Fine, but in the quoted reports, the NTSB is blaming a turn downwind, not a climb into a tailwind. You'd think if the culprit was severe windshear, then the report would say so. I agree with Robert that the way the reports cited above are written, they cannot be correct.
A change of the wind direction or speed with position (be it vertical or horizontal) or time is called windshear.
In both of this accidents, it is possible that an increase in tailwind as they climbed out of the ground boundary layer could have been a factor.
That, however, is "climbing into an increasing tailwind", and not turning downwind.
The very real "climbing into an increasing tailwind" that can be found after take-off is equivalent to the "descending into a diminishing headwind" on approach and landing. And as much as you can encounter an increasing tailwind when turning (and climbing) after take-off, you can also encounter a diminishing headwind when turning (and descending) into final, with equal aerodynamic implication, dangers and possible consequences. However, the "headwind turn" lacks the charm of the "downwind turn", and has never reached the myth level.
The reason from that, is that anybody taking as a frame of reference something other than the air itself (for example, someone standing on the ground) can clearly feel the wind in the face turn into a wind in the back as one turns 180°, and then wrongly extrapolate that effect to an airplane that is flying about the AIR itself, where the relative wind speed is always the airspeed and the relative wind direction is always straight headwind +/- slip angle (which is zero in coordinated flight), regardless of what's the speed and direction of the air about the ground (i.e, the wind).
That wrong extrapolation leads to think that one would face an airspeed reduction when turning downwind, which can lead to a stall, but an increase in airspeed when turning headwind, which would never lead to a stall. But both are wrong. The plane doesn't change its airspeed in a turn as a result of its change in direction relative to the wind.
The second report is particularly clear with the mistake: "The sudden change from a 24-knot left crosswind to a 24-knot tailwind during the pilot’s execution of the right-hand turn towards the downwind leg of the landing pattern [...] most likely induced an aerodynamic stall".
The investigator talks about a wind that was always 24 knots, and where supposed change of direction (from crosswind to tailwind) was not the result of a change in the wind itself, but of the aircraft turn. Make no mistake: as viewed from the plane (and sensed by the wings), the wind always comes from the same direction: 100% headwind (as long as it's in coordinated flight).
It's crystal clear that the investigator, in this case, had no intention of talking about a climb into an increasing tailwind, but instead fell in the trap of the mythical and non-existent "downwind turn" stall.
chuckyvt, sorry but your comment is absolutely wrong. And that's not an opinion. It's Physics.
Saying that "the inertia of the airplane is relative to the ground" is wrong. The inertia of the airplane is the same in any inertial frame of reference. If the ground can be considered an inertial frame of reference, then any other frame of reference that moves at a constant speed (constant in direction and magnitude) relative to the ground is an inertial frame of reference too. And with a steady wind, no matter what direction and how strong, the air is moving at a constant speed about the ground and hence it is an inertial frame of reference too.
But maybe an example will make that simpler to understand.
Say that you have an airplane flying East (x-axis) at 100 knots and makes a 180° turn to the West. Let's say that the turn takes 10 seconds (just because 10 is a nice number to make calculations, but you can take any other turn rate you like). Let's make an hypothesis that, no matter how much wind in the x-axis (West-East), the airspeed remains constant, and let's see what happens.
In the first case, let's consider that there is no wind.
The airplane starts with an airspeed of 100 kts East and ends with an airspeed of 100 kts West.
The change in airspeed in the West-East axis is 200 knots, which in 10 seconds means 20 knots per second (average) in this axis.
Since there is no wind, the ground speed also starts at 100 knots East and ends at 100 knots West. Again a change of 20 knots per second.
Second case, lets consider that the plane is initially flying with a tailwind of 20 knots and turns into the wind.
The airplane again starts and end with an airspeed of 100 knots but in opposite directions in the West-East axis, so that's again a change of 200 knots in 10 seconds, or 20 knots per second.
The ground speed, on the other hand, starts at 120kts East (100kts of airspeed + 20 kts of wind) and ends at 80kts West (100kts of airspeed - 20kts of wind) . Again a change of 200 kts in 10 seconds.
Finally, let's consider that it's initially flying with a headwind of 90 kts and turns downwind.
Again the airplane starts with an airspeed of 100 knots East and ends with an airspeed of 100 knots West for a total change of 200 kts in 10 seconds.
