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Research for this article about hydroplaning was gleaned from many sources. The biggest contributor was Thomas J Yager’s Research Memorandum Letter 85652 from the NASA research center in Langley, Virginia.  The Boeing aircraft company and the Flight Safety Foundation also provided generalized information.  Extracting and simplifying relevant information for the line pilot to help the decision making process when confronted with a potential hydroplaning scenario is the objective for this article.

Most pilots are familiar with the three types of hydroplaning:  Viscous Hydroplaning, Reverted Rubber Hydroplaning, and Dynamic Hydroplaning.

Viscous Hydroplaning can occur on all wet runways and is a technical term used to describe the normal slipperiness of wet runway surfaces.  It does not normally reduce friction to a point that inhibits wheel spin up after touchdown, or prevent the synchronous properties of the anti-skid system from functioning. It is the most commonly encountered source of low braking efficiency on wet runways. It is often mistaken as dynamic hydroplaning.  When a pilot experiences the effects of trying to turn-off a runway at a 90 degree intersection with a little too much forward speed, the resulting slide is the direct effect of viscous hydroplaning coupled with Newton’s first law. Oil and dusty surfaces can also have the same results.

Reverted Rubber Hydroplaning usually occurs when the wheels become locked on a wet or icy runway. The resulting friction causes heat which in turn melts the rubber compound in contact with the runway surface.  The heat generates steam which lifts the tire from the surface often leaving a light grey streak on the runway surface.

Chance encounters with this type of hydroplaning have been significantly reduced by the advances of anti-skid and Boeing designed auto-braking systems.

Dynamic Hydroplaning occurs when there is excess water on the runway and the aircraft ground speed equals or exceeds the escape velocity of the water drainage from the footprint.  At this point, choked water flow causes a buildup of dynamic pressure between the runway surface and the tire. A wedge of water then begins to partially or fully detach the tire from the runway surface. This results in a significant loss of braking efficiency and can lead to partial or full loss of directional control. This maybe further aggravated by cross winds especially at velocities over 10kts.

The British government first recognized the dangers of hydroplaning in the late 1950s. The Royal Air Force wanted to reduce the number of incidents they were experiencing under wet and standing water runway conditions.  A study discovered that grooving and crowning runways improved the braking efficiency and drainage capabilities. As a direct result of this research, the US and many other countries around the world have made grooved and crowned runway construction a standard.

NASA has graded the different types of runway surfaces based on runway   texture (macrotexture), and the depth of grooves and crowning.   Class 1 runway surfaces have the highest macrotexture depth values and Class 5 have the lowest macrotexture depth values. Since the potential for dynamic hydroplaning varies inversely with the runway surface texture, Class 1 surfaces are identified as having the least hydroplaning potential whereas Class 5 surfaces are considered to be the most susceptible.

Class 1:-  Deep grooves, 2% crown, open texture. Porous friction course overlay. The potential for hydroplaning on these surfaces is LOW.

Class 2:-  Shallow grooving, scoring and wire combing some large aggregate and asphalt. The potential for hydroplaning on these surfaces is POOR

Class 3:-  Heavily textured concrete, some mixed-gradation, aggregate asphalt. The potential for hydroplaning on these surfaces is FAIR.

Class 4:-  Lightly textured concrete, most small aggregate asphalt. The potential for hydroplaning on these surfaces is GOOD.

Class 5:-  Very little texture, rubber coated and heavily trafficked. The potential for hydroplaning on these surfaces is HIGH.

Where do these different surfaces fit in to the overall scheme of things? The US, Canada, and most of the countries in Europe, have runways that fit in to the first 2 classes.  Unfortunately, in certain countries in the Caribbean, Mexico and South America the level of construction falls in to the class 4 & 5 area. This means that the exposure to the risk of a hydroplaning encounter is significantly increased.

The one glaring example of this significant increased risk is Mexico City. The runway surface there falls between class 4 and 5. The density altitude increases the TAS well above the hydroplaning threshold speed Vp, see fig 1. The subtle indentations along the runway tend to puddle water especially after a heavy rain shower. This would then easily meet the FAA criteria for contaminated runways.  A runway is considered contaminated when more than 25% of the runway surface area (whether in isolated areas or not) within the required length and width being used is covered by surface water more than 1/8 of an inch or 3mm deep or slush and loose snow equivalent.

The conditions in the US are the best in the world.  Too often one can be lulled in to a false sense of security, especially if one has not operated an aircraft out of the United States for several months.

There are many variables that effect the hydroplaning equation. The following matrix shows the variables and puts them in an easy to understand process flow.

During ground and air operations, pilot techniques and control inputs together with certain aircraft parameters, including aerodynamics, engines, brake system and landing gear configuration  (single axle/tandem axle), interact to determine how much of the available tire pavement traction is utilized for stopping and directional control purposes. The generally accepted industry techniques to help diminish the negative effects of a dynamic hydroplaning encounter are divided between flare, touchdown and rollout.

In the descent approach portion, the aircraft is normally descending at about 700ft per minute approximately 11ft per second. The flare and touchdown arrests this sink rate to approximately 2 to 4 ft per second. Planning a moderately firm touchdown prevents an extended flare and, most importantly, in the hydroplaning scenario, promotes good strut compression, which in turn gives prompt wheel spin up, spoiler and auto brake deployment. It is of paramount importance to land on the center line to take the maximum benefit from the crowned runway surface and with no lateral drift. Both Boeing and Airbus have reduced demonstrated cross wind limits for contaminated runways. This reduction in cross wind limit can be explained by the difference in forces on an aircraft between a direct 5kt cross wind in comparison to a direct 10kt cross wind. These forces are proportional to the square of the velocity. In other words, a 10kt cross wind would quadruple the side force of a 5kt direct cross wind.

During the rollout it is important to initiate reverse thrust as soon as possible after main gear touchdown and increase that thrust up to the limits recommended for that specific aircraft. Reverse thrust is most effective at higher speeds and provides a powerful stopping force that is not dependent upon runway friction.  At higher ground speeds on DRY runways the total stopping force is split, aerodynamic drag/spoilers and reversers providing 55% and brakes 45%.  As the aircraft speed is retarded, the brakes are providing 80% of the total force. However on WET runways with spoilers and reverses deployed, that equation changes to 80% of the high speed stopping force (this % is higher during dynamic hydroplaning events).  As the aircraft slows, the brakes represent 70% of the stopping force.

All transport jets have multiple systems that should be utilized in a coordinated manner to help safely stop the aircraft within the available runway length. This means landing the aircraft within the touchdown zone on target speed and on the center line with zero drift.  In addition to making sure that the auto spoilers and brake system are monitored for correct deployment, followed by prompt activation of the reversers.

It should be noted that tandem axle landing gears, as applied to the B767/757 and the A330, each have their own wheel spin up logic. Pilots should refer to their particular aircraft manual to review these unique differences. It is possible during hydroplaning events that the front pair of wheels in a tandem axle configuration begin to hydroplane before the trailing pair. This is made possible by the front pair pushing enough water out of the way to leave the rear wheels in a partial or zero hydroplaning state.

Fig 1 The threshold speed Vp that a tire begins dynamic hydroplaning can be easily calculated. That equation for a non-rotating tire such as we have at touchdown would be 7.7 times the square root of the tire inflation pressure. A rotating tire such as we have on take-off would be 9 times the square root of the tire inflation pressure. It should be noted that tire pressures vary depending on manufacturers.

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