PILOT TECHNIQUE

Avoiding overrun accidents on contamined runways

Landing on slippery surfaces requires precise touchdown in addition to judicious use of brakes or thrust reverse.


Runway
Dry
Wet grooved
Wet nongrooved
Flooded nongrooved
Slush

Dry loose snow (0.5-3.0 in)

Packed snow
Thin Ice
Total landing distance (ft) ±100 ft
3496
3543
4177
4473
3896
4671
4377
5549
Distance increase %
0
1.33
19.5
27.9
11.4
33.6
25.2
58.7
Time (sec) ±0.67 sec
19.9
20.7
25.4
30.0
23.4
36.8
29.0
47.5

Landing distances, percentage distance increase and time to stop, over dry runway from TCH to full stop for a corporate BBJ landing at 144,000 lbs—other conditions as given above. Maximum-effort braking and thrust reversing until 60 KIAS is assumed (followed by idle reverse).

The main performance characteristics of a tire are traction, rolling and braking friction, and cornering ability. Airplanes do not have to worry about traction, but braking rolling friction is a crucial parameter in stopping.

The principal phenomena that lead to tire/road friction are adhesion and tire hysteresis. The dominant one—adhesion—is the result of instant intermolecular bonds between the rubber material and the runway surface (microtexture).

When there is a contamination interface layer (eg, a water film), the adhesion diminishes, resulting in an overall lower friction coefficient. This reduction in friction is what is commonly called “viscous hydroplaning.”

Sometimes that decrease can be a whole order-of-magnitude lower compared to dry surfaces. The friction coefficient is the ratio between the longitudinal friction force (braking) and the normal load (weight minus lift) and is a complex function of many parameters.

Dynamic and reverted-rubber hydroplaning (aquaplaning) are other phenomena that can completely destroy a tire’s ability to slow down the aircraft. Tire hysteresis can be imagined as the ability of rubber to instantaneously “hug” large-scale road protuberances (macrotexture) and thereby cause drag.

Tire slip and skid

For the tire(s) to create braking shear stresses in the contact patch, a tire slip has to exist. Tire slip is local relative motion between the tire’s center-of-mass and the circumferential tire speed at a point of contact with the runway.

This slip leads to “normal” wear and tear on tires, but is also essential in providing traction, braking friction and cornering capabilities. For a dry runway, peak friction occurs at 15–20% of slip, while on icy surfaces peak friction coefficient occurs at 5–10% and is much lower. (See upper figure on p 105.)

If the braking torque is larger than required for peak friction, the tire will transition rapidly into an unstable region in which quick deceleration will occur, resulting in 100% slip, or locked tire. (Again, see up­per figure on p 105.)

Sliding (skidding) friction is substantially lower than that of a rolling tire around peak braking friction. In addition, locked tires will lose all cornering ability, steering will become impossible, and the tire skidding will cause temperature and pressure buildup that can destroy it rapidly.

The resulting tire blow-out could lead to complete loss of control.

Antiskid systems

Modern, fully-modulated antilock braking systems (ABS or antiskid) prevent tire lockup and endeavor to maintain a braking coefficient of rolling friction close to the maximum possible. (See upper figure on p 105.)

This should not be confused with an automatic braking system (also abbreviated ABS), which delivers a predetermined level of deceleration (say, 8 ft/s2 or 0.25 G)—if possible automatically and without the pilot’s input—and also uses an antiskid braking system.

Modern fully-modulated ABS systems are about 90–95% efficient—meaning that they maintain peak braking efficiency for that percentage of time. The peak and the sliding longitudinal braking friction coefficient also depend on the rolling speed.

At higher speeds, the friction coefficient will be lower than at slower speeds—a fact that is often forgotten. (See lower figure on p 105.) Cornering is the ability of a tire to generate lateral force which is used for steering, eg, when the nose wheel is yawed in an airplane, or when resisting sideways forces due to crosswind and/or tilted surface.

Cornering force and the self-aligning tire moment are each a function of the slip angle or the angle between tire heading and travel track—a sort of tire’s “crab angle.” Cornering is crucial in resisting unwanted sideways motion due to crosswind.

Another important fact is that a tire can take only so much cornering and braking in the same time—a phenomenon which is often called “friction circle” or “friction ellipse.” More braking will deliver less cornering, and vice versa.

Formula I race cars distinguish themselves primarily by their in­credible cornering and braking performance. The best race tires can generate a 1.3–1.6 friction coefficient. (No, this is not a perpetual phenomenon.)

Due to the race car’s aerodynamic design and generation of negative lift, a Formula I car can deliver more than 4.0 G longitudinal and/or lateral deceleration on dry surfaces—a sort of “terrestrial aerobatics.”

Runway grooving—a NASA Langley-led method of improving surface drainage—has done a lot to minimize roadway and runway dynamic hydroplaning accidents, but is not a universal cure for all surface low-friction scenarios.

The table on this page shows landing distances for dry runway and runway with various contaminants for a “heavy” corporate Boe­ing BBJ landing at 144,000 lbs. In all instances, the air distance to main gear touchdown is 1115 ft and time elapsed from TCH is 4.64 sec.

Main gear touchdown occurs at 138 kts and the nose gear touches down 3 sec later at a groundspeed of 132 kts, during which time the airplane consumes an additional 653 ft of runway. Vertical touchdown speed is a gentle 96 fpm (1.6 fps) and the total distance (air plus ground) to stop is 3496 ft for a dry runway.

 

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