The ground speed, on the other hand, changes from 100-80=20kts East to 100+80=180kts West, again for a total change of 200knots in 10 seconds in that axis.
So as you see, assuming that the airspeed doesn't change leads to conclude that the change in airspeed and ground speed are the same, and that this holds true regardless of the wind. That the change in air and ground speeds is the same in the same amount of time, means that the acceleration about the air and about the ground are the same, showing that the inertia is equally present in both frames of reference.
Because these are all reversible conditions, the implication also works in the other way: assuming that the inertia is the same in both cases leads to conclude that the change in airspeed is the same than the change in groundspeed, regardless of the wind. And because we know that the change in airspeed is twice the airspeed when there is no wind (the magnitude does n0t change but the direction is the opposite), then with any wind you must find exactly that same change in the airspeed and groundspeed.
Now, no matter from what end we start, we start from an assumption: either that the airspeed doesn't change in the turn, or that the inertia is the same in both frames of reference. Can we rely on those assumptions?
Yes.
As you probably know, the second Newton's law of motion states that F = m x a.
To begin with, the mass of the airplane is the same in any frame of reference (even in non-inertial ones).
But, what is the force that makes the speed change 200 kts in 10 seconds, in all of the examples above?
It's the lateral component of the lift (the lift is tilted when the airplane banks and there is a lateral component of the lift).
The rate of turn is given by the airplane's airspeed and the bank angle. Since in all of the examples above the airspeed was the same and the turn took the same time (hence the same rate of turn), then the bank angle must have been the same.
Since the bank angle was the same, and the lift was the same in all cases (because the vertical component of the lift had to cancel the airplane's weight which was the same in all cases), hence the lateral component of the lift was the same in all cases.
Now, because the lateral component of the lift was the same and the mass of the ariplane was the same in all the cases, then the acceleration HAD to be the same in all the cases. And this holds true for the acceleration about the air and about the ground.
Not only is that consistent with the hypothesis of the inertia being the same in both frames of references (about the air or about the ground), but also (and not surprisingly, because one implies the other and vice-versa) with the result of the same change in speed of 200kts in 10 seconds, be it about the air or about the ground, and with any wind (as long as it is constant).
He didn't do this near the ground, did he?
Unless you are flying like Michael Goulian , you can not turn a GA airplane 90 degrees quickly enough to change directions, relative to the wind, to have any affect as described in this "down wind turn" scenario.
Sounds like the investigator has never been in a GA aircraft, let alone has any concept of what happens to an airplane in the air after it makes the transition from rolling down the runway to flying in the air.
Or perhaps he has been spending too much time at the airport hangar flying with the Sunday circlers.
The Palm, Prop Cavitation? This is air, not water. Air does not boil at low pressures. And losing altitude in a turn is a result of pilot error alone and nothing at ALL to do with wind direction. (Wind shear is another topic altogether) Ground speed picks up in a turn to downwind, so an inexperienced pilot may look out the window and notice a faster than normal GS and pull up, reducing airspeed. But that is it. There is no inertial change, no IAS change, no AOA drop, nothing. A turn is a turn is a turn, as long as it is coordinated. In this case, the NTSB report is flat out wrong.
Every pilot has noticed that quick drop in indicated airspeed while descending into the last fifty feet or so above the ground on approach. Moving from the steady state wind down into the turbulent wind just above the ground, where everything from buildings to trees to big vehicles roils the surface winds. Likewise, on climb out I've often seen the IAS value increase once out of the surface air layer.
Enough wind shear in a short distance can be a challenge for any pilot. If the surface wind and wind at 300 feet changed from upwind to downwind, that's as much as 40 knots of wind shear.
The T-6 appears to have begun turning left at very low level with an increasing bank angle, a recipe for an accelerated stall/spin and an out-of- control crash. Had the T-6 pilot simply climbed to a safe distance —500-1000 AGL — before turning, I doubt this accident would have happened.
The 172's pilot attempted a crosswind landing in conditions well beyond the manufacturer's recommended limit, with a c of g over 2" aft of the rear limit. In that condition the 172's stick force per G decreases, while pitch and roll control becomes increasingly light. Those two factors make it much easier to stall, regardless of the direction the wind is moving at any instant. Then the pilot appears to have lost control after abandoning the landing just 300 feet AGL, probably by not maintaining enough indicated airspeed to prevent a stall that may have been unrecoverable at that low altitude.
