Point of flare—the last 5 seconds

Greasing it on the runway can be done with more frequency.

By Nihad Daidzic

Bombardier Learjet 45 lands on Rwy 23 at FNC (Funchal, Madeira, Portugal). Touchdown accuracy is critical on slippery runways and during LAHSO operations.

I was flying as a passenger recently on an Airbus A340. The trip was uneventful and we arrived on time after a 10-hr transatlantic flight. However, something happened during landing that prompted me to write this article.

I have been doing research on landing dynamics and touchdown control and the A340 landing at ORD (O'Hare, Chicago IL) was a textbook case of what I am about to write.

This is what happened. From what I could judge through my passenger window, the pilot flying the airplane started normal flare at 35–45 ft gear height. Then, when we were very close to the ground, the pilot suddenly pitched up some more, likely in an attempt to arrest the descent rate and cushion touchdown.

Did it seem like the right thing to do? What actually happened is that we touched down quite hard, bounced and ended up several feet in the air, floated for another long 4 seconds and then touched down again with no further bouncing. The nose gear was lowered well past the 3000-ft marker.

The subsequent hard braking and noisy maximum thrust reversers proved that the flightcrew was taking stopping efforts very seriously, even on a long, dry ORD runway—and the passengers were hanging uncomfortably in their seat belts for the next 15 seconds.

An overlooked phenomenon

To me this seemed like a model case of adverse elevator effect (AEE) or reverse elevator response (also known as negative-altitude res­ponse)—a phenomenon almost completely forgotten, and, to the best of my knowledge, never mentioned in any flight training or line flying environment.

A typical landing, in which the pilot is trying to "feel" for the runway and "grease" it in a near-tangential touchdown, consumes a lot of runway. And, although such touchdowns may be smooth and pleasing to the passengers, the subsequent frantic braking certainly is not.

Touching down, without bouncing, at the proper runway point is essential for landings on contaminated runways and/or during land-and-hold-short operations (LAHSO).

Schematic of landing flare maneuver. The airplane does not actually follow the idealized curved flare but, due to AEE, follows the real flare curve. Angles and distances are highly exaggerated for better visual representation.

There are so many "adverse" effects facing pilots while controlling airplanes. Every fixed-wing pilot has experienced adverse yaw, while fewer understand the adverse aileron effect. As a matter of fact, the traditional airplane flight controls are all wrong—or at least their initial response is.

For a conventionally configured airplane with a tail elevator, in order to pitch up, an up elevator movement is required to create downward force on the tail. That additional tail-down force will create torque about the lateral axis and pitch the nose up.

Accordingly, that would increase the angle of attack (AOA) and coefficient of lift, increase lift and induce temporary vertical acceleration to change the flight trajectory in the vertical plane.

However, what's really happening on the short time scale is quite different. The initially unbalanced downward force on the tail will actually first accelerate the airplane downward. This is AEE. The reason why it is often called negative-altitude response is that the initial reaction to pull-up will be the opposite—downward acceleration, increasing descent rate, and loss of altitude (going below glideslope).

This is AEE at its best—and it is real! Stability aug­mentation systems (SAS) that intro­duce additional pitch (rate) damping may alter aircraft altitude and lag response, but they cannot eliminate it.

Some may think that AEE is an obscure academic phenomenon that does not affect real airplanes. In reality, one can observe its effect on every takeoff. During the takeoff rot­ation, additional downward force is created on the tail to rotate the airplane nose upward and the airplane momentarily becomes heavier.

Dissecting the flare/touchdown

Response of Boeing 747-100 to step pull-up maneuver. Initially, the airplane will accelerate downward, ROD will increase by 120 fpm (2 fps) and 1.6 sec will elapse before it regains the glidepath (albeit somewhat lower).

It is helpful to dissect the flare and touchdown maneuver. In the figure above, we show the schematics of the flare (roundout). Typically, we think of the flare as a continuously smooth and curved vertical path which starts with the pilot pulling back on the yoke/stick. In reality, it is more complicated than that.

Due to the AEE, the airplane will initially accelerate downward, increase its rate of descent (ROD) and steepen the glidepath. So the real vertical trajectory looks more like the blue dashed line than the "ideal" flare curve. In addition, one can see why late flare might be a bad idea (red-dashed line)—which is what very likely happened in the aforementioned A340 landing.

It may sometimes be better to accept the existing ROD rather than try to pitch up when too close to the ground, as that will only accelerate the airplane CG downward. Push­ing the yoke forward might be a better course of action in such cases.

If this seems unnatural, remember that so was learning to push the yoke forward when the nose dropped in stall.


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