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I was recently answering this question and for some reason it made me think of this fairly famous aviation saying

The three most useless things to a pilot are the altitude above you, runway behind you and fuel on the ground

Now, I get altitude above you - you may as well use some of it lest you meet a mountain. I also get fuel on the ground - if in any doubt fuel up before you go.

But I'm not sure I understand why "runway behind you" is useless in this context

Could someone explain?

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    $\begingroup$ *sigh* Please people, stop asking how to use a runway behind you. For what its worth I was interested if this was a literal saying or if it was figuratively speaking about something other than the obvious answer. $\endgroup$ – Jamiec Dec 11 '19 at 8:34
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    $\begingroup$ Altitude below you is useful for lots more things than "not flying into a mountain". The main use is if you are at a nice high altitude AGL and a problem develops, you have time to think about what to do next, and possibly discuss the issue with co-pilot/ATC and then formulate a plan. $\endgroup$ – Martin Bonner supports Monica Dec 11 '19 at 9:46
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    $\begingroup$ The saying is a bit confusing because 2 of the 3 examples are clearly referring to flying at altitude and the "runway" one only applies to being in the process of takeoff or landing. A "runway behind you" can be very useful if you are high enough to turn around and land on it, especially if it's the closest one in an emergency $\endgroup$ – JoelFan Dec 11 '19 at 20:12
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The runway ahead of you is useful to accelerate and - if needed - to decelerate.

If you enter (or touch down) the runway in the middle, you will not be able to benefit from the part of the runway behind your entering point. It is the same as if that part would not exist. Therefore it is useless.

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    $\begingroup$ unless the runway behind you has arrestor wires on it. $\endgroup$ – Sirex Dec 10 '19 at 19:07
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    $\begingroup$ ... and you caught one... @Sirex $\endgroup$ – Harper - Reinstate Monica Dec 10 '19 at 21:56
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    $\begingroup$ @Harper-ReinstateMonica or you needed to not catch one (not equipped). Unlikely but I was reminded of this famous incident $\endgroup$ – Chris H Dec 11 '19 at 11:12
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    $\begingroup$ @Sirex Even then, it's not the runway itself that is useful, but the wires. $\endgroup$ – HiddenWindshield Dec 12 '19 at 17:22
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It refers to takeoff from an intersection rather than using the full runway length. The part of the runway behind you is now useless. It is part of the saying because the TODA (takeoff distance available) is now reduced and this needs to be taken into account when doing performance calculations. It is e.g. discussed in this thread on pprune.org.

One could also interpret it to refer to landing: When touching down half way down the runway, the part behind you is useless for stopping.

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    $\begingroup$ OTOH, if you happen to be departing an airport with an 11,000 ft runway in say a Piper Cherokee, most of that runway is pretty useless to you :-) $\endgroup$ – jamesqf Dec 9 '19 at 17:59
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    $\begingroup$ Not if you lose an engine 20' off the deck. $\endgroup$ – Michael Hall Dec 9 '19 at 20:31
  • $\begingroup$ @Michael Hall: A good bit of it is, 'cause I can sure land in a lot less than 9000 ft from 20 ft AGL. $\endgroup$ – jamesqf Dec 10 '19 at 5:19
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    $\begingroup$ Yes, but there is a big difference between useless, and not necessary. $\endgroup$ – Michael Hall Dec 10 '19 at 6:50
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In case of engine failure during the takeoff run, it might be possible to land immediately and stop if there's enough runway. But if the run was started partway down the runway, there will be less room to do that, possibly resulting in a more severe result.

The point of all three "useless" items is that, by using them before a problem arises, the results of the problem may be greatly mitigated. It's an indirect way of recommending making use of the resources available to be prepared.

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I've always known the saying as "The three most useless things to a pilot are the altitude above you, runway behind you, and the fuel you had"

What the saying is really talking about, is give up your margin grudgingly, especially when you don't need to.

This works for all aspects of life.

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All three of the useless data are contrasts with useful information:

  • the altitude above you

What matters is how much air is below us.

  • runway behind you

We're only interested in how much runway is available in front of us.

  • fuel on the ground

We care about how much fuel is in our tanks.

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