What is your exact definition of “wind shear”? As I read it, it is not a constant or predictable phenomenon, just like turbulence. If it occurs, it can be at any time, in any direction, in any strength. So I think one can only be prepared for it by keeping some extra margin at all times.
Or do you mean the more or less constant shear associated with fronts, especially with cold fronts? Isn’t that limited to the time in which the front passes? Relevant only to heavier and IFR-equipped planes?
Wind shear (see example graph above) occurs whenever there is wind. It is the wind gradient with height.
Wind gradient effects are much more pronounced at higher wind speeds, e.g. 20kts plus. I don’t think anyone would land in a tailwind of that speed. The normal circumstances in which one would land with a tailwind are in light wind conditions, perhaps where terrain or approach aids suggest a landing in one direction, for example Salzburg, or Annecy.
Picturing this in extremis, descending from say a 10 kt through a neat boundary to a 0 kt wind will indeed suddenly increase the relative speed by 10 kt hence the IAS and more relevant for the discussion, the lift force at the same angle of incidence (by square!).
However as said, at the wind speeds a pilot would think at such a landing option is not going to give any significant gradient.
Technically, there is of cause always wind shear, but you don’t talk about it as such unless the wind gradient is great enough to (potentially) make a noticeable change to the flight path.
As byteworks says, the main cause of airspeed stability for a trimmed aircraft comes from the change in lift, with the consequent exchange between height (potential energy) and speed (kinetic energy).
Landing into wind, if you lose e.g. 10kts of headwind on the way down short final, at C172 approach speeds you might end up roughly 50ft lower near the bottom than you would have “everything else being equal”, which probably means 100-150 yards shorter, depending on the glide ratio.
Conversely with a tailwind, and no corrective action, you would land 100-150 yards longer, or perhaps even more because you are used to the opposite effect when landing into wind.
(Numbers not exact)
Another reason (second or third) for the longer landing distance is that it is so easy to misjudge the final approach because of the unfamiliarly sight picture on the low final needed.
Some pilots have surprising strong feelings about downwind landings. One of my instructors in past (an ex military one) regarded it as a cardinal sin, whereas others just see it as an option for the cases where the only ILS is downwind and the weather is below circling, or one has to land uphill.
The most dangerous thing about landing downwind is if the landing is aborted and there are obstructions in the departure end of the runway. The climb gradient is reduced and very difficult to judge, and I can think of two fatal accidents in Denmark caused by a late go-around and obstacles, during downwind landings. So “cardinal sin” was arguably not an exaggeration in those cases.
How much tailwind will you accept? Obviously the length of the available runway / obstacle free gradient on the go-around end comes into play, but often there is plenty runway available.
The landing distance charts in the POH of the aircraft I regularly fly have the graph only go up to 5 knots tailwind. Does that mean it is illegal to land with more tailwind?
Tailwind Straight-In approaches are easy to overcook by the way.
In some aircraft there is a POH limitation.
On some aircraft there is a tyre speed limitation which can be a problem with a tailwind.
Finally, as has been previously mentioned there is a climb performance issue in the event of a go around, particularly OEI.
When I land the Cirrus at the little glider field next to my company in the Czech Republic I prefer to land uphill with a little tailwind than downhill with a headwind. With 10 knots of headwind I land downhill though, but since the headwind is rarely 10 knots i prefer to land with tailwind most of the time.
The last time i flew there the wind was +30 knots (gusts to 40) from the east.. so i landed downhill and stopped after about 100 meters ….
