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If in an emergency and a modern passenger aircraft had to glide, will it make a big difference if the aircraft had a headwind rather than having to glide downwind to nearest airport?

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    $\begingroup$ Any tailwind will increase your gliding distance. Any headwind will decrease your gliding distance. The stronger the wind, the more it will have an effect. $\endgroup$ Aug 3 at 20:07
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    $\begingroup$ Welcome to aviation.SE! I suggest you have a look at this question and this one to see if they help. $\endgroup$
    – Pondlife
    Aug 3 at 20:10
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    $\begingroup$ @Pondlife the first link doesn't consider wind and the second is wrong in that it only considers best glide speed. The speed-to-fly is what we want to know for how far the plane can glide, and that is heavily influenced by head and tailwinds. $\endgroup$ Aug 3 at 21:25
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Maybe

The only way I can think to characterize a "big difference" for an airplane which has lost its engines is "Can the plane make it to the runway in time?"

If that's the question, then the answer has to do with the plane's glide polar.

Theory

The way to read the below chart is that the airspeed is on the x-axis and the sink rate is on the y-axis. You'll notice that as airspeed decreases, sink rate decreases; up until that sharp hook, which is when the plane starts stalling.

The best glide range occurs when the straight line is tangent to the L/D curve. (green curve in the below picture)

enter image description here

The straight line's starting point is determined by the atmospheric conditions:

  • Calm air: the starting point is the origin (red line)
  • Sinking air: the starting point moves up the y-axis (yellow line)
  • Rising air: the starting point moves down the y-axis (not shown)
  • Tailwind: the starting point moves back the x-axis (black line).
  • Headwind: the starting point moves forward on the x-axis (purple line)

Application

So the question of how much of a difference it makes comes down to how far the headwind vs tailwind moves the tangent line's starting point. You can see that the stronger the headwind, the steeper the tangent line. You can also see that a headwind has a very similar effect to sinking air, only the difference is that you never fly out of the sink.

As a results, a headwind has a surprisingly large impact on glide ratio. How much is dependent on how far the plane needs to fly and how much of a headwind it has vs. its glide polar.

Unfortunately, as is the way of these things, a headwind hurts much more than a tailwind helps. From my experience in gliders, I'd hazard a guess that a headwind which is only 10% of the best glide speed will have a >20% impact on range. 10% of an airliner's speed is quite fast, but it's not crazy fast and if it were flying in or near the jet-stream the plane could easily be fighting headwinds much stronger than that.

One last consideration in determining the glide range is how winds will change in both heading and direction as the plane descends. Check out the altitude slider at www.windy.com to get a feel for how the global winds vary with altitude.

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    $\begingroup$ The answers are amazing! $\endgroup$
    – Tevios
    Aug 4 at 16:32
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    $\begingroup$ @Tevios glad it's helpful. Don't forget to upvote answers which are helpful, and to designate one as the accepted answer (if so appropriate). Although I'd give it some time yet, no need to rush. Others may yet be preparing their answers, too. $\endgroup$ Aug 4 at 20:17

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