# Aircraft turns from headwind to tailwind - loss of IAS? **URL:** \ **Category:** Factual Questions\ **Created:** [December 23, 2009, 10:59pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386 "2009-12-23T22:59:27Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [December 29, 2009, 8:20pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/41 "2009-12-29T20:20:55Z")
Wait… > [@](#): > > the Pilot was the victim of a personal assault by a group of five youths in an unprovoked attack. The Police report indicated that during the attack he received a blow to the back of his head and another to the middle of his back. There were no visible injuries, and as far as they were aware, medical attention was not sought by the Pilot. The Avon and Somerset Constabulary report states that he was suffering from shock. He got struck in the back of the head prior to flight and was flying in a state of shock? Um, sounds like an accident waiting to happen. Flying in poor visibility with bad wind conditions, low and with insufficient instrumentation? And somehow airplane physics was the cause\* of the crash? * * * \*Okay, techically, physics is the cause of every crash, just like physics is the cause of every not-crash. Still, I think you know what I mean. 😉 ---
**Author:** ![Hi\_Medlo](https://avatars.discourse-cdn.com/v4/letter/h/a183cd/32.png) [@Hi\_Medlo](https://boards.straightdope.com/u/Hi_Medlo)\ **Post date:** [December 29, 2009, 9:43pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/42 "2009-12-29T21:43:42Z")
A little thought experiment, Elvis: Let’s say we have a small model aircraft trimmed to fly in smooth, level circles in calm air. We do this trimming in a large enclosed space, say the hangar deck of an aircraft carrier. Now, we ask the captain to put the carrier in motion at a speed of, say, 15 knots. Once everything is stablized we launch out model again. Will we be able to infer anything about the carrier’s speed and direction by the flight path? Will it rise as it flies in the direction of the ship’s motion and fall going the other way? Will it maintain it’s position relative to the earth and drift towards the stern (if we’re going forward)? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [December 29, 2009, 10:21pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/43 "2009-12-29T22:21:18Z")
> [@ElvisL1ves](#): > > Transiently, and slightly, but yes, certainly. And in the latter case, there will be a similar increase in IAS. It’s just F = m a, where a is relative to the gravitational field, IOW the _ground_. At the speeds you’re talking about (i.e. non-relativistic), F = m a is unchanged by a change of the frame of reference. This transient difference is either an illusion or due to other effects, not due to a constant velocity of the air mass relative to the Earth. There’s no basis for that in Newtonian physics. ---
**Author:** ![Waffle\_Decider](https://avatars.discourse-cdn.com/v4/letter/w/77aa72/32.png) [@Waffle\_Decider](https://boards.straightdope.com/u/Waffle_Decider)\ **Post date:** [December 29, 2009, 10:59pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/44 "2009-12-29T22:59:26Z")
> [@ElvisL1ves](#): > > Well, an abrupt change in aircraft orientation can have the same effect on the wind relative to the aircraft as a gust does. However, this has nothing to do with the mythical downwind turn. If the wind is steady, the change in IAS after your abrupt change in orientation is always going to be the same regardless of the wind direction. In a steady state, the aircraft is already “blowing with the wind”, so to speak. From the perspective of its airspeed indicator, the ambient wind is exactly zero. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [December 30, 2009, 3:19am UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/45 "2009-12-30T03:19:48Z")
> [@ElvisL1ves](#): > > Transiently, and slightly, but yes, certainly. And in the latter case, there will be a similar increase in IAS. It’s just F = m a, where a is relative to the gravitational field, IOW the _ground_. > > If that isn’t clear, try this: You’ve seen an IAS change in a headwind or tailwind gust, right? And it’s settled back down when the gust has been sustained for a few seconds, right? Well, an abrupt change in aircraft orientation can have the same effect on the wind relative to the aircraft as a gust does. If it didn’t, there would be no wind-shear accidents, since aircraft would never suddenly lose airspeed because of them. Any abrupt change in aircraft orientation will have exactly the same effect on the aircraft regardless of what the wind is doing over the ground. There is no way for the aircraft or pilot to know what the wind is doing relative to the ground, and they need not know except for the purposes of navigating between ground features. I did address your instantaneous change earlier and showed there is no difference between headwinds, tailwinds, and nil wind situations. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [December 30, 2009, 4:01am UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/46 "2009-12-30T04:01:07Z")
