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Hydroplaning

Introduction:

  • Hydroplaning is an extremely under appreciated and dangerous hazard which exists not only during, but after inclement weather
  • Simply put, hydroplaning is when the airplane's tires skim atop a thin layer of water on a runway, decreasing their contact and therefore effectiveness
  • Occurs where standing water is in excess of 0.1 inches
  • Deep tread or channels may allow up to 2 inches of water before hydroplaning occurs
  • If you experience hydroplaning, GO-AROUND!
  • Speed depends on tire pressure, not weight
    • This is because a heavier airplane creates a larger "footprint" spreading the load
    • Minimum total hydroplaning speed (knots) = 9 x square root of tire inflation pressure (psi)

    Vhydroplane = 9 √ (tire pressure) 

  • There are three forms of hydroplaning:
    • Dynamic
    • Viscous
    • Rubber reversion

Dynamic:

  • Dynamic hydroplaning occurs when standing water on a wet runway is not displaced from under the tires fast enough to allow the tire to make pavement contact over its total footprint area
  • The tire rides on a wedge of water under part of the tire surface
  • It can be partial or total hydroplaning, meaning the tire is no longer in contact with the runway surface area
  • It is possible that as the tire breaks contact with the runway that the center of pressure in the tire footprint area could move forward
  • At this point, total spin-down could occur and the wheel stops rotating, which results in total loss of braking action
  • The speed at which this happens is called minimum total hydroplaning speed
  • The formula that is used to compute hydroplaning speed is:

Viscous:

  • Viscous hydroplaning can cause complete loss of braking action at a lower speed if the wet runway is contaminated with a film of oil, dust, grease, rubber or the runway is smooth
  • The contamination combines with the water and creates a more viscous (slippery) mixture
  • It should be noted that viscous hydroplaning can occur with a water depth less than dynamic hydroplaning, and skidding can occur at lower speeds, like taxiing during light rain, applying the brakes and rolling over an oil spill
  • With regards to rubber, consider that rubber is found primarily on the approach and departure end of the runway

Rubber Reversion:

  • Rubber reversion hydroplaning is less known and is caused by the friction-generated heat that produces superheated steam at high pressure in the tire footprint area
  • The high temperature causes the rubber to revert to its uncured state and form a seal around the tire area that traps the high-pressure steam
  • It is theorized that this condition would occur on damp runways or when touchdown occurs on an isolated damp spot of a dry runway, which results in no spin-up of the tires and a reverted rubber skid

Suggestions:

  • Do not be afraid to delay landing
    • Under zero wind conditions, most runways have adequate cross-fall (rounding of the runway surfaces or crown) to provide drainage under quite high rates of precipitation
    • It appears that drainage can be seriously affected in crosswinds above 10 knots; however, a 15- to 20-minute waiting period after a downpour is usually sufficient to drain the water
  • Be knowledgeable of the many variables associated with landing under wet runway conditions:
    • Landing weather forecast
    • Aircraft weight and approach speed
    • Hydroplaning speed
    • Conditions of tires - if the tread depth of the tires on an aircraft is greater than the depth of the water on the runway, then hydroplaning will not occur. Knowledge of the general condition of the tires (why we do pre-flights) should be helpful in a qualitative sense when potential hydroplaning conditions are expected
    • Brake characteristics
    • Wind effects on the aircraft while landing on a wet runway (crabbing)
    • Runway length and slope
    • Glide path angle
  • Do not exceed 1.3 Vs plus wind additives at the runway threshold
  • Establish and maintain a stabilized approach
  • Use maximum flaps to provide minimum approach speeds
  • Be prepared to go around from the threshold
  • Do not perform a long flare
  • Do not allow the aircraft to drift during the flare
  • Touch down firmly and do not allow the aircraft to bounce
  • If a crosswind exists, apply lateral wheel control into the wind
  • Keep the aircraft centerline aligned with the runway centerline
  • Anti-skid braking should be applied steadily to full pedal deflection when automatic ground spoilers deploy and main wheel spin-up occurs. Do not modulate brake pressure
    • The anti-skid system will not operate until the main wheels of the aircraft spin... don't lock your brakes before touchdown
  • Be prepared to deploy ground spoilers manually if automatic deployment does not occur. Spoiler deployment greatly assists wheel spin-up during wet runway operations by materially reducing the wing lift and increasing the weight on the wheels, thus shortening your stopping distance
  • Apply maximum reverse thrust as soon as possible after main gear touchdown; this is when it is most effective
  • Get the nose of the aircraft down quickly
    • Do not attempt to hold the nose off for aerodynamic braking
  • Apply forward column pressure as soon as the nose-wheel is on the runway to increase weight on the nose-wheel for improved steering effectiveness. Do not, however, apply excessive forward column pressure because the down elevator will, to some extent, unload the main wheels and decrease braking effectiveness
  • If the aircraft is in a skid, align the aircraft centerline with the runway centerline if you can. Get off the brakes to maximize cornering capability and bring the aircraft back to runway center
    • If you are in a crab and cannot align aircraft centerline with runway centerline and attempted cornering is not effective, get out of reverse thrust to eliminate reverse thrust component side forces tending to push the aircraft off the side of the runway

Case Studies:

  • NTSB Identification: ERA13CA394: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's loss of directional control during takeoff due to right main landing gear contact with a pool of standing water on the runway which resulted in a runway excursion
  • NTSB Identification: CEN13LA02: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's decision to continue the landing after touching down long and on a wet runway that reduced the airplaneā€™s braking capability, which resulted in an overrun

Conclusion:

  • The flight is not over until the aircraft is chocked and the engine(s) turned off
  • With regard to viscous hydroplaning, consider where you will see that rubber build up
  • Rubber is not only on the approach end, but on the departure end of the runway
    • If you find yourself behind the aircraft scrambling to stop and you slam on the brakes over this rubber, you may do more harm than good!

References:

  • None