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If taking off in q tailwind is unsafe (due to loss of lift from reduced air speed), then why is it safe flying in tailwind, such as in jet stream?

Pondlife
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Harshil Sharma
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3 Answers3

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I once had a flight instructor tell me, "As soon as you get airborne, there's no wind." While this is not strictly true in all cases (wind shear is still a thing), it does hold true for flying with the wind. The aircraft is moving in the moving air mass, so the fact that the wind is "coming from" behind you only helps you get to your destination faster. Aircraft can fly at the same airspeed no matter which direction the air mass is moving.

Greg Hewgill
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“Wind” is simply an air mass moving over the ground. When flying, planes do not experience wind per se because they are flying at a (mostly) fixed speed relative to that air mass.

We speak of a “headwind” or “tailwind”because the movement of the air either reduces or increases the plane’s speed relative to the ground.

We want to takeoff and land with a headwind because the reduced ground speed means we need less runway to achieve the same speed relative to the air, whereas a tailwind means we need more runway.

StephenS
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  • From what I think, more important than runway length required, a tailwind will require more engine power to maintain sufficient airspeed to prevent a stall. Which is what makes me think that a jetstream can theoretically reduce an aircraft's airspeed and may remove all lift. It's not a problem if jetstream is at 300 kph and the aircraft is moving at 800 kph IAS. Can, theoretically, would a 800 kph tailwind possibly stall the aircraft? – Harshil Sharma May 24 '21 at 05:44
  • @HarshilSharma sudden increase of tailwind can stall an aircraft, see e.g. Delta Air Lines flight 191. The jet stream is, however, hundreds of kilometers wide and kilometers high, so even though the wind speed can be high, the change when entering or leaving it is gradual, so the aircraft will catch up it easily, the airspeed remaining constant and just change in ground speed indicating the presence of a jet stream. – Jan Hudec May 24 '21 at 08:31
  • @HarshilSharma Two side notes: 1. transport aircraft never move at 800 km/h indicated. 800 km/h true airspeed is rather low, but due to the low air density up in the cruise flight level the indicated airspeed is only around 250 knots (460 km/h). 2. in cruise with flaps retracted the margin to stall isn't that large, so a 300 km/h wind shear would almost certainly stall the aircraft—fortunately the jet stream is never sudden. – Jan Hudec May 24 '21 at 08:35
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    @HarshilSharma, no, tail wind does not require more engine power to maintain sufficient airspeed. The aircraft flies relative to the air, so the power to airspeed is always the same. The only problem with tail-wind is that the runway is moving aft under the air mass while the aircraft is taking off or landing, so more of it is needed. – Jan Hudec May 24 '21 at 08:37
  • @HarshilSharma Incorrect. I normally cruise at 75% power, which results in an airspeed of 110kt. If there is a 10kt tailwind, that means a groundspeed of 120kt. If there is a 10kt headwind, that means a groundspeed of 100kt. My plane stalls at an airspeed of about 45kt; my ground speed is irrelevant. – StephenS May 24 '21 at 12:41
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If taking off in q tailwind is unsafe (due to loss of lift from reduced air speed)

This is incorrect. The airspeed required to take off is the same, no matter the speed of the wind relative to the ground.

Taking off in a tailwind is potentially unsafe because the required ground speed is higher. If you take off in a tailwind, you may run out of runway before your airspeed is high enough.

You cannot run out of runway while in flight.

Anonymous Physicist
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  • Why was this downvoted?! – Michael Hall May 24 '21 at 14:45
  • @MichaelHall -- Maybe someone thought the quote was correct-- in the sense that at any given point in the take-off run (specified either in terms of elapsed distance, or elapsed time), the airspeed will be less when there is a tailwind. I'd say the answer would be improved by deleting the very first sentence after the quote. – quiet flyer May 24 '21 at 16:55
  • But the airspeed will NOT be less, the ground speed will just be greater. (Unless you say airspeed compared to ____.) This will affect time to accelerate, ground roll distance, tires speed, etc. – Michael Hall May 24 '21 at 18:48
  • The word "less" always implies a comparison, so I stand by my comment. Of course, the airspeed at the moment of liftoff will not be less, normally. The quoted statement is vague enough that you one can argue either way, that it is correct or not correct. – quiet flyer May 24 '21 at 19:17
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    No, it doesn't imply any comparison! If you rotate at 80 knots you should ALWAYS rotate at 80 knots whether you have a headwind, tailwind, or no wind. Period. Therefore airspeed will NOT be less - instead ground roll, ground speed, and time to accelerate will all be increased with a tailwind. This is the fundamental misconception that non-flyers can't seem go get over, and it baffles me because it really isn't complicated... – Michael Hall May 24 '21 at 19:33
  • OK, so to clarify, you absolutely cannot simply say that "airspeed will be less" without a modifier. Because with a tailwind the statement "airspeed will be less after 1000' of ground roll" is true, but "airspeed will be less at rotation" is obviously not. – Michael Hall May 24 '21 at 19:37