In both cases, the pilots appear to have lost control of perfectly running aircraft.
I agree with Goyer, and Garrison, that this downwind turn stuff is nothing but an urban myth, something accident investigators have no business using to explain crashes that appear to be direct results of poor pilotage and inattention to aircraft loading procedures and c of g calculation.
Douglas M
Surrey, British Columbia
The "myth of the downwind turn" is a myth for the airplane and concerns indicated airspeed (what high flying pilots use to indicate whether they are flying or not.)
The reality of the downwind turn problem concerns ground speed and is of great concern to contact flying pilots at low AGL altitude in maneuvering flight. When spraying a field, we always work crosswind and make repetitive swath runs from the downwind border to the upwind border. Coming out of a swath run, we turn downwind first so that we can make the somewhat greater than 180 degree return to the next swath row fifty feet upwind with an upwind base to final turn. Turning upwind first, with a wind greater than five knots, would not allow a ground track that would keep us in sight of the target (next swath row, runway, whatever) and allow the angle of bank necessary to hit or overfly that target. The greater angle of bank necessary to maintain the required ground track at the greater ground speed of a downwind condition would much more likely cause a mush and eventually a stall.
The same physical problem, increased ground speed requiring a greater angle of bank, occurs on every crosswind landing when we allow intimidation to cause us to make a pattern ending in a downwind base to final turn. There is no regulation preventing good wind management at non-controlled airports. The airplane doesn't care which way you make base to final. It is fine with landing well downwind of the runway following a down wind base to final turn (using the same angle of bank as would be used on the upwind base to final turn.) The problem is that the pilot really wants to land on the runway, so he will bank more in the downwind turn to get there.
There need be no fight about "the myth of the downwind turn." Instrument flight, other than beyond the MAP on a non-aligned approach, involves no downwind turn problem. It is a myth. Contact flight (using no instrument) at low altitude (crop dusting, bush, pipeline patrol, takeoff and landing) requires wind management to control excessive ground speed mitigation of our ability to hit targets and miss obstructions. Not all landing sites (airports, spray strips, bush strips, forced landing fields, etc.) have obstruction and terrain free approaches. It is easier to hit what we want to hit and miss what we want to miss (on the ground) at a slower ground speed. Call it the "reality of the downwind turn."
And yes, it does show up in incidents and accidents. Some us live in a maneuvering flight mode for every hour of every flight. All of us go there when we takeoff and land.
I tell all my crop duster and pipeline patrol students, however, "When he comes out of his aircraft showing teeth rather than a wagging tail, always say, 'you're probably right.' Just keep repeating, "you're probably right" until he shakes his head and walks away." Concerning an aircraft that is high and piloted by one totally unconcerned with the earth below, you are right.
Some of those accidents sound more like poor piloting technique to me.
Try this experiment. Fly around under the hood with a safety pilot in a steady wind and see if you can tell when you are upwind/crosswind/downwind. I will bet any amount of money that with just a 6-pack of instruments and your "feel" you can not tell the difference.
When you are driving a boat in circles on a large moving river, can you feel the "acceleration" when going from upstream to downstream? Nope. There is no acceleration. Relative to the bank there is, but that is not a "felt" acceleration, just a perceived one. Your only frame of reference is the water, just as the air is while flying. (Substitute submarine for boat if you are about to argue that a boat is on top of the water and not in it....)
Inertia is not relevant to this discussion at all. The inertia of the aircraft through the air is CONSTANT as long as the TAS is constant, no matter what.
Would you hit the ground harder going downwind that upwind if you crashed? Yes, and that is a different, but interesting discussion in itself:)
The amazing part of this article is the number of people and possibly 'licensed' pilots who are trying to argue the validity of this myth. To those who claim this is a valid concern, answer me this. I filled an empty 16 ounce glass with 8 ounces of ice then top off the glass with water. When the ice that is floating above the rim of the glass melts, how much water will overflow the glass ?
If that question stumps you, maybe this will be easier. What is heavier, a pound of lead or a pound of feathers ?
Enquiring minds want to know.
There is a good discussion of this topic in the forum section on this site. http://www.flyingmag.com/forums/technical/turning-upwind-down-wind-and-a...
And holy cow chuckyvt really needs to review some basic theory on this topic. I sure hope it is a joke.....
