T/O overruns and veer-offs on slippery runways with crosswinds
Engine and tire failures are primary causes of high-speed rejected takeoff accidents.
(L) Accelerate-stop (AS) and (R) accelerate-go (AG) of a twin-engine jet aircraft on a wet/slippery runway with significant crosswind. (Diagram not to scale.)
How can a pilot maintain runway track while performing an RTO on a slippery runway with XW? Relaxing left rudder pressure will allow the reverse thrust torque to turn the airplane downwind, which automatically directs some reverse thrust upwind. Runway centerline tracking can then be restored and maintained while still delivering maximum longitudinal frictional braking. However, this maneuver must be performed in a timely manner—ie, before the airplane is too close to the runway edge.
To go or not to stop
The idea of reducing V1 when operating on slippery runways seems reasonable, as frictional deceleration will suffer. On the other hand, it will now take longer in terms of distance and time to accelerate to liftoff. If all or some tires fail below V1, it is still recommended to continue with takeoff, although the subsequent landing may be problematic.
A heavy, FLLTOW, twin-engine airplane at high density altitudes may have an OEI acceleration of only 1–2 kts/sec, which will decrease further during rotation because of the large increase in induced drag. More than 15 sec may elapse between the sudden engine failure and actual liftoff.
As we can see from the right-hand illustration in the lower diagram on p 56, continuing the OEI takeoff could be equally hazardous when operating on a slippery runway with a crosswind. In this example, the left (downwind) engine has failed and the right engine continues to produce maximum thrust. The torque created by asymmetric thrust must be offset by applying right rudder.
A 30-kt XW coming from the right side, together with the rudder lift, may generate a force of almost 30,000 lbf, with a maximum 15,000 lbf cornering force opposing it—and possibly less.
At relatively high speed and continuing with the takeoff, the effective weight on the wheels is reduced significantly. Not much tire(s) cornering force is available and the airplane may begin to accelerate rapidly sideways. In less than 7 sec of continuing OEI takeoff, the airplane may already veer off if the pilot does not arrest sideways drift.
This could be yet another race against time and inertia. Forward acceleration is weak, allowing wind and compensating horizontal rudder force more time to conspire against the pilot. Trying to turn slightly into wind to direct some thrust against it will require more rudder deflection and may not stop the sideways motion entirely. The question remains whether the airplane will lift off before the downwind tires depart the runway.
When not to take off
(An operational takeoff diagram can be designed which prohibits takeoffs at a certain combination of effective runway friction coefficient (or airplane Mu) and crosswind component. (Diagram not to scale.)
As the above 2 takeoff scenarios illustrate, a wet/contaminated runway can be lethal when combined with crosswind and an engine and/or tire failure, whether the decision is made to stop or to continue.
Engine or tire failures close to V1 speeds are rare. Only a fraction of takeoffs are performed on wet (or contaminated) runways with XW coexisting, and the probability that an engine and/or tire will fail at high speed is remote. However, aviation history is full of accidents that were "practically impossible."
Clearly, there are 2 additional powerful factors affecting takeoffs—crosswind and runway effective braking friction. Effective braking friction coefficients are difficult to estimate. Several methods exist to measure it, but none is reliable enough to be applied to all aircraft types.
Many airlines and corporate flight departments have no SOPs that address this takeoff hazard adequately. For aircraft dispatching purposes, one can design and use a diagram similar to the one presented in the figure on this page. Naturally, as the crosswind increases, the runway must be less "slippery." And, of course, other operational factors such as antiskid and/or thrust reverser inoperative must be considered. Company-specific operational safety margins can be built in. This provides a good balance of operational economics and maintenance of safety.
Runway excursions during takeoffs and landings are still the leading cause of airplane accidents. Overruns and veer-offs during takeoffs in transport-category airplanes are less frequent than those in landing scenarios, but are usually deadlier.
The combination of crosswind and a slippery runway can be lethal depending on which engine and/or tires failed and whether the pilot decides to abort or continue takeoff. Pilot training for such scenarios is still inadequate. Contaminated runway ground run simulation models are not accurate enough in existing FSSs for realistic pilot training.
However, several FSS manufacturers and operators are currently addressing this issue. Good SOP policies combined with in-depth theoretical and practical FSS training could help reduce takeoff/ RTO overrun and veer-off accidents. Pilots of corporate jets with fuselage-mounted engines may have a somewhat easier problem than pilots of airplanes with wing-mounted engines, but it would be unwise to dismiss this serious threat.
Having said all this, we must be fair and recognize that situations will exist where no amount of flight training can prepare pilots for every possible takeoff problem. This is especially true when we consider the limited time available for the actual process of recognition/decision/action.
Nihad Daidzic is president of AAR Aerospace Consulting, located in Saint Peter MN. He has worked for many years on the US and European space programs and is also a tenured university professor of aviation and of mechanical engineering.