Point of flare—the last 5 seconds

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

A "new" flare technique would use sharper pull-up, followed soon after by a short push-over pulse. Timing and heights are critical and vary with airplane type. Angles and distances are highly exaggerated for better visual representation.

AEE has been known to airplane designers for a long time. It was previously studied for airplanes with large moments in pitch inertia and the case when the airplane was in level flight. Analytical and experimental studies (including piloted simulator studies) have shown the airplane accelerating downward initially and losing altitude for pilot step, impulse or ramp pitch-up maneuvers.

To be sure, at 20,000 ft no one was really concerned that the airplane CG would dip momentarily by, say, 3–5 ft for a second or so before starting to climb! While the negative force on the elevator is generated rapidly, the pitch-up rotation, the subsequent increase in AOA and lift, upward acceleration and the change of flightpath takes some time (response lag).

Many studies have concluded that AEE does not have a critical effect on longitudinal (pitch) handling qualities, although NASA-funded studies have shown that the Space Shuttle orbiter would dip about 20–25 ft initially in the step pull-up maneuver.

That is why longitudinal maneuvering dynamics is not the greatest feature of NASA's soon-to-be-retired reusable orbiter. And AEE could be a real problem when landing future supersonic/hypersonic space-planes with thin delta wing configuration that use elevons for pitch control.

Interestingly, what every previous scientific study missed about the reverse elevator effect is that dipping a couple of feet or descending momentarily at 100–200 fpm before reversing it and starting climb is not the worst consequence of AEE.

The most unfavorable effect is the time delay introduced by AEE, where the longitudinal dynamics may result in a 1.0 to 1.5-sec response lag before the initial reverse elevator effect is neutralized. Since we know that flare and touchdown are actually very short maneuvers lasting only a few seconds, delays on the order of 1.0 sec during landing maneuvers are critically important.

For a transport category airplane descending at, say, 700–800 fpm and 30 ft above the runway with the pilot initiating flare, a 1.0-sec delay in aircraft response after the step pull-up maneuver means that the airplane will lose an additional 12–13 ft before anything happens.

So what do pilots do? They add this height subconsciously and start the flare higher in order to compensate for the "nothing's-happening-when-I-pull-on-the-yoke" effect. However, starting flare higher (and necessarily more gently) results in longer and larger scatter in touchdowns.

This is also the result of the inability and uncertainty of human vision to judge heights accurately in the 30–60 ft region. Even with radio altimeters, a mere 300-millisecond delay in pilot response to height call­outs can result in large touchdown ROD deviations. That is why it is almost impossible to achieve any consistency in manual landings and why practical experience is so helpful.

To investigate AEE, a simple, linearized, longitudinal "flat-earth" model of short-term landing flare dynamics was developed in which pitch damping, pitch stiffness and aeroelastic effects were neglected—meaning that we have ignored the "phugoid" and "short-term" damped pitching motion.

For example, the numerical results of Boeing 747-100 simulations are shown in the lower figure on p 111. As the elevator is suddenly displaced up (step input), the ROD increases from 800 to 960 fpm (after 0.6 sec) before dropping back to the original 800 fpm (13 fps) a whole 1.1 sec later. During this time the airplane has descended 15 ft and steepened its glidepath. It is as if the airplane just rotated in pitch (as if pinned in its CG) with no effect on the vertical trajectory.

In airplane stability theory this is called "pure pitching motion." The 747 steepened the glidepath initially and then regained it 1.6 sec after the step pull-up maneuver, now de­scend­ing at 400–500 fpm. The mathematical model from which the results were calculated be­comes in­creas­ingly inaccurate for longer times.

Aerospace engineers knew about the reverse elevator response and looked for the ways to minimize it. On the other hand, some pilot, perhaps accidentally, found the way to use AEE. As any Boeing 727 pilot knows, the idea is to push the yoke forward just before touchdown.

It's often thought that smooth touchdowns are achieved by pushing on the yoke, because the main landing gear being behind the airplane CG will result in upward rotation and thus reduce touchdown impact. It is true that the landing gear upward movement will change the vertical contact speed—however, that in itself does not change the airplane CG vertical speed.

Although pull-up AEE leads to touchdown difficulties, it can also be used to pilot advantage. As much as the pull-up maneuver would actually cause the airplane to accelerate downward initially, pushing the yoke forward would cause the airplane to climb initially and thus reduce ROD.

New flare/touchdown technique

I believe that more precise and consistent touchdowns can be achieved with the flare-touchdown technique in which we use AEE to our advantage. An airplane can start the flare at a lower flare height (say, 20 ft instead of 30 ft), by pulling 1.20–1.25 g (2° per sec) and pitching the nose up to, say, 7°.

Immediately afterwards, or after a short hesitation, depending on the height above the runway, a step pushover maneuver of about 0.5 sec will result in an additional 150 to 200-fpm reduction in vertical speed, thus cushioning the landing.

Pilots may not like anything "mechanical" in flying—but honestly, it is difficult to develop any "feel" for landing flare as it is such a short maneuver and the airplane is quite lazy to respond.
Also, a further vertical speed reduction of 120 fpm or more can be used from the main gear (behind airplane CG) derotation, although that alone will not affect the airplane's CG vertical speed.

Since the flare started lower and the sharper executed pull-up causes more prominent AEE, the airplane effectively rotates and slows vertically by staying practically on the glidepath. The timing and proper height of the maneuver are critical. Pushing over when still too high may lead to hard landings.

Touchdowns using this "new" technique would occur not far beyond the point where the straight glidepath (say, 3°) intercepts the runway. Also, derotation of the nose in pushover will result in touchdown attitudes of 4° to 5°, still providing sufficient ground clearance for the nose gear. The momentum of derotation will also result in faster lowering of the nose gear, which is often done too slowly in daily operations, unnecessarily consuming useful runway and delaying braking efforts.

To sum up, the suggested landing-flare technique could consistently yield 1000-ft-plus savings in used runway. Touchdown would be more accurate and consistent, which is critical as it occurs in the high-speed landing portion where every second airplane consumes 200–250 ft of the runway. This could result in safer LAHSO ops and reduced overruns on contaminated runways. It could also reduce maintenance cost for brakes, reversers, airframe, etc.

More analytical and computational studies are required, however. Extensive ground-simulator piloted studies involving different airplane types are necessary too. Especially important for safety would be to understand and identify disadvantages of using this technique.

Certainly, no one wants to increase the frequency of hard landings, ballooning, floating and/or bounces. Direct lift control (DLC) would be another, probably better, method to improve touchdown accuracy, but very few airplanes today have it installed.

Nihad Daidzic is associate professor of aviation, adjunct professor of mechanical engineering, and chair of the Aviation Dept at Minnesota State University,
Mankato MN. He is also president of AAR Aerospace Consulting in Saint Peter MN.


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