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I am currently training for my commercial Pilot check ride and I am trying to execute the short field spot landing. Under FAA ACS standards you have to hit your spot within -0 to + 100 feet. I live in New Mexico USA where the winds can be quite unpredictable and I am having a hard time knowing where to aim to hit my spot. It seems that with a sizeable headwind the airplane does not want to float as it would if there was not any wind. In fact with a 15 knot headwind it seems like the plane does not want to float at all, almost like ground effect isn't there.

My understanding of ground effect is that it is basically the planes wake being reflected back to the wings creating a high pressure area under the wings and consequently increasing lift. However, if there is a headwind when landing, wouldn't the wind blow the wake of the aircraft away from the wings and reduce ground effect? Reversely if you have a tailwind wouldn't that blow the aircraft's wake further under the wings? That would explain why a plane seems to float forever when landing with a tailwind.

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  • $\begingroup$ When you say when there is a headwind the plane "does not want to float", are you speaking of a reduced "float" time, or a reduced "float" *distance"? $\endgroup$ Jul 10, 2022 at 4:26
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    $\begingroup$ As Robert stated at the end of his answer, practise makes perfect. To me it seems you just haven't got the hang of it, put more hours into it and you'll get it. The reluctancy to float is due to the gradient effect. You are losing airspeed as wind dies going down, it always does. You need to aim differently depending on wind, and ATIS will give you the wind. $\endgroup$
    – Jpe61
    Jul 10, 2022 at 8:03
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    $\begingroup$ One thing about New Mexico is alot of it is a mile or more high. So you're dealing with greater TAS from altitude too. As far as the glide path (which you can get as good as possible without "resorting" to throttle or flaps/slipping) you aim long for a headwind but less "float" distance. Tail wind is opposite. Maybe the mind confuses distance with time a little when floating. But I would bring someone along to help with IAS. Invariably, if you round at the same IAS, you go farther down the runway with a tailwind. Time never seems the same with a little adrenaline going. $\endgroup$ Jul 10, 2022 at 8:48
  • $\begingroup$ ground effect is that it is basically the planes wake being reflected back to the wings No, that is a misunderstanding. When calculating ground effect, a second wing is positioned such that it looks like a reflection of the real wing in the ground plane. This is a trick to achieve a flow pattern where streamlines at ground level are parallel to it. What really happens is that downwash is reduced because it is blocked by the ground. $\endgroup$ Jul 10, 2022 at 13:43
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    $\begingroup$ There is no need to aim at a different spot on the runway based on wind. Aim where you intent to land, period. Just anticipate needing more power to stay on glideslope the stronger the wind is. "wouldn't the wind blow the wake away..." sure, at the same speed and direction it is blowing the airplane for a net zero difference. $\endgroup$ Jul 10, 2022 at 21:46

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What you have described may be a gradient effect. When descending in a headwind, windspeed can be considerably lower near the ground than at 100 feet. If it is possible (perhaps with your co-pilot) monitor airspeed all the way down. Make sure that, even though groundspeed looks the same, you are not losing airspeed due to wind gradient. Short final and rounding are busy times. You could lose 5-10 knots IAS without even realizing it.

This will depend alot on local knowledge from experienced pilots. If the terrain and wind direction favors a gradient, you should know about it before getting too "cute" with short field technique.

It would really help learning to hit the spot rather than trying to shave a few knots off the approach. A lull, or gradient, could put IAS dangerously low. Descending into a gradient, one must pitch down to maintain airspeed. This will put your target in a different spot in the windshield. The pilot may try to keep the target in the same spot, resulting in a pitch up and loss of airspeed. The downwind case is of course opposite.

There are 2 good ways to "hit a spot" in wind, either by slipping or with throttle modulation. It will obviously be harder than no wind, and practice will make perfect.

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  • $\begingroup$ Thanks Robert for the answer. When landing on windy days here the plane can tend to sink rather than float when flying close to the runway. This can take me off guard causing me to land well short of my spot or even causing me to bounce the landing. I knew that headwind decreased the closer you go to the ground but the sink can be so strong sometimes I thought something else may be going on with ground effect. Its probably just a loss of headwind like you said. The winds here can be very gusty and brutal. $\endgroup$ Jul 15, 2022 at 1:48
  • $\begingroup$ @DanielHaverporth adding a few knots to approach speed on windy days might help. This would be a perfect lesson with an instructor and well worth it. A little skip from being a bit fast on touchdown is much better than the "pancake" from being too slow. Good luck! $\endgroup$ Jul 15, 2022 at 6:07
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  • If there is a wind gradient (decreasing wind speed with altitude) over the last few tens of feet to the ground (which there almost always is), and if the plane is descending to some degree in what you are calling the "float", rather then flying absolutely horizontally, then yes, absolutely, a headwind (gradient) will tend to subtract energy from the plane (increasing the sink rate and decreasing the float time as well as distance), and a tailwind (gradient) will tend to add energy to the plane (decreasing the sink rate and increasing the float time as well as distance.)

  • I practice alternating upwind/downwind landings in a closed-circuit situation on a routine basis, and I find this effect to be very pronounced.

  • The effects described above are not dependent on ground effect per se-- they would be be present even if you were landing on some sort of perforated sieve that created friction that slowed the horizontal component of the wind, but did not actually create a true "ground effect" in the manner that a solid surface does.

  • Moving on to consider actual "ground effect"-- it seems likely that a wind gradient would also, somehow, have some sort of effect on the ground effect. How could it be otherwise? Either the ground is altering the dynamics of the adjacent airmass, or it is not. And any change in the magnitude of the ground effect will affect the aircraft's rate of energy loss, even in a completely horizontal "float". What the effect of the wind gradient on the ground effect would be, is not completely obvious, but it seems unlikely that it would be absolutely zero.

  • If you wish to state that there is no wind gradient at all in your situation, then you are essentially saying that there is no friction between the wind and the ground. In that case, the airmass doesn't "know" anything about its speed relative to the ground-- the "ground" could actually be a conveyor belt turning hundreds of miles per hour in some arbitrary direction, and the airmass would be completely unaffected. In such a case, naturally, the motion of the ground relative to the airmass has zero effect on the aerodynamics of the landing aircraft. Or to put it another way, in such a case, the wind speed and direction, relative to the ground, has zero effect on the aerodynamics of the landing aircraft. But such cases can only be hypothetical-- in the real world, there is always some amount of gradient in the speed of the wind near the ground.

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