T/O overruns and veer-offs on slippery runways with crosswinds
Engine and tire failures are primary causes of high-speed rejected takeoff accidents.
To stop or not to go
Essential takeoff distance definitions. (Diagram not to scale.)
Let us now dissect 2 potentially dangerous takeoff scenarios in which a tire and/or engine has failed in a twin-engine jet on a slippery (ie, wet or contaminated) runway with significant crosswind. Both scenarios of AS and accelerate-go (AG) are illustrated in the lower diagram on p 56.
A pilot may be in great peril when the runway is slippery with significant XW. The left-hand illustration in this figure shows the case of an AS attempt. The upwind (left engine) has failed and the reverse thrust is deployed on the right (downwind) engine.
Reverse thrust is not only necessary in the case of wet or contaminated runways—it is now also explicitly endorsed in regulations. In order to compensate for the torque of the asymmetric reverse thrust trying to turn the airplane downwind (to the right), left rudder must be applied.
This not only creates a neutralizing torque for directional control—it also superposes a sideforce to the force of the wind. Most modern airplanes will turn into the wind during a ground roll, offsetting somewhat the need for rudder input. The only force opposing the combined lateral forces of wind and horizontal rudder lift is the lateral (tire cornering) resistance.
The combined action of cornering and longitudinal (braking) action can never exceed the maximum value as defined by the tire friction circle (or ellipse). The more braking is applied, the less cornering force will exist to prevent sideways motion.
There are 3 powerful forces—wind, rudder lift and reverse thrust component—all working against a pilot. It can take just a few extra seconds for a tire to reach the downwind runway edge, after which control may be completely lost in a high-speed veer-off. This scenario is just as hazardous as a landing ground run with thrust reversers on a slippery runway with significant crosswind.
ASDR and TODR on dry and wet runways. (Diagram not to scale.)
Let us imagine a hypothetical 400,000-lb airplane (similar to a Boeing 767-300ER) rejecting takeoff at about 155 KCAS at SL ISA. Without describing the mathematical model and a set of tedious calculations, an assumed 30-kt XW generates a side force of about 11,000 lbf.
A total sideforce due to wind and displaced rudder is then about 21,000 lbf. The only force resisting is the tire cornering (lateral) force. At high speeds, and with the lift-dump system deployed, no more than about 50% of the airplane weight may be on tires.
The cornering force will have to "compete" with the braking force for the "piece of rubber." If the effective friction coefficient—which accounts for the theoretical maximum friction coefficient, antiskid efficiency, etc—is about 0.1, the maximum lateral resistance force—about 15,000 lbf—may be created by the tires.
This leaves about 6000 lbf unbalanced force to accelerate the airplane sideways. Ice or hydroplaning would produce even lower effective braking friction coefficients.
The calculation of friction forces in viscoelastic tire material is complicated and depends on many factors, of which slip and slip angle are among the most important.
After a few seconds the airplane will drift sideways and be displaced on the downwind runway side. The pilot may notice this and push the left rudder some more to "bring the airplane back" on a centerline—and in doing so make a fatal mistake.
If the airplane starts drifting toward the downwind runway edge, a "normal" pilot reaction may be to increase left rudder pressure and turn the airplane into the wind and away from the downwind runway edge. However, not only will this increase the horizontal rudder force (if it didn't reach a physical stop already)—it will also have the effect of turning the reverse thrust vector further downwind.
Together that will result in 9000-plus lbf of unbalanced sideforce. In the next few seconds the airplane will be displaced significantly and about to reach the runway edge with the downwind tire(s).
Since the airplane is accelerating sideways, the more the pilot tries to turn the airplane into the wind, the faster it moves sideways. Simultaneously, maximum-effort braking will lower cornering resistance, and the airplane will slide (or skid) sideways regardless of where the nose is pointing.
As the airplane slows down, the cornering force may increase. Will the cornering force increase and catch-up with this vicious cycle, or will the airplane veer off? Chances are not good, as the cornering force can only increase by relaxing the longitudinal braking, resulting in a possible RTO overrun.
Veer-off and overrun accidents on slippery runways with crosswind confirm that, unfortunately, this scenario is quite possible. In addition, blowing the downwind tire in such circumstances can lead to a rapidly deteriorating situation. Not only is the braking friction reduced on the downwind wheel, but the braking force from the good upwind tires will rotate the airplane even more into the wind.
What if only the downwind tires were to fail close to V1 and the pilot elected to reject the takeoff? The situation then is similar to landing on a slippery runway with thrust reversers in a crosswind—already covered in a previous article (Pro Pilot, Sep 2009)—with the added problem of asymmetric braking.