Mark in Idaho: a pound of lead weighs more than a pound of feathers if the masses are the same. the feathers weight less due to their buoyancy resulting from the volume of air it displaces- for the same reason a pound of feathers would float in water while a pound of lead would obviously sink. This of course does not include the effect of the solar torque force or time dilation due to density differences.....:)
Over many years and several thousand hours, I've had 3 unintentional stalls. Two were at low altitude on final and the third was in cruise at 5000 feet. Three different planes were involved. I have related these before.
The first was as a student with instructor in a 150. We were carrying half the gust factor as recommended, but we hit a non typical gust. The bottom fell out and I punched the throttle. I have very quick reflexes, but when the throttle hit the panel I discovered the instructor and I were holding hands<:-))
With the addition of power the 150 was almost immediately flying again.
The second was in a Cherokee 180 after several hundred hours. This too was on final from a gust that was way more than typical or forecast. The ASI dropped off scale and I was on the express elevator down. All I could do was firewall it and hope that what nature had taken away, nature would give back.
The wind did pick back up, byt the old Cherokee was in ground effect, only a couple of feet off the ground, just short of the end of 06 when it finally started flying again.
The third was in the old Debonair, headed for Bo specific training pyut on by te ABS in conjunction with the Air Safety Foundation at Port Columbus in Ohio.
It was a beautiful sun shiny spring day with a clear blue sky. Light winds and no forecast turbulence except some light stuff at my destination. I was moving at about 190 MPH (old airplane) at 5000 feet, fat, happy, warm, sleepy, and complacent when I hit a very hard bump. A couple of seconds later I hit another one and instinctively pulled the throttle back. A couple seconds later I hit a really hard bump and found myself looking straight ahead “at the ground”! I eased the nose back up and maintained Va for a couple of minute, expecting to find another set of three bumps. I went back to cruise after those bumps didn't materialize.
These were all due to wind shear albeit the last, most likely from something BIG having passed through the area shortly before I did.
KHTL is an excellent place to experience changes in wind speed and direction close to the ground. With a Northerly wind it spills over the trees to the north of the runwat. Brush and a slight rise to the S of thr runway will cause the wind to roll up and reverse. When on final for 27 you have a wind from the North. If you don't land right at the end of the runway you hit turbulence and an abrupt wind shift to the South followed by another abrupt shift to the North when about 50 feet off the runway. For the unsuspecting this can make for a very exciting landing, particularly with gusts from all directions. I've been 90 degreed a number of times up there.
Great place to take those who think they have cross winds mastered,
I've mentioned win shear, wind speed variations and directional changes close to the ground. All good reasons to cause problems, but unlike the “down wind turn” they really do cause a change in the relative air speed.
I find it troubling that FAA personnel would refer to “the down wind turn” or lack the writing skills to make their intent clear if it was otherwise.
Sylvestre, that is all 100% incorrect. Sorry bud, your intuition is sound, but the reality of the situation is much different. Turning downwind in a steady wind does not change your airspeed, not matter the rate of turn, as long as the turn is coordinated (and thrust is added to counter the increased drag from the higher AOA inherent in any level turn). Neither the speed of the wind nor the speed of the aircraft has any relevance to this topic.
I am sure you are asking where the energy came from that increases your inertia relative to the earth when you turn downwind, as you would obviously hit harder with a higher ground speed if you crashed. The answer is the initial speed of the air mass. When you are holding on the runway before takeoff with a 30kt headwind, your plane must resist the 30kt force pushing it backwards. This can be done with brakes, weight (inertia) or thrust, but when you accelerate to take off with a rotation speed of 70kts lets say, you only really accelerate 40kts over the ground. Your inertia relative to the air is 70kts, but only 40kts relative to earth. The extra 30kts is being provided by the moving air mass. When you turn downwind, the air mass's inertia moves to the positive side of the equation and ground speed increases, but because both the air mass and the aircraft are moving at a constant rate, there is no net acceleration.
Same for putting a boat in a moving river. To keep it stationary relative to the shore so you can get in, a force must be applied in a direction opposite the direction of the moving water mass. You now have some inertia relative to the water, but zero relative to the earth. Let go of the shore, and you will feel an acceleration as the inertia relative to the water drops to O and increases relative to everything else in the universe, most relevantly the shore.
I grappled with the counter-intuitiveness of this at one point too, but after some thought experimenting the physics became clear. And trust me.....I'm no rocket surgeon. I mean brain scientist.
