Landing on Liquid Ice

Landing on Liquid Ice

The Pilatus PC-24, which is the first business jet designed to take off and land on unpaved runways, undergoes water pond testing. Although turbine aircraft undergo testing under extreme conditions, no aircraft is immune to hydroplaning, especially
if not flown at the proper speed.

No aircraft is immune to hydroplaning. Know your hydroplane speed, plan ahead and use good technique when landing on wet surfaces.

A light jet overran the runway at Chicago Midway Airport in mid-July. Perhaps because the new-model jet was the first of its type delivered to a customer, images of the aftermath received widespread distribution in aviation-oriented media. Preliminary reports state: “Information from video footage and ADS-B data suggest that the aircraft landed on Runway 31C and steered off the left side of the runway just before reaching the Engineered Material Arrestor System (EMAS) pad at the end of the runway.”

Light rain was reported at the time of the overrun, and the pavement in the photo appears quite wet. Witnesses state it had rained heavily before the jet, arriving from Philadelphia, attempted landing on the 5,141- by 150-foot (1567 by 45.7 meter) Runway 31C. The FAA’s Airplane Flying Handbook tells us:

Dynamic hydroplaning occurs when there is a film of water on the runway that is at least one-tenth inch deep. As the speed of the airplane and the depth of the water increases, the water layer builds up an increasing resistance to displacement, resulting in the formation of a wedge of water beneath the tire. At some speed, termed the hydroplaning speed (VP), the water pressure equals the weight of the airplane and the tire is lifted off the runway surface. In this condition, the tires no longer contribute to directional control and braking action is nil.

Dynamic hydroplaning is related to tire inflation pressure. Data obtained during hydroplaning tests have shown the minimum dynamic hydroplaning speed (VP) of a tire to be 8.6 times the square root of the tire pressure in pounds per square inch (PSI). For an airplane with a main tire pressure of 24 pounds, the calculated hydroplaning speed would be approximately 42 knots.

It is important to note that the calculated speed referred to above is for the start of dynamic hydroplaning. Once hydroplaning has started, it may persist to a significantly slower speed depending on the type being experienced.

Reverted rubber (steam) hydroplaning occurs during heavy braking that results in a prolonged locked-wheel skid. Only a thin film of water on the runway is required to facilitate this type of hydroplaning. The tire skidding generates enough heat to cause the rubber in contact with the runway to revert to its original uncured state. The reverted rubber acts as a seal between the tire and the runway, and delays water exit from the tire footprint area. The water heats and is converted to steam which supports the tire off the runway.

Reverted rubber hydroplaning frequently follows an encounter with dynamic hydroplaning, during which time the pilot may have the brakes locked in an attempt to slow the airplane. Eventually the airplane slows enough to where the tires make contact with the runway surface and the airplane begins to skid.

The remedy for this type of hydroplane is to release the brakes and allow the wheels to spin up, then apply moderate braking. Reverted rubber hydroplaning is insidious in that the pilot may not know when it begins, and it can persist to very slow ground speeds (20 knots or less).

Viscous hydroplaning is due to the viscous properties of water. A thin film of fluid no more than one-thousandth of an inch in depth is all that is needed. The tire cannot penetrate the fluid and the tire rolls on top of the film. This can occur at a much lower speed than dynamic hydroplane, but requires a smooth or smooth acting surface such as asphalt or a touchdown area coated with the accumulated rubber of past landings. Such a surface can have the same friction coefficient as wet ice.

When confronted with the possibility of hydroplaning, it is best to land on a grooved runway (if available). Touchdown speed should be as slow as possible consistent with safety. After the nosewheel is lowered to the runway, apply moderate braking. If deceleration is not detected and hydroplaning is suspected, the nose should be raised and aerodynamic drag utilized to decelerate to a point where the brakes do become effective.

Apply brakes firmly until reaching a point just short of a skid. At the first sign of a skid, release brake pressure and allow the wheels to spin up. Maintain directional control as possible with the rudder.

In a crosswind, if hydroplaning should occur, the crosswind will cause the airplane to simultaneously weathervane into the wind as well as slide downwind.

