiting factor, so less braking may be required reducing the possibility of a brake/wheel fire or tyre deflation. Again, safety improvement.
With the software we use, adding runway length often results in a lower flap setting (increased speeds) and improved V2 (even more speed). As brake energy is proportional to speed squared, that can be quite significantly more in the event of an RTO.

3. What may conspire to kill you on the day may not actually be the certified standards of engine failure/stop/go scenarios. In fact, none of these should kill you because the aircraft is certified not to. What may kill you is a multiple failure, an error (eg thrust calculation error), a sticking brake, tyre failure, manipulation or handling error, or other event that is not possible to plan for. In that case, having more runway to sort it out may save you.
Agreed. Although I would add that there are many examples of aircraft failing or nearly failing to get airborne from very long runways due to too much thrust reduction, which might not have happened if full thrust or thereabouts was expected to begin with off a much shorter strip.
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