Sylvestre, accelerating your aircraft in the [also moving] mass of air within which you are flying requires energy. You can buy that energy with either fuel, or altitude (or both). No other forms of payment are accepted.
Bbgadzos, it would seem that if the NTSB meant to that the pilot slowed the aircraft due to the illusion of speed (while ignoring attitude and the ASI), then they should have said so. It would have been much more informative. And it would have provided a clear object lesson: fly the wing.
Let's just keep it simple kids. Go on out on a nice post cold front day with a nice north westerly flow in progress . Say 25 kts at 6000. Establish yourself In steady flight on a North west heading. Roll into a turn either way maintaing altitude and a constant bank. Once the airspeed stabilizes with constant pitch power and bank do you notice any airspeed gains or losses corresponding to particular headings ? Of course not.
Think about it.If we did, every time we changed course we would see a corresponding airspeed change while in cruse. And the greater the wind speed aloft the greater the airspeed changes would be. Up in the jet stream at flight levels where wind velocities can approach 180 kts the effect would have to be tremendous. Fortunately this effect does not exist.
Now if you want to talk about how sudden CHANGES in WIND velocity due to altitude gradients or gust fronts effect aircraft performance that's a different matter. But then as others have pointed out that would be " wind shear".
Not loss of performance due to turning in a particular direction.
Just my observations after 53 years and 27000 something hours of trying to keep the blue. " on top".
The NTSB is simply a group of guys doing their jobs. Some do it better than others. This particular gentleman just misunderstands the physics of this subject and is letting intuition get the best of him. I saw this many times with my primary students. Turning to downwind while doing ground reference maneuvers they visually notice an increase in ground speed, and incorrectly correlate that with IAS. They assume that if their speed is increasing and power output is constant, the only reason for the increase in speed is a descent. Subconsciously they pull up in attempt to arrest the supposed descent which will in fact decrease airspeed, putting the aircraft in a slower than normal condition. There is no inertial change, no acceleration or deceleration, only a reaction to an observed increase in ground speed. There are many analogies to stress this point, but in the end the physics prevail. The only time stall becomes an issue is with wind shear or mountain wave. My day job plane flies at 370 and close to called the coffin corner. When we encounter mountain wave while maintaining altitude, the descending side of the wave can cause a precarious decrease in airspeed and has caused a number of high altitude stalls in the past. Other than that, steady wind has NO effect on IAS or inertia.
I tried an experiment in MS Flight Simulator that helps explain what's happening in a downwind turn. I set the sim up with a 100 knot steady wind and myself in a Cessna 172 at 3,000 feet over Seattle. I then made a series constant bank 360 degree turns for about 20 minutes.
The result? My airspeed never changed but my ground speed sure did. After the flight I checked my ground track. My turns all looked the same: I basically corkscrewed away from Seattle farther and farther out over the water as the wind took me for a ride.
I also tried this experiment at a reduced power setting and 80 knots. Despite the 100-knot wind, I never stalled - though I might have flown backward a few times.
Those who still believe in the downwind turn should themselves these questions:
Why is it that these "downwind turn-related" accidents always seem to involve aircraft manueuvring at very low altitude? Have you even heard of anyone stalling, spinning and crashing/burning/dying due to a change of direction from into the wind to crosswind to downwind at, say, 1000 feet or more? Me neither. We routinely takeoff into headwinds sometimes quite strong, then within a minute we turn crosswind, then downwind when flying circuits (Canadian version of flying the pattern), so why aren't there a spate of these type of crashes every month? Who doesn't think low flying will sooner or later get you, when you get complacent, mentally lazy or distracted? At 300AGl, as little as five to fifteen seconds of distraction or inaction could cause a crash.l
As many others have pointed out above, at low altitudes changes in groundspeed might easily be mispercieved sufficiently to cause them to pitch upward and stall. That's why the manufacturer of that plane put attitude indicators and airspeed indicators and altimeters and VSIs right in front of you!!! They're there so you can confirm by instrument what your eyes are already telling you. VFR does n0t mean you never check your gauges.
The only pilots I know of who routinely fly low are aerial sprayers and helicopter pilots. The rest of us should climb to at least 500 feet AGL before turning crosswind or downwind, unless a specific IFR procedure calls for an earlier turn. For example, a SID at JFK either did, or still does require for a turn off runway heading just 400 feet AGL. Airline pilots do that a hundred thousand times a year, so where are the airliner downwind turn accidents?