Under Pressure

NASA has tested the hydroplaning phenomenon extensively. It publishes a table that correlates tire pressure to the speed at which dynamic hydroplaning will occur. Note the main wheel tire pressure for the airplane you’re flying, and compare that pressure to the actual touchdown speed, which should be very close to the stalling speed as adjusted for airplane weight. You may find that your normal landing puts you close to a hydroplaning speed.

There is extremely little margin between the full-stall, essentially short-field landing touchdown speed and the NASA hydroplaning speed. You need to be very careful and land much slower than many pilots routinely land to avoid hydroplaning on a wet runway in an airplane like that!

Note that these are groundspeeds, the speed the airplane’s tires are traveling across the wet surface. This becomes important as we return to the case of the Midway light jet.

Wet Runway

METARs around 1836Z, the time of the mishap, are listed below. I’ve highlighted some important information.

KMDW 121853Z 18009G17KT 10SM -RA FEW060 BKN110 BKN130 OVC200 23/21 A2991 RMK AO2 LTG DSNT NE SLP120 P0007 T02280211 $ 

KMDW 121753Z 21011KT 10SM -RA FEW040 SCT090 BKN120 OVC200 23/21 A2996 RMK AO2 LTG DSNT ALQDS RAB37 SLP135 P0001 60060 T02280206 10256 20206 51006 $ 

KMDW 121653Z 20011KT 10SM
FEW045 SCT060 BKN110 OVC150 23/21 A2995 RMK AO2 LTG DSNT W AND NW RAE41 TSE09 SLP133 P0001 T02280211

The light jet, landing on Midway’s Runway 31C, touched down with a variable and at times gusty wind. 100 degrees to as much as 130 degrees off the runway heading, a crosswind-to-tail wind landing. The trend established by the NASA Tire Pressure versus Hydroplaning Speed table, suggests this to be a potential contributing factor, and certainly enough to remind us to think about dynamic hydroplaning in any airplane we fly.

Mitigation

One-tenth inch of water on a runway is all that is needed to induce dynamic hydroplaning.

To minimize the hazard of hydroplan-ing when landing on a wet runway:

  • Fly the proper touchdown speed as slowly as possible to just above the
    stall speed at the moment of touchdown to preserve a hydroplaning speed margin.
  • Land aligned with the runway centerline with zero sideslip using appropriate crosswind control inputs. This is a basic requirement for
    passing the Private, Recreational and even Light Sport Pilot check ride, so
    it should be your routine for all landings, and then all your landings become good practice for landing on a wet runway.
  • Touch down as close to the approach end of the runway as possible to maximize available landing distance. We normally aim for the touchdown zone markers, which are usually 1,000 feet from the runway threshold. But that reduces available stopping distance by 1,000 feet. In the case of Midway’s 31C that make it effectively a 4,141-foot runway, about 20 percent less stopping distance when hydroplaning is a risk.
  • Plan a “firm” but smooth arrival, to put the tires solidly against the pavement. Don’t try to “grease it on” if the runway is wet.
  • Hold the elevator after touchdown to maximize aerodynamic braking. But when the nose does come down, don’t push the wheel down and cause the airplane to wheelbarrow, or induce a pilot-induced oscillation.
  • Avoid applying brakes at or above the NASA-critical speed for your airplane. Land at a speed and with remaining runway distance that permits coming to a stop with little or no braking. Once below hydroplaning speed for your airplane, brake firmly without causing the tires to skid. Treat a wet runway like you’d treat one with a film of ice.
  • Fly the proper touchdown speed and aligned with the runway centerline with zero sideslip using appropriate crosswind control inputs. Plan a “firm” but smooth arrival, to put the tires solidly against the pavement.
  • Execute an immediate go-around if you detect hydroplaning upon touchdown, unless you have a runway much longer than your computed landing distance with a very healthy margin.
  • Divert to a more suitable airport if a wet runway is combined with a significant crosswind or component. You might hydroplane off the side or end of the runway.
  • Some pilots advocate retracting flaps to put more weight on the wheels, increasing braking and directional control. Attempting to retract flaps during the landing roll is a common cause of inadvertent landing gear retraction in retractable gear airplanes. I recommend against this practice in retractable gear airplanes.

Hydroplaning is one of those things we read about but we really can’t practice unless we’re doing it for real. Any time the runway is wet, think about the possibility of dynamic hydroplaning and adjust your technique to avoid the threat. 

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