AIR SAFETY

Avoiding veer-off accidents on contaminated runways

Landing on slippery surfaces with significant crosswinds demands expert piloting skills.


Landing on a slippery surface for variable crosswinds and zero sideways displacement, speed and acceleration on touchdown. Lateral displacement is calculated for 10 sec elapsed time after touchdown. Displacements highlighted in red define likely veer-off depending on runway width.

This situation will give the pilot more time to control longitudinal track, but the runway overrun scenario becomes more likely as friction braking is agonizingly weak. Summarized results of lateral displacement 10 sec after all wheels are on the ground, and for various magnitudes of crosswind component, are shown in the table on p 56, which considers the case with and without thrust reversers.

As long as the tire lateral force can defeat the sideways “push” by the wind, lateral displacement will be zero. If the airplane lands 10 ft off the centerline, that number should be added to tabular values.

A sideways drift velocity and/or acceleration on touchdown would make things much worse. We should also not forget that it takes several seconds to put the nose gear on the runway and start the reversers after touchdown, all while a crosswind is working diligently to push the airplane sideways on a slippery runway.

Conclusion

Millennium Aviation Hawker 700 on a snowy tarmac at HEF (Manassas VA). When the runway is covered with snow or not cleaned properly it is often impossible to visually identify the centerline. This can only complicate the situation.

The model presented here is based on a differential equation of motion in lateral direction using Newton’s Law. Several simplifying assumptions have been made in order to reduce the enormous complexity of the problem.

Obtaining the airplane’s sideways cross-section area and the drag coefficient is very difficult. Airplanes are neither designed for nor capable of flying sideways, other than for small sideslips. Does it then make sense to land on the upwind half of the runway?

The data certainly would indicate so, but the problem is that we don’t know what the actual wind will be on the runway. We might have just a little bit too much “correction” so that airplane exits sideways on the upwind part. How could we then blame crosswind for that?

The most important lesson(s) we can learn when landing on slippery runways with crosswind is to put as much weight on the tires as soon as possible. Touchdown should be done on the runway centerline (if you can see it) with no lateral speed or acceleration.

Use aerodynamic controls and thrust reversers (if your aircraft is equipped) and turn into the skid to control the runway track and obtain maximum longitudinal deceleration.

Thrust reversers can be your worst enemy in such situations, unless you possess the skill of an expert car driver. In general, landings on contaminated runways with significant crosswinds are best avoided.

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

 

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