Every pilot meets it in the flare: you cross the threshold with the power at idle, raise the nose, and the airplane — which has been descending obediently all the way down final — suddenly declines to land. It skims. It floats. If you carried extra speed, it floats and floats, eating runway while you sit there renegotiating.
The FAA's handbook describes the sensation the way generations of instructors have: it feels "as if there were an air cushion" trapped between the wing and the runway. As a description of the feel, that's exactly right. As a description of the mechanism, it isn't — nothing meaningful is being compressed under you, and the airplane is not resting on a pillow. What's actually changed is the price of lift.
The bill that shrinks
In What Holds the Airplane Up we followed the deal: the wing makes lift by turning a river of air downward. That downward river — the downwash — is also a cost. Tilting the flow down tilts the wing's total force back a little, and the rearward lean is a drag force: induced drag, the drag you pay purely for producing lift. (It gets a full article of its own one tier up.)
Bring the wing close to the ground and the ground interferes with the billing. The earth is a boundary the flow cannot pass through, so the wing's downwash pattern and the swirling vortices trailing from its wingtips can't develop the way they do in free air. Less downwash for the same lift means the wing's force tilts back less — the lift is cheaper. Same lift, less induced drag, and at approach speeds induced drag is a big slice of the total. Your airplane suddenly needs less power — or, with the power already at idle, sheds speed more slowly than it did fifty feet up. That's the float.
How strong, how low
The effect follows a smooth curve that steepens dramatically as you get low. The handbook's numbers: with the wing about one full wingspan above the surface, induced drag drops by a negligible 1.4 percent. At one-quarter of the span, the reduction is 23.5 percent. At one-tenth of the span — flare height for most trainers — it's 47.6 percent. Nearly half the induced-drag bill, waived in the last few feet.
| Height above surface | Induced-drag reduction |
|---|---|
| 1.0 × wingspan | 1.4% |
| 0.25 × wingspan | 23.5% |
| 0.10 × wingspan | 47.6% |
Notice the shape of that table: the effect just keeps growing as the gap closes. There's no special altitude where it switches on and no maximum you pass through — only a curve that's negligible a span up and dramatic in the flare. It also explains why low-wing airplanes float more insistently than high-wing ones: their wings ride closer to the runway, further down the same curve.
Where it bites
On landing, the arithmetic is simple: extra approach speed is energy, ground effect lowers the rate at which the energy drains, and the runway behind you is gone. Fly the recommended speed and the float is a brief courtesy; carry ten extra knots "for safety" and you've bought a long, expensive glide one foot up.
The takeoff version is nastier. An airplane can be dragged off the ground in ground effect at a speed too slow to fly out of it. Climb a wingspan up and the discount expires — induced drag returns in full while the airplane is slow, heavy, and low. The handbook's sequence is blunt: airborne with a deficiency of speed, marginal climb performance once out of the effect, and in extreme conditions a settle back onto the runway. Mishandle it from there and you're auditioning for a stall. The airplane didn't "lose the cushion." It flew back into the full price of lift before it had the speed to pay it.
So keep the cushion as poetry for what the flare feels like — the handbook does. Just file the mechanism where it belongs: the ground doesn't push you up. It makes lift briefly cheap, and the airplane, like anyone handed a discount, takes its time leaving the store.