> [@billfish678](#): > > Yeah, you are technically right at some measurable but very small level, but IMO this kinda stuff causes more problems than it solves. That’s all I’ve said, including the part where it’s very small. I’ll even add that it doesn’t help a pilot fly the plane any better if he does understand it, in normal flying conditions, if that soothes anybody’s feelings. It is not a “very small” effect in aerobatics, or in a “normal” airplane caught in a microburst, though. I’ve always thought that the more a pilot knows about flying in general, including the physics behind it, the better a pilot he’ll be in general. I can’t agree that it causes problems, sorry. > [@Hi\_Medlo](#): > > Will we be able to infer anything about the carrier’s speed and direction by the flight path? Will it rise as it flies in the direction of the ship’s motion and fall going the other way? Will it maintain it’s position relative to the earth and drift towards the stern (if we’re going forward)? Is the air the model is flying in moving with the ship as well? If so, then the (slow) transient effects of moving the air over the model plane would tend to mask, or even overpower, inertial effects. If the air were not influenced by the ship’s motion (good luck engineering that one, btw), then yes, you’d have an inertial navigation system conceptually similar to INS’s already used on many ships and aircraft. > [@ZenBeam](#): > > At the speeds you’re talking about (i.e. non-relativistic), F = m a is unchanged by a change of the frame of reference. What change of FOR? Acceleration and inertia are relative to _gravity_, IOW the ground. If you’re comparing a ground-based to an aircraft-based FOR, then an aircraft-based FOR will see the turn as a change in relative wind velocity (speed plus direction vector), which is a transient effect. In a ground-based FOR, a turn will look like a change in net lift and thrust vectors, which are also transient effects. > [@Waffle\_Decider](#): > > In a steady state, the aircraft is already “blowing with the wind”, so to speak. From the perspective of its airspeed indicator, the ambient wind is exactly zero. A turn is not a steady state condition. IAS certainly will stabilize back to whatever the aircraft is configured for after the transient effects from the turn die out, again assuming a steady wind, sure, but it’s that transient state that is under discussion. [quote=“1920s Style “Death Ray”, post:45, topic:522386”] There is no way for the aircraft or pilot to know what the wind is doing relative to the ground [/quote] The aircraft does know what direction gravity is, and it does have inertia relative to gravity. Even with a change in orientation, its inertia will tend to keep it moving in the same direction relative to the earth. Newton’s First Law. > [@](#): > > I did address your instantaneous change earlier and showed there is no difference between headwinds, tailwinds, and nil wind situations. You addressed only steady-state conditions, which are not under disagreement (well, they actually used to be, back in the Dawn of Flight days - Jimmy Doolittle actually got an MIT PhD for explaining the difference between airspeed and groundspeed in steady state). But we’re discussing transients. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [December 30, 2009, 4:11am UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/47 "2009-12-30T04:11:59Z")
> [@ElvisL1ves](#): > > That’s all I’ve said, including the part where it’s very small. I’ll even add that it doesn’t help a pilot fly the plane any better if he does understand it, in normal flying conditions, if that soothes anybody’s feelings. It is not a “very small” effect in aerobatics, or in a “normal” airplane caught in a microburst, though. > > I’ve always thought that the more a pilot knows about flying in general, including the physics behind it, the better a pilot he’ll be in general. I can’t agree that it causes problems, sorry. > > . But when more pilots \*\*misunderstand and misapply \*\*this nearly irrelevant factoid that doesnt apply to normal flying than pilots that are enlightened by this nearly useless factoid that doesnt affect them then the problem this knowledge causes IS problematic. Its the aeronautic equivalent of observing that, yes, some very small fraction of people DO die in car accidents because they were trapped in the car because they were wearing their seat belts when it caught fire and they could not get out. If pilots were told to think “dangerous down wind turn” means don’t fucking look at the ground, keep an eye on your IAS above all else it would be all good…but instead a good fraction get all wound up in this 99th percentile physics voodoo that only confuses most and obfuscates the real issues… ---
**Author:** ![Waffle\_Decider](https://avatars.discourse-cdn.com/v4/letter/w/77aa72/32.png) [@Waffle\_Decider](https://boards.straightdope.com/u/Waffle_Decider)\ **Post date:** [December 30, 2009, 5:06am UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/48 "2009-12-30T05:06:27Z")
What does gravity have anything to do with this discussion? Gravity acts on the vertical axis. The acceleration due to gravity on the horizontal plane is zero and can be safely ignored for this discussion. > [@](#): > > A turn is not a steady state condition. IAS certainly will stabilize back to whatever the aircraft is configured for after the transient effects from the turn die out, again assuming a steady wind, sure, but it’s that transient state that is under discussion. Yes, your instantaneous change in orientation is not a steady state condition. The IAS will indeed change quite a bit. The aircraft will keep travelling backward until its thrust and drag has consumed all of its speed in the original direction. However, since we’re assuming that the wind is steady, the aircraft is already moving together with the wind throughout the maneuver. As far as the aircraft is concerned, there is no wind. The effects of wind on this maneuver will be exactly zero, as far as IAS goes. There will be a large change in the IAS, but the change will be exactly the same whether you’re turning upwind, downwind, or whatever. Wind only matters if the aircraft is on ground, where we do very much care about groundspeed, or when the wind is not steady, like in a gust or microburst. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [December 30, 2009, 5:23am UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/49 "2009-12-30T05:23:11Z")