Finally, being more disciplined and circumspect and learning to fly the numbers would prevent a lot of low-level accidents: : a specific power setting (about 75%), plus a certain attitude ( five to 15 degrees up on your attitude indicator) will create a positive rate of climb every time, regardless of changes in wind direction as you climb.
Sometimes I think the skill level of the GA pilot population needs to be raised at least a few notches. Too many pilots are dying or maiming themselves and others in preventable accidents where they should know better. Either we collectively improve our safety record, or the regulator will attempt to do it for us through additional laws and further expense.
Or we could all take up golf. That' safe, isn't it? Not!!!!
Douglas M
Surrey, BC
You got us. We're very ashamed of saying that thing we didn't say.
Now, if you believe that wind shear and the downwind turn are the same thing . . . then you need to read a bit more carefully. Sorry again for saying that gravity didn't exist. (or was it air?)
There are a number of plausible theories here, but in general, density altitude, out-of-CG condition, low-level turbulence, and sloppy flying are always good bets when it comes to accidents such as these. I will not speculate on these. I will say, however, that neither had anything to do with downwind turns.
Guys who still speak of the "inertial" effects of the downwind turn.
Do we agree that a steady wind is a mass of air moving at constant speed about the ground?
Do we agree that the mass of air inside the fuselage of an A380 at cruise is moving about the ground at the same constant speed than the A380's groundspeed itself? (say some 500 kts)
Do we agree than, given the two statements above, the air inside the fuselage of the A380 at cruise is effectively a 500kts steady wind?
Now please explain me why if I launch a single-digit-knots paper plane to make a circular flight (could be a turn or a loop) inside an A380 at cruise, the plane neither vaporzies nor is smashed against the rear bulkead when it's subject to such an acceleration that suffers a change in it's airspeed of some 1000 kts within a few seconds?
Answer: because it doesn't change. Or try this on your next flight while at crusie: walk a circular pattern in the aisle or galley. Do you find any problem holding a constant speed about the airplane's floor, that is the same than against the air inside the plane (that is, your airspeed), despite the fact that the plane and that air are moving at 500 kts about the ground?
Once again:
The A380 is parked on the ground. When the 5 kts paper plane makes a 180 from moving forward to moving to the back of the plane, it's airpseed vector changes from 5 kts forward to 5 kts rearward. A change of 10 kts. The ground speed changes from 5 kts forward to 5 kts rearward. Again a change of 10 kts.
The A380 is cruising at 500 kts. When the 5 kts paper plane makes a 180 from moving forward to moving to the back of the plane, it's airpseed vector changes from 5 kts forward to 5 kts rearward. A change of 10 kts. The ground speed changes from 505 kts forward to 495 kts rearward. Again a change of 10 kts.
That is enough to rest may case. But for those who want to go more advanced. People who decide to go on must be aware that, while I tried to make it as simple as I could (a chalk and blackboard would have helped), what follows is not very simple and can be a tough read involving some concepts of dynamics in circular motion.
The change in the airspeed VECTOR is always the same: 10 kts in the longitudinal axis. The change in the groundspeed VECTOR is always the same: 10 kts in the longitudinal axis. The change in the airspeed MAGNITUDE is always the same: zero. The change in the ground speed MAGNITUDE is NOT always the same, but it doesn't need to be: The acceleration is the change in the speed VECTOR, not in the speed MAGNITUDE. THE ACCELERATION IS ALWAYS THE SAME AND THE CHANGE IN SPEED IS ALWAYS THE SAME (be it airspeed or groundspeed, be it with no wind or whatever steady wind).
The reason why the MAGNITUDE of the airspeed vector doesn't change is because the acceleration is provided by a force, and that force is the lateral component of the lift when the airplane banks, which, by definition of LIFT, is always perpendicular to the airspeed vector (lift is the component of the total aerodynamic force that is perpendicular to the airspeed vector, the other component, parallel to the airspeed vector, is, you guessed, drag). And when the force and acceleration are perpendicular to to the speed vector, it has the ability to change only its direction, not it's magnitude. (Disclaimer, if more power is not added to keep the airspeed constant, the airspeed will diminish because of the increased induced drag caused by the increase of lift needed to sustain the increase in load factor that happens in a turn, but that's another story: Whatever you make to keep the airspeed constant with no wind also works to keep the airspeed constant when turning in any steady wind, and with any I mean any wind direction and any wind speed).
