An air accident report (Warning: 1.3MB PDF) was published yesterday, into the tragic crash of a light aircraft in the Wicklow Mountains in rough weather, resulting in the death of 4 people. The report’s conclusion is that, in difficult weather conditions and mountainous terrain, the plane stalled with insufficient height available for recovery, and impacted terrain.
p.20 of the report contains the following:
This sounds wrong to me. It is suggesting that the indicated airspeed (speed relative to the air) will change depending on whether the plane is flying in the same direction as the wind (over ground) or a different direction.
My understanding is that the movement of the air over the ground, and the groundspeed of the aircraft, are irrelevant to the aerodynamic performance of the aircraft. To put it another way, if a plane can maintain constant airspeed in a turn in still air, it should also maintain constant airspeed throughout a turn in steadily moving air, by making exactly the same control inputs.
I accept that in this particular case, the air was not moving steadily, and there was severe turbulence, mountain waves, and gusting. But the report seems to claim that a turn from upwind to downwind direction will in general result in a loss of IAS.
In my experience, airspeed remains more or less the same and the ground speed increases or decreases.
Now here’s the thing: I haven’t flown in a while, and I stopped flying fixed-wings when I learned how to fly helicopters. Helicopters fly low and slow, so there are issues that we don’t really have to deal with. But I did learn fixed-wings where the wind would routinely blow 20 or 30 knots (fortunately, pretty much right down the runway). I’ve made many turns from a headwind to a tailwind in the pattern and in turns-around-points without noticeable loss of airspeed. Groundspeed, as you mention, is another matter.
Mountains have currents that can be dangerous. One of the helicopters I’ve flown crashed due to them. The pilot was making an approach to land on a ridge. The procedure is to come in high and approach steep. He came in too shallow and couldn’t climb out of a downdraft. He hit the side of the ridge short and rolled 100 m down the hill. (He was OK. Helicopter was destroyed.)
I did not read the report, so I don’t know how they concluded how the aircraft stalled. He could have been trying to climb over an obstruction and got slow, or there might have been wind shear.
I’m a flight instructor, but not an aerodynamicist, so I’m not sure I can answer this with precision. But I’ll give it a shot.
Theoretically, yes - turning from a headwind into a tailwind will result in a loss of indicated airspeed, and a gain of groundspeed.
You may not notice this effect unless you’re really looking for it and it’s a pretty strong wind. I have noticed it, usually with students as they practice landings in strong winds. It’s especially noticeable when the plane is already too slow prior to a turn from base to final.
I believe the reason for this has to do with how airspeed indicators work. The pitot tube measures incoming “ram” air, and this is compared to ambient air from the static port. A headwind will naturally provide more ram air to the pitot tube; turning away from it will decrease it.
This effect is usually brought up in discussions of wind shear and microbursts. There are a couple of questions on the FAA written tests specifically asking about indicated airspeeds as one encounters the severe headwinds and tailwinds associated with a microburst.
No this is wrong. The aircraft moves within the airmass and the movement of the airmass over the ground has absolutely NO effect on the air speed of the aircraft. If it did imagine what would happen to aircraft flying at high altitudes in winds of 100 knots or more, when they turned downwind they’d be falling out of the sky, they’d certainly have wild fluctuations in IAS, but this does not happen.
I don’t have time to read the whole 30 pages of the report but the section quoted is either poorly worded or the investigator doesn’t know his stuff.
There are some dangers with manoeuvering at low level in windy conditions and some of them may be mistaken for a loss of IAS when turning downwind.
When turning from into wind to downwind there is an increase in ground speed and when at low level the pilot may perceive the increase in ground speed as a lowering of the nose and instinctively raise the nose. This will cause a loss of IAS, but it is due to the nose being raised not the wind.
When flying through airmasses moving at different speeds you may get a loss in IAS. So if you were to climb or descend into an increasing tailwind or decreasing headwind you would experience a decrease in IAS, but this is due to a change in the airmass you are flying through. This type of thing is common because the wind is slower close to the ground due to friction and so when climbing and descending at low levels it is common to experience changing wind conditions.
You may experience a similar thing if you were to turn in a valley. The turn may take you toward rising terrain so as you got closer to the ground the wind would likely be slower and you may experience changes in IAS due to the change in the airmass you’re flying through.
Turbulence can cause fluctuations in airspeed.
But note that in all these cases, the change in IAS is due to a change in the air you are flying through.
There is one other thing that decreases IAS during a turn. When in a turn the wing is banked and therefore the lift vector is at an angle. As the vertical component of the lift vector must equal the weight of the aircraft to maintain level flight, the lift vector has to be increased. In short, when turning you need more lift to maintain altitude and one of the side effects of lift is drag. Extra drag causes a decrease in IAS if it is not compensated for with an increase in thrust. This will happen in any level turn and is unrelated to wind.
Mach Tuck what you are noticing from your students is probably due to the change in wind as they descend.
hibernicus your understanding of IAS in a turn is correct.
I didn’t read the full report either, but isn’t this the situation we’re theoretically talking about?
You seem to be saying that this would cause a change in IAS, and I thought that’s what I was saying too (perhaps with less precise terminology). And would it not be ultimately due to interaction with the pitot tue, since it is indicated airspeed we’re talking about?