> [@billfish678](#): > > But when more pilots \*\*misunderstand and misapply \*\*this nearly irrelevant factoid that doesnt apply to normal flying How can it be misapplied? Only in extreme conditions can it even be noticed. > [@](#): > > If pilots were told to think “dangerous down wind turn” means don’t fucking look at the ground, keep an eye on your IAS above all else it would be all good And they are told just that, as students. Weren’t you? > [@](#): > > but instead a good fraction get all wound up in this 99th percentile physics voodoo that only confuses most and obfuscates the real issues… The conceptual problem that IME usually gets student pilots into trouble is grasping the difference between airspeed and groundspeed and grasping that only the former matters in terms of aircraft control and performance, not awareness of transient inertial effects that they can’t even detect in the kinds of maneuvers a student performs. Hell, adverse yaw and the power curve are harder to understand and those do matter at even basic levels. ---
**Author:** ![Hi\_Medlo](https://avatars.discourse-cdn.com/v4/letter/h/a183cd/32.png) [@Hi\_Medlo](https://boards.straightdope.com/u/Hi_Medlo)\ **Post date:** [December 30, 2009, 1:07pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/50 "2009-12-30T13:07:01Z")
Yes, the air on the hangar deck is moving with the ship…that’s the whole point of the thought experiment. That package of air is exactly like a steady-state wind, and the aircraft will fly with it, still in its steady turn, without rising or falling regardless of its speed relative to the earth. Another thought experiment: Our aircraft is flying 1000 feet above ground level, so it has a given amount of potential energy relative to the ground. If we fly over a 300 foot deep valley, the potential energy suddenly rises to 1300 feet worth. If ground reference matters, shouldn’t the aircraft lose altitude to maintain the same potential energy? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [December 30, 2009, 2:43pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/51 "2009-12-30T14:43:12Z")
> [@ElvisL1ves](#): > > What change of FOR? Acceleration and inertia are relative to _gravity_, IOW the ground. If you’re comparing a ground-based to an aircraft-based FOR, then an aircraft-based FOR will see the turn as a change in relative wind velocity (speed plus direction vector), which is a transient effect. In a ground-based FOR, a turn will look like a change in net lift and thrust vectors, which are also transient effects. You’re arguing that there is a difference when the air mass is stationary WRT Earth and when the air mass is moving WRT Earth. That change of frame of reference. **The physics of the turn in the frame of reference of the air mass in each of those two cases is identical, because Newton’s laws are unchanged.** Looking at both cases in the frame of reference of the Earth obscures this somewhat. I bolded that sentence, because it’s the essential point that you need to overcome to argue that there is a difference. This statement of yours in particular, “Acceleration and inertia are relative to _gravity_, IOW the ground” seems to be the source of your confusion. This is false. Newton’s laws are unchanged by Galilean transformations. [See here](http://en.wikipedia.org/wiki/Galilean_relativity), especially point 5 [here](http://en.wikipedia.org/wiki/Galilean_relativity#Newton.27s_theory_versus_special_relativity). ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [December 30, 2009, 5:09pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/52 "2009-12-30T17:09:09Z")
Dude. Inertia does not exist relative to the wind. It’s relative to the earth. Find a good physics text and look up “inertial frame of reference”, okay? ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [December 30, 2009, 5:17pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/53 "2009-12-30T17:17:59Z")
> [@Hi\_Medlo](#): > > That package of air is exactly like a steady-state wind, and the aircraft will fly with it, still in its steady turn, without rising or falling regardless of its speed relative to the earth. Better explain where the aircraft’s inertia enters into the picture, then. Does it have any? > [@](#): > > Our aircraft is flying 1000 feet above ground level, so it has a given amount of potential energy relative to the ground. If we fly over a 300 foot deep valley, the potential energy suddenly rises to 1300 feet worth. PE is relative to anything you like, but you have to be consistent about it. In this case, you’ve shifted your datum relative to the earth’s center of mass, from which the aircraft’s weight derives, and that shift accounts for all the change in PE you’re calculating. The aircraft’s position relative to the gravitational field it’s operating in hasn’t changed a bit. Remember that PE itself isn’t important, _changes_ in PE are, due to its conversion to other forms of energy. PE is only potential, it doesn’t “exist” (for lack of a better term) until it’s converted. In your scenario it isn’t converted at all, and PE is exactly the same before and after. High-school stuff again, friend. ---
