Avoid a wild ride on a rollout
As my teenage son might say, hydroplaning sounds kind of cool, like the latest water-sport craze to hit Waikiki, or a new kind of dance. But if you've ever experienced it in an airplane, you know the truth: It's decidedly uncool. In fact, it's a great way to transition from "in control" to "out of control" during a landing rollout. So understanding some of the dynamics taking place when your airplane (or any wheeled vehicle you happen to be piloting) is hydroplaning is a good place to start if you want to avoid it in the first place.
Speaking of dynamics, the people who name these kinds of things decided that there are three distinct varieties of hydroplaning: dynamic, viscous, and reverted-rubber. Dynamic hydroplaning occurs when the tires of your aircraft encounter standing water that is deep enough to cause the tires to break contact with the runway surface. Essentially the tires can't displace the water fast enough, causing a wedge of water to build up that the tires then ride upon like a surfboard. In a worst-case scenario the tires spin down completely. When this happens you can stand on the brakes all day long (or until you run out of runway, whichever comes first) and they won't help you even a little bit.
Viscous hydroplaning is deserving of its own name because it can take place at much lower speeds and doesn't need standing water to occur. All it requires is a damp smooth-surface runway or taxiway that happens to be coated with a thin film (as little as one-one thousandth of an inch deep) of oil, old rubber, or other contaminants. The tire surface can't penetrate this thin film and make contact with the runway. If you happen to apply the aircraft's brakes in these conditions — the rubber-coated ends of some runways will do nicely — you could find yourself with little or no braking ability.
Reverted-rubber hydroplaning requires several key ingredients. First, the runway must be wet. Second, you need plenty of heat to turn the water into superheated steam — tire friction from locked brakes will do the trick. The steam causes the tire rubber to revert to its uncured soft state, effectively creating a seal between the runway and the tire. The high-pressure steam gets trapped by the seal, and the aircraft slides along on top of this cushion of steam like a hockey puck across an ice rink. As with the other types of hydroplaning, braking capability can drop to practically nothing. Once reverted-rubber hydroplaning starts, it can persist down to very slow speeds. Telltale white scrub marks left on the runway are an indication that reverted-rubber hydroplaning has taken place.
Hydroplaning first attracted the serious interest of researchers nearly half a century ago when the phenomenon was formally investigated in the United Kingdom. The British (who seem to have their own word for everything) first referred to it as aqua-planing. Within a few years researchers in the United States got involved, including some who eventually carried on this work at NASA. That agency is now recognized as the world's leading authority on the subject.
It was NASA scientists who discovered that the speed at which dynamic hydroplaning occurs is directly related to tire pressure. In general, tires can hydroplane at any speed that is roughly equal to or greater than nine times the square root of the tire pressure. A light aircraft with tires inflated to 30 pounds per square inch could thus hydroplane at 49 knots or above. A jet with 175-psi tires could hydroplane at 119 knots or higher. It's worth noting that for every two to three pounds of tire underinflation, your aircraft's minimum hydroplaning speed will decrease about 1 knot.
Researchers also figured out that runway design plays a big role in reducing the frequency of hydroplaning incidents and accidents. Nowadays the majority of air-carrier airports in the United States have grooved and crowned runways that help channel water away during a downpour. While many general aviation airports also have grooved runways, they are not nearly as commonplace as at the larger airports.
Tire-tread depth and design greatly affects the likelihood of hydroplaning too. Research has shown that as little as one-tenth of an inch of standing water is all that's needed for dynamic
hydroplaning to occur. The deeper a tire's tread, though, the more easily it will whisk water away and the less likely it is to hydroplane. For a tire with good tread depth, higher groundspeeds and deeper water depth are required to hydroplane, compared with one worn smooth. Once 80 percent or more of a tire's tread depth has worn away, you can expect its propensity to hydroplane to increase markedly.
Crosswinds can make a hydroplaning situation even more challenging for a pilot. In fact, hydroplaning incident and accident reports show that far more aircraft have gone off the sides of runways than off the far end. On a dry runway, the coefficient of friction between the aircraft tires and the runway — sometimes referred to as cornering force — is usually more than adequate to allow a pilot to control the aircraft in a crosswind. In a hydroplaning situation with little or no cornering force available, all bets are off. There isn't much to keep the airplane "stuck" to the runway. And flight control inputs become less effective as the aircraft slows down. To make matters worse, crosswind forces act in proportion to the square of the crosswind velocity. This means a 15-knot crosswind produces nine times as much side force on an airplane as does a 5-knot crosswind. So conditions conducive to hydroplaning, plus a strong crosswind, ought to set off warning alarms in your mind.
If you happen to fly a jet, you're somewhat better protected against hydroplaning. Jet braking systems include antiskid control, which greatly minimizes the possibility of reverted-rubber hydroplaning (usually caused by locked brakes). High tire pressures in the 150-to-200-psi range raise the minimum hydroplaning speed accordingly (although correspondingly high landing speeds can negate this factor). And many have engine thrust reversers that help slow them below dynamic hydroplaning speed more quickly. Finally, their ground spoilers help to rapidly place the aircraft's full weight on the main wheels after touchdown, which improves braking effectiveness.
But these features alone don't guarantee immunity from hydroplaning, as the crew of a Mitsubishi Diamond Jet discovered in the following incident at Sugar Land Regional Airport in Houston. "Upon touchdown on Runway 35 at SGR the pilot experienced nil braking action. The aircraft veered to the right on rollout. At this time both pilots attempted to regain directional control by use of rudder and brakes. The aircraft exited the north end of the runway and came to rest in a ditch. The aircraft sustained minor damage. Pilot reported this incident was caused by tire hydroplaning and loss of brake effectiveness due to wet runway conditions."
The crew of an Israel Aircraft Industries Westwind 1124A also learned the hard way how quickly things can turn sour once an airplane starts hydroplaning:
"Weather was marginal VFR and heavy rain showers for this flight. On landing, the aircraft began hydroplaning as pilot initiated reverse thrust. The left thrust reverser deployed, but the right side did not. The aircraft began a slow skid left of the centerline of Runway 16. The pilot stowed the left reverser as the airplane departed the left side of the runway, and he regained control with nosewheel steering. The aircraft continued off the left side of the runway, striking a taxiway identifier sign, then crossing the Runway 34 access taxiway, striking a runway threshold light, and coming to a stop."
How can you reduce the chance of a hydroplaning encounter of your own? Having an idea of the hydroplaning speed range of your aircraft is a good place to start. In most cases you won't be able to land at a speed below hydroplaning speed, so minimizing the time you'll spend at or above it is your goal. Plan to cross the fence no faster than 1.3 V S for the highest available flap setting, plus any wind additives required. Know as much as you can in advance, such as the condition of the tires, whether the runway is grooved (this can be found on AOPA's Airport Directory Online), the amount of crosswind you're facing, and just how much standing water might be present on the runway. Understanding what you're up against might prompt you to make a precautionary go-around, or even consider another runway or airport altogether for landing.
Now isn't the time to try for a greaser either. A moderately firm landing (without a bounce) is best so as to more quickly get the aircraft's full weight on the wheels. If your aircraft has antiskid braking, take full advantage of it by steadily applying (and not pumping) the brakes until stopping is assured. And if you have other helpful features available to you like reverse thrust and ground spoilers, use them early in the landing roll when they are most effective.
With a little planning and good technique, you can minimize the chance you'll find yourself along for the ride on your next landing rollout.
Vincent Czaplyski holds ATP and CFI certificates. He flies as a Boeing 737 captain for a major U.S. airline.
Related Articles
