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 with a 30-kt crosswind. The lateral reverse thrust component is assumed to be a constant 500 lb. Reverse thrust is applied once all the tires are on the ground, while the airplane may be accelerating sideways from the point of touchdown.

A crosswind may rotate the airplane downwind or upwind, depending on the directional stability on the ground, where the main landing gear serves as the point of rotation—albeit a weak one.

To the imagined horror of the flightcrew, the airplane will be sliding sideways toward the runway edge while still pointing straight.

The only thing a pilot can do to avoid veer-off is to steer the airplane into the wind using aerodynamic controls and add forward thrust. However, the aircraft is on a slippery runway and the friction braking is almost nonexistent.

Applying forward thrust to fight the crosswind will ruin that little bit of deceleration available from the brakes. If caught unprepared, a pilot might have to choose the lesser of 2 evils—skidding off the runway sideways or overrunning the runway end.

An attempt at a go-around after all the wheels are on the runway might be an even worse alternative. (See Pro Pilot, Dec 2008, pp 110–113.) If the airplane is equipped with thrust reversers, as should be the case when attempting to land on contaminated runways, the crew might face the situation illustrated by scenario (b).

A crosswind will normally turn the airplane nose into the wind due to directional stability—although some corporate jets might swing downwind—but now the thrust reversers work against the pilot and will literally pull the airplane off the runway sideways.

The figure shows (in sketch form) the external forces acting on an airplane. Obviously, the sideways drift will be even more pronounced. Several aviation experts state that the pilot should get out of reverse thrust and apply forward thrust to maintain runway track—but this might not be the best advice in all circumstances.

For one thing, there might not be enough time to re-stow the reversers and apply forward thrust. This method also neglects the fact that we need reverse thrust to slow down. Even if disengaging thrust reversers would ultimately lead to regaining the runway track, the airplane might not be able to stop before it reaches the runway end.

Turning into skid

So why not do what expert race car drivers do and turn into the skid? We could use nose gear, rudder and aileron, and/or decrease reverse thrust on the upwind engine to force the airplane to turn downwind, as illustrated in scenario (c).

At the high-speed portion of the landing run only 6–10% of the airplane weight is on the nose gear, and there might be no cornering force on the nose gear tire to affect the airplane’s heading.

Landing on a slippery surface, again with a 30-kt crosswind. No reverse thrust was used (or available). This data can be used from the point of touchdown.

Using aerodynamic controls and/ or reverse thrust turns the nose of the plane downwind and into the skid. Simultaneously, the reverse thrust vector turns into wind and, by controlling the ground “crab” angle, we can easily regain runway centerline and maintain runway track, all while slowing down with almost maximum reverse thrust!

Steering with ailerons in such a situation might work quite nicely. It is like a sailing maneuver in floatplanes. Turning the control wheel into the wind will often turn the airplane’s nose downwind. In addition, pushing on the downwind rudder will help swing the airplane.

A full left rudder and control column (stick) into wind might be needed to defeat the airplane’s directional stability on the ground from a right crosswind. If necessary, decreasing reverse thrust on the upwind engine will help swing the airplane nose downwind, although a typical corporate jet with rear-fuselage-mounted engines will experience little torque from asymmetric thrust.

By turning into skid and keeping full reverse thrust, the pilot can control the magnitude of sideways motion and regain runway centerline. However, this is an extreme maneu­ver that should never be learned on an actual landing.

A flight simulator or flight training device that accurately simulates the airplane’s ground dynamics can be used to train pilots in handling such extreme situations. To provide a better feel of how powerful this “crosswind trap” on a slippery runway can be, let us do some calculations using a 30,000-lb corporate jet landing on an ice-covered runway with a 30-kt crosswind.

The tire lateral force resisting sideways slide is about 630 lb for a sliding friction coefficient of 0.03 and 70% of the weight on the tires. The sideways drag force created by the wind will be about 770 lb for a midsize corporate jet.

If a runway drift angle into wind is about 6° the lateral reverse thrust component will be about 500 lb. Maximum reverse thrust will be approximately 5000 lb, which is 50% of the maximum forward thrust of 10,000 lb.

A net lateral force of about 650 lb exists and is now accelerating our 30,000 lb corporate airplane sideways. Some results of the calculations are shown in the left-hand figure on p 56. If the airplane lands 10 ft off the centerline, and/or with a touchdown sideways drift of 3 ft/sec (1.8 kts), the situation be­comes much more dire.

In this worst-case scenario it will take slightly over 7 sec to move 50 ft sideways—perhaps already enough to put the downwind main tire in the grass and mud and cause complete loss of directional control.

But even if the pilot lands the airplane on the centerline with no side drift, veer-off will still occur as the landing run will take more than 25 sec, giving the wind “enough” time to push the airplane sideways.

In reality, the airplane’s “ground” crab angle may become much larger—10 to 15°—pulling the airplane faster off the runway edge with thrust rever­sers engaged. Calculations of lateral displacement without thrust rever­sers are shown in the right-hand figure on p 56.

 

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