As we know it, aircraft are extremely sensitive to gust during its final landing stage. They can lose airspeed if the wind changes to tail, ad thus loose lift or need to adjust their thrust in order o regain the lost speed. On the other side, the opposite happens when encountering a headwind.

On the contrary, we don't take that into account when we consider a fighter jet. When a jet fly's at 20,000 with headwind of 50 knots, turns 90 degrees to the left and has the wind from the right- is it considered to be a change in wind direction or because the whole system is moving along with the plane and the change is more gradual- the aircraft feels no change in actual lift which is derived from the true speed it feels?

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    $\begingroup$ It’s a single question, based on a misconception, with three good answers. Voting to reopen. $\endgroup$ – Koyovis Feb 7 '20 at 22:12
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    $\begingroup$ Don't see any reason do downvote the question. When you say it didn't include any research I must disagree- as I thoght that "When an aircraft is traveling in an airmass- everything is relative to an airmass". As I have encountered several casas (And I have more than 5000 horurs accumulated on aircraft- this is not such a trivial question- especially when you encounter this in flight. As for an example- Yesterday flying at 39000' wind gained 20 knots more from the back- the TAS (and CAS of course) decreased, and the engines were ordered to add thrust until regained speed. Ideas? $\endgroup$ – Stan Feb 8 '20 at 7:54
  • $\begingroup$ Further- you as an engineer should be aware that asking question which seem to be trivial most of the times do hide in them a more serious debate. Having said the- down-voting a question is irrelevant in a place that should be a place of debate and looking for the truth. $\endgroup$ – Stan Feb 8 '20 at 8:02
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    $\begingroup$ I absolutely agree with Stav and @Koyovis, as this question boils down to basic physics and terminology. It is a very good question, the merrygohappyminusone -guys are triggerhappy again. $\endgroup$ – Jpe61 Feb 8 '20 at 9:15

Before I get into the question about the turning fighter jet, a few words about airplanes' being extremely sensitive to gusts during final stages of landing. If this was actually true, I'd be dead by now. Seriously. Airplanes are sensitive to gusts during landing, I'll give you that, but I think it's exaggerating beyond reason to say it is extreme.

Now, back to one of the most persistent myths in aviation:

The myth of the downwind turn

The jet in your question does not make a full turn from headwind to downwind, but it does not matter, the physics are the same as in the myth of the downwind turn.

As you stated the plane is moving along with the airmass. So, for the plane, aerodynamically there is no head-, side-, or downwind. Only us on the ground can feel it, as we are locked into to the coordinate system of the earth with our feet.

The airplane is locked into the coordinate system of the airmass, and as long as the airmass itself does not experience acceleration, the airplane flying in it can turn in whichever way, and it will not notice any change in the wind direction.

The direction of the wind is locked into the coordinate system of the earth, so in your example there is no change in wind direction. Not for the jet aerodynamically speaking, and not in the sense of navigation (relative wind changes, but you use the same formulas for the calculations anyway). What does change is the groundspeed of the aircraft, and this is the reason wind (and gusts especially) causes some concerns for landing planes. Takeoff is also affected, but not quite as much as landing.

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    $\begingroup$ The answer is almost perfect. Gusts (sudden change) do affect the airspeed, not just groundspeed. Similarly, vertical gusts affect the angle-of-attack. $\endgroup$ – ymb1 Feb 7 '20 at 22:57
  • $\begingroup$ Can you please elaborate what is the difference between being locked to an airmass and turning inside it, in contrary to having the airmass changing direction? I mean literature preferably :) I have been flying for 15 years now and never thought about the possibility that a changing direction inside an airmass can cause changes in airspeed, but somebody made me wonder :) $\endgroup$ – Stan Feb 8 '20 at 8:46
  • $\begingroup$ @ymb1 I'll stitch that in (a bit later), a very good remark. $\endgroup$ – Jpe61 Feb 8 '20 at 8:52
  • $\begingroup$ "..the airplane flying in it can turn in whichever way, and it will not notice any change in the wind direction." Except for changes in Angle of Attack and Angle of Sideslip. $\endgroup$ – Koyovis Feb 8 '20 at 9:01
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    $\begingroup$ Ok I will have to rephrase it. Will take some time though :) $\endgroup$ – Stan Feb 8 '20 at 9:38

It's a change in wind direction but it's not considered windshear. The change in "wind direction" will be imperceptible to the pilot and passengers other than a change in the plane's groundspeed and even that is only detectable with a groundspeed readout.

