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?
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9$\begingroup$ Does this answer your question? Why is tailwind during final approach and landing so dangerous? and see also: How do jet streams affect the fuel consumption? $\endgroup$– randomheadCommented May 24, 2021 at 1:23
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$\begingroup$ Do any of these questions help? Here, here, here $\endgroup$– PondlifeCommented May 24, 2021 at 3:28
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$\begingroup$ Cruising speeds are also generally quite a lot higher than landing / take-off speeds - so the effective loss of lift is not relevant - 100% - 2% is still 98%... At landing speed 100% - 40% - well you get the idea .... $\endgroup$– Mr RCommented May 24, 2021 at 4:02
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$\begingroup$ BTW, I downvoted because the statement concerning loss of lift from reduced airspeed is not clear. Yes, you will have reduced lift at lower airspeeds, but with a tailwind you will simply take off at a higher groundspeed further down the runway. Hint: Airplanes don't rotate after a fixed ground roll distance, they rotate at a predetermined airspeed. Always... $\endgroup$– Michael HallCommented May 24, 2021 at 19:40
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$\begingroup$ Another hint: If you are flying in the jet stream at 300 knots of indicated airspeed you will have the same IAS no matter whether you have a tailwind or a headwind. The ONLY thing that changes based on direction of flight is ground speed. $\endgroup$– Michael HallCommented May 24, 2021 at 20:15
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3 Answers
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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.
answered May 24, 2021 at 1:23
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$\begingroup$ Or how fast the air mass is moving. $\endgroup$– jamesqfCommented May 24, 2021 at 3:57
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$\begingroup$ What about a tailwind matching the max speed IAS an aircraft can gain, and thus reducing IAS to 0, with no the wings generating no lift? $\endgroup$ Commented May 24, 2021 at 5:46
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2$\begingroup$ With tailwind you fly at same IAS but increase ground speed. You look at IAS and ignore ground speed. $\endgroup$ Commented May 24, 2021 at 6:07
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$\begingroup$ @HarshilSharma In that case, the IAS of the airplane wouldn't change, but its ground speed would double. $\endgroup$ Commented May 24, 2021 at 13:21
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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.
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$\begingroup$ 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? $\endgroup$ Commented May 24, 2021 at 5:44
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$\begingroup$ @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. $\endgroup$ Commented May 24, 2021 at 8:31
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$\begingroup$ @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. $\endgroup$ Commented May 24, 2021 at 8:35
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2$\begingroup$ @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. $\endgroup$ Commented May 24, 2021 at 8:37
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$\begingroup$ @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. $\endgroup$– StephenSCommented May 24, 2021 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.
answered May 24, 2021 at 11:10
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$\begingroup$ @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. $\endgroup$ Commented May 24, 2021 at 16:55
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$\begingroup$ 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. $\endgroup$ Commented May 24, 2021 at 18:48
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$\begingroup$ 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. $\endgroup$ Commented May 24, 2021 at 19:17
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2$\begingroup$ 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... $\endgroup$ Commented May 24, 2021 at 19:33