Now, why does the MAGNITUDE of the groundspeed vector changes when there is a steady wind and the plane turns? The easy answer is that it has to, because the groundspeed vector is the vectorial sum of the airspeed vector and the windspeed vector. And when you sum two vectors and one of them changes only it's direction, the resultant vector necessarily changes it's magnitude (believe me, or draw some arrows and check it, or go study some vectorial algebra). Hoeve,r that doesn't help understand WHY it changes, just to accept that it does.
So going more to the roots, the reason why the magnitude of the groundspeed vector changes when there is a steady wind and you turn, you need to imagine an airplane turning viewed from above.
And I'm sorry, Robert, but Maddogdriver went too far with the over-simplification when he said "Nope. There is no acceleration. Relative to the bank there is, but that is not a "felt" acceleration, just a perceived one. Your only frame of reference is the water, just as the air is while flying."
No frame of reference is "special". Anything that you explain using one frame of reference, you must be able to explain using any other one. One could be more practical, intuitive, or simplify the calculations. But that's all. And in this case in particular, the acceleration vector (the only one, let's leave the qualificators like "felt" and "perceived" at a side) is the same in any and all inertial frames of reference, and the water or the air moving at constant speed about the ground are as inertial as the ground itself, so any acceleration about the air or water is also there about the ground.
Let's start with no wind. The plane is flying East and it banks. The lateral component of the force aims North and, since that's perpendicular to the airspeed vector, it has the ability to change only the direction but not the magnitude of the speed vector. When the plane has turned 90° it's heading North, now the lateral component of the lift aims West. Again perpendicular to the speed vector, again changing only the direction of such speed vector but not its magnitude. And that applies to both the airspeed and groundspeed because, with no wind, they are the same. And the trajectory of the plane about the air (thing that it was leaving a trail of poofs of smoke) or about the ground (say that it was dropping paint on the ground) are exactly the same: perfect circles that superimpose perfectly one over the other.
Now let's say that there is a wind from West to East. Again the airplane banks and the lateral component of the force aims North, which is perpendicular to both the airspeed and groundspeed vectors, and hence has the ability to change only their direction but not their magnitude. But that's about to change for the groundspeed vector. As soon as the plane starts to turn.
Let's take a snapshot at the point when the plane has turned 90° and is heading North.
The lateral component of the lift now aims to the West and, since the airplane is heading North, again it's perpendicular to the speed vector and has the ability to change only the direction but not the magnitude of said speed vector. But what speed vector?
When the airplane is heading North, the AIRSPEED vector is heading North. However, the airplane is now in a crosswind (from West to East, remember), and hence the airplane is drifting right. It's ground trajectory, it's ground track if you will, is not due North, but deviated somehow to the East. How much will depend on the magnitude of the airspeed and of the windspeed. But it's ground track, and hence the direction of the groundspeed vector, is not North but diagonal somewhere between North and East. However, the lift vector cares very little about what frame of reference we are using, and will still aim West in any of them (because it has to be perpendicular to the airspeed vector, remember?, and the direction of the airspeed vector is, again, due North). So figure this, and here is the KEY:
The force that is providing the acceleration needed to turn, and hence the acceleration itself, is aiming West, and the groundspeed vector is aiming somewhere diagonal between North and East. The acceleration is NOT perpendicular to the gorundspeed vector. It has a component that is perpendicular (that will make the groundspeed vector change its direction) but also a component that is parallel to the groundspeed vector (that will make it change its magnitude). Not only that, but since the acceleration aims West and the groundspeed vector aims somehow east, the acceleration is REDUCING the magnitude of the groundspeed vector.
Make no mistake: The acceleration vector is THE SAME in ANY inertial frame of reference. It is how you descompose that acceleration in a component perpendicular to the speed vector and another parallel to the speed vector what changes with the frame of reference: about the air (airspeed vector) or about the ground (groundspeed vector).