  • $\begingroup$ Next time you fly look at you CAS and Ground speed when the tail wind increases. There is a reason I asked such an easy question :) I'ts not that easy. $\endgroup$ – Stan Feb 8 '20 at 8:00

At cruising altitude and airspeed, a change in wind is almost a non-issue. Wind velocity (direction and speed) changing will not noticeably affect airspeed. It will, however, affect groundspeed. Windshear (a sudden and/or drastic change of wind velocity) will affect airspeed momentarily until the plane’s momentum is overcome by the aerodynamic forces acting on it. This will be most apparent as turbulence or loss of performance. It is also most apparent during slow flight due to being so close to your performance envelope edge. The sudden gain then loss of altitude when flying through a microburst is a good example of that.

Flying low and downwind over a mountain is another good example. The sudden downward rush of wind on the leeward side will force your airplane to lose altitude. The plane’s performance remains the same. The cause is that the airplane is flying in an air mass (packet of air) that is itself rushing downward. The plane is just along for the ride. The pilot is forced to drastically increase the plane’s climb performance through the airmass in order to stop their descent before impacting the ground. Like throwing a fish bowl on the ground. If the fish does not jump up, out of the bowl, it will hit the ground with the bowl (never mind that fish can’t fly).

During landing, your slow airspeed and proximity to the ground combined with the vertical component of your momentum makes any loss of performance dangerous. It could lead to a sudden increase in descent rate at the wrong moment or a stall if you are too slow. A change in wind velocity as little as 5-10 knots can have an effect on the plane. That’s why pilots are encouraged to add half of the gust factor (the difference between the steady wind and the peak wind) to their approach and landing speeds.

In your example, it looks like you are concentrating more on changes in relative wind velocity based on changes in aircraft heading instead of windshear. That is different. The aircraft’s airspeed will remain the same. There will be no loss of performance relative to the airmass because the aircraft is flying IN the airmass. The airplane does not care about its performance in relation to the ground unless it is on the ground.

Picture this, you are in an empty B747, in smooth air, full throttle, close to Vne. Someone suddenly releases a homing pigeon at the very back of the airplane. As the little bird races to the front of the big bird, is he supersonic? No. Because he is moving inside the relatively still air of the B747.

  • $\begingroup$ Your example at the end is not good because you are talking about an object that does not experience external air. From your answer above- I am not asking it is dangerous to lose speed or not during cruise, but how does a change in wind direction - whether caused by the airmass changing or by a very quick jet with a turn radius of 30 degrees per second- affect the momentary lift of the aircraft, it tries to maintain the same altitude and engine setting. $\endgroup$ – Stan Feb 8 '20 at 9:05
  • $\begingroup$ And from my experience- together with affecting groundspeed which is obvious- it momentarily affects the TAS and IAS and such, until proper adjustment. just like in a wind shear during landing- in which by all theory and experience- first you encounter the front of the downdraught- which is strong headwind- causing the plane to gain lift and close it's engines a bit in order to maintain the airspeed. $\endgroup$ – Stan Feb 8 '20 at 9:07
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    $\begingroup$ Well, that is my point. The air around the pigeon, on the inside of the B747, is moving at around .9 Mach. Does it feel like .9 Mach to the pigeon. He is inside an airmass, flying his own personal airspeed. $\endgroup$ – Dean F. Feb 8 '20 at 17:07
  • $\begingroup$ @DeanF. Your pidgeon example is excellent!!! If that does not clarify the dilemma of flying in a moving airmass, I guess nothing will... $\endgroup$ – Jpe61 Feb 8 '20 at 17:18
  • $\begingroup$ Yes, it's understood.. it took me some time to actually understand the reason for that. the concept of relative speed I clearly understood, but from my logic you could see that my misconception was that by pulling a lot of G's, the aircraft actually proceeds the particles in which it travels, meaning- changing its center of reference. What helped me understand was something I read in an article, saying that a turn, no matter how quick, maintains equilibrium and all the masses inside a system do travel together unchanging the relations between them. In contrary to let's say, magic. $\endgroup$ – Stan Feb 8 '20 at 18:25

Ok, I was wrong. In order to better understand what is happening, other than saying that the plane flies embedded in air mass, it is easier to understand that the turn, no matter how fast it is- maintains equilibrium. meaning- it is not abrupt in no time, but a collection and a gradual turn into the tailwind or vice versa. This is easier understood by looking at each particle of air in which the jet fighter is flying. every second millisecond etc.- that the plane turns, the particles travel with it by the same velocity in relation to the ground, making it move altogether. The situation would be different if the aircraft turn 90 degrees in no time, making the air around it change direction in no time- like happens in a windshear or inversion.

  • $\begingroup$ Best to morph your two anwers into one, or delete one of them. $\endgroup$ – Koyovis Feb 9 '20 at 1:03

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