TAP017: Aquaplaning

There are three types of aquaplaning - viscous, rubber reversion, and dynamic.

Viscous

This occurs when a thin film of contaminant creates a break in the contact of the tyre with the runway surface. This type normally only occurs on unusually smooth surfaces such as the runway touchdown zone where there is an excessive build-up of rubber. Viscous aquaplaning can occur even in damp conditions at high and low speeds. Because there's no actual contact, no marks are left on the runway.

Reverted rubber

This type of aquaplaning occurs when a stationary tyre (so either 'locked up' during braking or at touchdown) is dragged across a surface causing friction at the contact point. The heat produced by the friction boils the water on the surface creating steam. The pressure of the steam lifts the centre of the tyre off the surface leaving the edges still in contact creating a seal which traps the steam, this then melts the rubber and reverts it to its unvulcanised state. Friction levels during this type of aquaplaning are the equivalent of icy runways. The tyre will have 'bubbled' rubber deposits on it and the runway will show marks in the form of being pressure washed as the tyre effectively 'steam cleaned' it.

Dynamic aquaplaning

Now this is the most common type of aquaplaning and the one that's most likely to affect us. Aircraft in general are prone to this one because it's a relatively high-speed phenomenon that occurs when there is a film of water on the runway that is at least 2.5 mm deep. As the speed of the aircraft and the depth of the water increase, the water layer builds up an increasing resistance to displacement, resulting in the formation that wall of water beneath the tire we mentioned earlier. Once the tyre speed gets to the point where it can no longer displace the water quick enough it starts to aquaplane. At some speed, termed the aquaplaning speed (Vp), the upward force generated by water pressure equals the weight of the aircraft and the tire is lifted off the runway surface. In this condition, the tires no longer contribute to directional control, and braking action becomes very poor once in this state. 
When we use the landing distance calculations, aquaplaning is taken into account when contaminated performance is selected. Airbus says "Performance data for landing on runways contaminated with standing water, slush and snow include accountability for the reduced wheel braking on the contaminated runway including negligible wheel braking above the hydroplaning speed." 
As there is no surface contact during dynamic aquaplaning, there are no marks left on the runway surface or the tyre.

The minimum speed for dynamic aquaplaning (Vp) in knots is about 9 times the square root of the tire pressure in pounds per square inch (PSI). The pressures on our airbus' vary depending on the MSN number but there is a placard on the back of each main undercarriage strut with the required pressure. As an example though, if an A319 has a pressure of 200 PSI, then the aquaplaning speed would be 127kts, surprisingly similar to the sort of speeds we touchdown at! A locked up wheel will aquaplane at much lower speeds - as low as 7.7√P which would be only 108kts! And once aquaplaning has started it can continue at speeds well below this.

If you touch down with some crab angle on a dry runway, the aircraft automatically realigns with the direction of travel down the runway.

But on a contaminated runway, the aircraft tends to travel along the runway centerline with the existing crab angle. This is then compounded by the side force created by the crosswind component on the fuselage and the tail fin which tends to make the aircraft skid sideways (downwind) off the centerline.

If full reverse is applied as is recommended, you could end up in a situation where you're skidding down the runway at an angle and no amount of rudder will straighten you up. This is because the reverse thrust results into two force components, a stopping force aligned along the aircraft direction of travel (runway centerline), and a side force, perpendicular to the runway centerline, which further increases the tendency to skid sideways. As the airspeed decreases, the rudder efficiency decreases and is also made worse by the airflow disruption created by the engine reverse airflow.

To get out if this situation it's quite counterintuitive. The harder the wheel braking force, the lower the tire-cornering force, so if the aircraft tends to continue skidding sideways. Releasing the brakes (by taking over from the autobrake) increases the tire-cornering force and helps to maintain or regain directional control.

Selecting reverse idle cancels the effects of reverse thrust (the side force and rudder airflow disruption) and helps in regaining directional control.

Once directional control has been recovered and the runway centerline has been regained:

• Pedal braking can be applied as required, and

• Reverse thrust can be reselected.

In conclusion then, if it is thought that there is a possibility of aquaplaning, then a positive touchdown should be made using MED autobrake and full reversers. It should also be remembered that if aquaplaning starts to occur, braking coefficient will be the equivalent of an icy runway. If unsure, as mentioned before, the landing performance calculations can be selected to a contaminated state to take aquaplaning into account.

If a crabbing skid is experienced after touchdown and directional control is lost,
cancel reverse and release brakes
Regain directional control and the centerline
Reverse thrust and pedal braking can then be reapplied