Avoiding veer-off accidents on contaminated runways
Landing on slippery surfaces with significant crosswinds demands expert piloting skills.
By Nihad Daidzic
Learjet 25D lands in rain at DOV (Dover AFB, Dover DE). Pilots landing in medium to strong crosswinds and with poor runway friction are often surprised by the magnitude of veer-off tendency.
A Citation jet touches down on the centerline of a 5000-ft runway covered with 2 inches of fresh snow on top of a crust of refrozen slush—and with blistering winter crosswinds in excess of 20 kts.
Soon after touchdown, the airplane starts skidding sideways and within a short time exits the runway—still sideways—collapsing the main landing gear while moving at significant forward speed.
The aircraft does not have thrust reversers and the flightcrew is unable to do anything to prevent the aircraft veer-off. At first they believe that the airplane brakes have malfunctioned. This is a typical runway veer-off (or skidding-sideways) scenario.
The main ingredients required for such an accident or incident to occur are at least moderately strong crosswinds and runway contamination providing low tire/surface friction coefficient. Runway contamination can take the form of ice or snow, or just being sufficiently wet.
Sometimes light rain, mixed with the dirt and dust on the runway, can create a very slippery surface. Also, when the rubber deposit on the runway becomes moderately wet it will become very slick (due to the hydrophobic nature of rubber) and offer little friction.
A particularly dangerous scenario can occur during the tires’ dynamic or reverted-rubber hydroplaning while experiencing moderate—or stronger—crosswinds. The problem with slippery contaminated runways is that both the longitudinal and the lateral tire friction components will suffer.
In the Citation accident referred to above, the airplane might have been able to stop on the available runway without a crosswind. The problem is that the airplane has to move sideways only about 50 ft to depart the runway, while in a forward direction there may be 4000 ft or more available for stopping.
Indeed, almost 50% of all aviation accidents or incidents occur during the landing phase, and the most common landing accidents are veer-offs and overruns. Using thrust reversers (if available) to improve longitudinal deceleration can have a severe adverse effect on runway tracking.
On the one hand, slippery runway conditions require thrust reversers as a dominant deceleration force—but, on the other hand, using reversers may lead to the airplane losing directional control, sliding and skidding off the runway sideways. This is yet another pilot’s “coffin-corner.”
Thrust reversers—friend or foe?
Imagine an airplane landing on a runway which is slippery due to runway contamination such as ice, snow or standing water.
There will be little braking friction available to use the runway surface for deceleration or to provide cornering (lateral) force. About 70–75% of the longitudinal deceleration will come from the reverse thrust alone—if it is available.
If the airplane lands straight, with the linear momentum aligned with the runway, it will maintain direction unless external force(s) act on it. Since there is little effective friction on the slippery tire/runway interface, there will be no way to change the linear momentum of an aircraft significantly, other than by using thrust reversers—again, if they are available.
Landing on a slippery surface with significant crosswind. In scenario (a) there are no thrust reversers. In (b) the aircraft is “weathercocking” into wind with adverse thrust reverse, while (c) illustrates turning into the skid to prevent veer-off while still utilizing the full advantage of thrust reverse to stop on a slippery runway.
Angular momentum around the airplane’s vertical axis can be changed by using aerodynamic controls, rudders and/or ailerons (in some airplanes) or asymmetric (differential) thrust, which provides torque to rotate the airplane.
Regardless of the change of angular momentum, the airplane will continue moving in the same direction. A crosswind blowing at 30 kts and acting sideways on a midsize corporate jet might generate 700–1000 lb of lateral (side) force.
This might not seem a lot, but on a slippery runway there might not be much cornering (lateral) force on the tires to resist the sideways sliding motion, and the airplane might skid accelerating downwind.
If the ground drift on the runway is about 3° and the downwind main tire is, say, 40–60 ft from the runway edge (on a 100 or 150-ft runway), it might take only 10–15 sec to exit the runway sideways if no thrust reversers are used.
With thrust reversers, an airplane can be pulled off the runway in as little as 3–5 sec. A tire generates shear stresses caused by the tire-surface slip to provide longitudinal braking and slip angle to provide the cornering (lateral) force.
If the tire is turning while braking, the total tire friction force must then be split into 2 perpendicular components. The “friction circle” defines how much tire friction can be shared between the braking and cornering efforts.
If one wants maximum braking, then no cornering force will be provided. If one wants maximum cornering, such as on high-speed turn-off, then little or no braking force will be delivered.
Race tires, such as those for Formula I, can be designed to provide more braking or more cornering efficiency, in which case the “friction circle” becomes the “friction ellipse.” Let us now analyze typical scenarios of an airplane landing on a slippery runway with significant crosswind, as illustrated in the diagram on this page.
If the airplane is not equipped with thrust reversers we have the situation sketched in scenario (a). The airplane is moving sideways pushed by the wind, and little tire lateral force exists to resist it.
Since the tire slip angle is large, it almost won’t matter whether the tire is rolling or locked. The lateral force is basically equal to the effective weight on the tires times the sliding coefficient of friction.
The sliding coefficient of friction, on ice-covered runways or while hydroplaning (aquaplaning), may not be larger than 0.03 at typical corporate jet touchdown ground speeds. Despite having lift-dump spoilers deployed, the full weight of an airplane will not be on tires at those speeds.
Actually, even with spoilers deployed there may be no more than 60–70% of the landing weight on tires. Back to scenario (a). While sliding sideways, the airplane’s longitudinal axis will be more or less aligned with the runway.