So, in summary, you were flying East with pure tailwind, turn left, now you are passing North and notice (by calculation, not that you feel anyhting special) that the acceleration doesn't change the magnitude of the airspeed vector but it is changing (reducing) the magnitude of the groundspeed vector. No wonder that by when you reach West (completing a 180° turn and changing from pure tailwind to pure headwind) the airspeed will not have changed and the groundspeed will have reduced (exactly by 2 x windspeed, by the way).
iused2fly- valid point. The low altitude flying involved is a very good indicator of the real issue here. You would think that a jet holding at a fix at 35,000ft in the jet stream with winds of 120kts+ that there would be a lot of stalls going on. A ground speed change of 240kts from upwind to downwind is pretty severe, and if there really was a deceleration in the turn, it would be violent.
A more dedicated me would love to calculate the supposed acceleration experienced by a 180 degree turn in 30 seconds with a 240kt ground speed acceleration during that turn.
Also, a 60 degree bank is 2 g's. Period. No matter what. If you are really accelerating when turning down wind, theoretically a 60 degree bank would result in more that 2 g's, which does not happen. Ever. That fact alone should settle this argument.
Sylvestre.........holy cow. So planes with propeller are more stable......than what? Stability in any direction has nothing to do with the method of thrust production. And I am not sure what happened or what you did wrong during your turn while retracting flaps, but "absolute airspeed" has nothing to do with it. My guess is your feet were flat on the floor.
Hey, ever noticed how your airspeed seems to drop rapidly when doing an uncoordinated turn in a certain direction- either left or right? Know what causes that? It's an interesting phenomenon that can lead to some misconceptions about airspeed.
Sylvestre........I'm not really sure what to say to that. I fly airliners and there is no danger of turning inside a fast moving mass of air. At all.......other than it possibly being a bit turbulent. And stability is simply an aircraft's ability to remain in or return to a given state after disturbance. Thrust will equal drag in level unaccelerated flight. The method of thrust might determine how quickly equilibrium is returned, but this still has absolutely nothing to do with wind or ground speed. I'm sorry, but you are wrong.
And your ability to accelerate at high altitude is simply due to a lack of excess thrust. Aircraft weight and air temperature determine cruise altitude, based on the thrust available.
Turn rate is a function of bank angle and true airspeed. Bank should be limited at high altitude, but that has NOTHING to do with wind. Bank increases load factor (read: weight) of the aircraft, and therefore AOA and the thrust required to maintain altitude. Transonic airflow over the wing is also a factor.
Long story short, wind is only a factor if you want to figure out how long it will take you to get from A to B and how much fuel you will burn.
You need to research this stuff before you start preaching the dangers of turning downwind. People do not need to be afraid of things that do not exist.
RandyL: the authors are saying that the only cause for an accident on the down wind turn is an OPTICAL ILLUSION, nothing to do with any sort of inertial change or acceleration. There are many optical illusions out there, but they have no physical effect on the plane.
Sylvestre: Please read up, or better yet ask someone else to review your ideas and see what they think. I can't keep saying the same thing again and again.
Gabriel: I have absolutely no idea how any of what you posted above has any relevance to this article or our discussion. Thrust vectors, headings turns, fpm, all this is irrelevant. Here are the facts;
A plane in a turn, regardless of wind speed or direction, will always experience the same acceleration for a given airspeed and bank. NOTHING ELSE CAUSES AN ACCELERATION, DROP IN AIRSPEED OR ANYTHING OF THE SORT. ESPECIALLY THE WIND.
Talking about what is going on relative to the ground serves only to affect time and fuel burn to a point in space. THAT IS IT.
There is no point in talking about a wind vector, tailwind component, turn direction, heading change, speed tables, angle of attack.
All of your speed components and heading change examples, while impressive in their convolution and circuity, are nonsense. And I mean this with the utmost respect for your desire to get this all straight.
THERE IS NOTHING DIFFERENT BETWEEN A DOWNWIND TURN AND AN UPWIND TURN, OTHER THAN A CHANGE IN GROUND SPEED.
If I could say this in a more simple way, I would.
Peter Garrison's makes a perfectly valid argument, as a physicist I can vouch for it. Once you are in the air there is no such thing as head or tail wind, at least not from the aircraft point of view. The absurdity of talking about head/tail wind during such a turn reminds of reading thousands of foolish comments on the internet about why aircraft can't take off from a moving conveyor belt (the famous puzzle). If you cross a mountain ridge and there are wind shears (because of altitude or time change) that's a different story but in a perfectly steady wind field the aircraft can't sense any head or tail wind, it is a pure lunacy to argue otherwise.