**Author:** ![Hi\_Medlo](https://avatars.discourse-cdn.com/v4/letter/h/a183cd/32.png) [@Hi\_Medlo](https://boards.straightdope.com/u/Hi_Medlo)\ **Post date:** [December 30, 2009, 5:28pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/54 "2009-12-30T17:28:23Z")
The aircraft’s inertia is relative to the air it’s flying in…and in my example, also relative to the walls of the hangar deck, since they and the air are moving at the same speed. What if we changed the path of the airplane to crash into one of the hangar walls? Would we expect a different result depending on which wall we hit? Would it strike the rear wall harder than the front? If its inertia is relative to the earth we would. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [December 30, 2009, 5:39pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/55 "2009-12-30T17:39:30Z")
> [@Hi\_Medlo](#): > > The aircraft’s inertia is relative to the air it’s flying in Wrong. I lack the patience to explain Newtonian physics here when there are many more efficient ways for you to learn it for yourself. Sorry. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [December 30, 2009, 5:58pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/56 "2009-12-30T17:58:44Z")
> [@ElvisL1ves](#): > > Dude. Inertia does not exist relative to the wind. It’s relative to the earth. > > Find a good physics text and look up “inertial frame of reference”, okay? Your ignorance is staggering. It also appears to be wilful, so I think I’m done here. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [December 30, 2009, 6:10pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/57 "2009-12-30T18:10:41Z")
> [@ElvisL1ves](#): > > Dude. Inertia does not exist relative to the wind. It’s relative to the earth. No - there is nothing special about the earth-based frame of reference. Galileo figured this out quite some time ago. He employed a thought experiment involving basic physics on board a ship. The aircraft carrier hangar bay is an updated example of the same thing. For a more extreme example, consider flying at a steady speed and altitude in an airliner. Is there anything fundamentally different about walking around, or tossing a ball at 550 knots as compared to doing the same thing while stationary at the gate? Galileo said: absolutely not. ---
**Author:** ![Anne\_Elk](https://avatars.discourse-cdn.com/v4/letter/a/22d042/32.png) [@Anne\_Elk](https://boards.straightdope.com/u/Anne_Elk)\ **Post date:** [December 30, 2009, 10:03pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/58 "2009-12-30T22:03:30Z")
Here’s a link for an Irish pilots message board concerning the crash in the OP. I noticed that there are a few down-wind turn zealots in Ireland as well. [http://www.flyinginireland.com/forum/viewtopic.php?f=1&t=3665](http://www.flyinginireland.com/forum/viewtopic.php?f=1&t=3665) ---
**Author:** ![Hi\_Medlo](https://avatars.discourse-cdn.com/v4/letter/h/a183cd/32.png) [@Hi\_Medlo](https://boards.straightdope.com/u/Hi_Medlo)\ **Post date:** [December 30, 2009, 10:51pm UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/59 "2009-12-30T22:51:01Z")
[http://www.aeroexperiments.org/brainteasers.shtml](http://www.aeroexperiments.org/brainteasers.shtml) ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [December 31, 2009, 12:52am UTC](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386/60 "2009-12-31T00:52:21Z")
> [@ElvisL1ves](#): > > Find a good physics text and look up “inertial frame of reference”, okay? An inertial frame of reference is categorically NOT unique to the Earth. I wonder if you’ve been confused by an aircraft inertial reference system which measures acceleration relative to the Earth, but it ONLY does this because it has been initialised while stationary on the ground. If you were to initialise it while drifting at 10knots it would measure accelerations relative to the initial drift. You are correct that it is high school physics, but you don’t understand it. > **[Inertial frames and Newtonian mechanics (from Einstein Light)](https://www.phys.unsw.edu.au/einsteinlight/jw/module1_Inertial.htm)** > > An explantion of Galilean relativity, electromagnetism and their apparent incompatibility; an explanation of Einstein's relativity resolves this problem, and some consequences of relativity. Educate yourself at the above website. > [@](#): > > This may be stated in a few different ways: > > A frame of reference that moves with constant velocity with respect to an inertial frame is also an inertial frame. > For example: Newton’s laws work in the physics lab, which is fixed to the Earth. Therefore they also work in a train moving with constant velocity with respect to the Earth. (ie if you set up your experiments in a train moving smoothly, without turning, speeding up, slowing down or bumping, you will get the same results.) > > One could also state it this way: > > If the laws of mechanics work in one frame of reference, then they also work in a frame of reference that moves with constant velocity with respect to the first frame. > (ie if your first frame is inertial, the second frame is inertial.) If you can juggle on the platform, you can juggle in the train, or even in a plane, even if a bystander sees you and the balls whizzing by at high speed. An airmass moving at a steady speed and direction is an intertial frame of reference. [Previous page](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386.md?page=2) [Next page](https://boards.straightdope.com/t/aircraft-turns-from-headwind-to-tailwind-loss-of-ias/522386.md?page=4)