But I may be misunderstanding you. If so, could you put what you’re saying in the context of the classic microburst example we always see in FAA publications? I wouldn’t be surprised to find that it is oversimplified.
The key is that in the part of my post you quoted, the airmass itself is changing. When in a level turn though in a constant wind there is no change in the airmass and therefore no change in IAS. When flying at 100 knots the pitot tube is having 100 knots of air pushed into it, this doesn’t change when turning downwind.
The microburst example is another one where the airmass itself is changing. Assuming you fly under a thunderstorm from one side to the other and experience the full sequence of windshear from the microburst, you initially fly into an increasing headwind and experience performance increasing windshear, you then stabilise for a while in the new conditions prior to flying out the other side and experiencing an increasing tailwind which causes performances decreasing windshear. That’s a very real concern and can cause a significant loss in airspeed which can be a problem particularly when already flying slowly such as when on approach to land.
But the scenario the OP was asking about is when flying in conditions with a strong but constant wind.
If you fly a Tiger Moth at 80 knots in a stable 300 knot wind and turn from into wind to downwind the Tiger Moth will keep chugging along at 80 knots through the turn, it won’t lose IAS and stall when that 300 knot wind becomes a tailwind.
This question comes up frequently in model aircraft circles (where a 20 mph wind is a much larger fraction of cruise speed than in full scale) and has resulted in some of the longest, bitterest arguments I’ve seen.
Run!
Yes, but only briefly. The key phrase in the cite is “if the turn is made quickly”. The airplane’s CG has inertia relative to the earth that is not necessarily going to change as fast as its orientation relative to the earth or the wind. In the extreme case, imagine it instantaneously yawing through 180 degrees so the tail is pointing in the original direction of flight. The plane will have the same groundspeed, in the same direction, as it had a moment before, due to its inertia, but the airspeed will change by 2x the wind speed. After some time, the inertia-induced transient effects will dissipate and the airplane will restabilize at its previous airspeed (assuming no configuration or power changes).
My relatively stale pilot experience - you are flying in the airmass. Your speed is realtive to the airmass. Ground speed is something you see as the ground passing below, but you have to subtract (or add) wind speed.
The trouble is, windy conditions are rarely regular. The windier it gets, the more likely the wind is irregular - watch a field of long grass or the fine dust of snow particles in an open area; wind seems to come in bursts. You can usually feel it too… If you are travelling close to stall speed going with the wind, or the gust variation is extreme (microbursts are this, iirc) then suddenly the air you are travelling in is going almost as fast as you and you have no lift. If you are lucky, this passes and you then regain lift, if your attitude has not suddenly become nose-down in the meantime. Erratic conditions are no time to be cutting it close.
Elvis, if you instantaneously yawed the airplane 180 degrees on a windless day you’d experience the same thing. It has nothing to do with wind or inertia relative to the earth.
An airplane operates relative to the air, nothing else.
This is not true. The extreme case of instantly yawing through 180 degrees violates all of the physics that governs how the aircraft flies in the first place, making the analogy irrelevant and the conclusion wrong.
Turning quickly loses you airspeed due to increased drag. Turning from into wind to down wind has zero effect on the aircraft, not briefly, not theoretically, none.
This is true close to the ground, steady windy conditions are common at higher altitudes though but the downwind turn believers tend to forget this. Ah, they say, the only aircraft flying at those altitudes are airliners and they go so fast that it doesn’t matter. Well, not really, gliders fly at high altitudes in some pretty strong winds at times and doing very tight turns if they’re trying to stay in some lift and at relatively slow speeds. They’d be a prime candidate for the dreaded downwind turn to put them into a stall, but it doesn’t happen.
I spend most of my working life flying at low level in strong steady winds over water and have never had any loss of airspeed turning downwind that was any different from the loss of airspeed in a turn in nil wind
Downwind turn accidents always happen low, always within sight of the ground. Why? Because the pilot’s eyeballs see the ground whipping by as the aircraft flies downwind, the pilot thinks “whoa! too fast” and slows down, then when turning away from downwind, the aircraft appears to be turning too slowly in relation to the terrain, so the pilot tightens an already slow turn. Stall, spin, NTSB! The syndrome is even worse when the terrain is rolling hills and the horizon is indefinite. The aircraft really doesn’t care if it’s going into or downwind, only that enough air is going over it’s wings.
A plane is flying North at 200 kts IAS. There is no wind. The plane does a practically instantaneous course reversal.
I believe the plane will continue moving North, slowing as the engines accelerate the plane Southward. It will pass through 0 kts IAS on its way to 200 kts South, losing altitude in the process.
Your thought experiment fails here. There ain’t no such animal - airplanes can’t do instantaneous course reversals. If you were (say with some sort of bizarre wind tunnel setup) to expose an airplane to a sudden 200kt relative wind blowing from tail to nose, it would almost certainly suffer catastrophic damage.
1920s Style “Death Ray” has it right. When turning in air that is moving at any steady velocity, an airplane behaves just as it would in still air. Put another way, without looking outside the airplane, there is no way to do turns and from that tell which way a steady wind is blowing.
