Does Headwind on Landing affect Ground Effect?

Question

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.

Answer 1

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.

Answer 2

• 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.