During Adverse Conditions, Decelerating to Stop Demands More from Crew and Aircraft Hydroplaning, gusting cross winds and mechanical failures are only a few of the factors that contribute to runway overrun accidents and incidents after landing or rejecting a takeoff. Improvements in tire design, runway construction and aircraft systems reduce risks, but crew training remains the most important tool to stop safely. by Jack L. King Aviation Consultant Decelerating an aircraft to a stop on a runway traction during wet-weather operations and can become significantly more critical in ad- the use of anti-skid braking devices, coupled verse conditions, such as heavy rain in mar- with high-pressure tires, has reduced greatly ginal visibility with gusting cross winds. Add the risk of hydroplaning. Still, accident and the surprise of a malfunction, which requires incident statistics confirm that several major a high-speed rejected takeoff (RTO) or a con- runway overrun accidents each year are caused trolled stop after a touchdown on a slightly by unsuccessful braking involving either a high- flooded runway, and a flight crew is challenged speed landing or an RTO on a wet runway to prevent an off-runway excursion. surface; the factors involved in decelerating to a controlled stop are very similar in these Research findings and technological advances two situations. in recent years have helped alleviate, but not eliminate, the hazards associated with takeoff Overrun Accidents and landing in adverse weather. The U.S. Na- tional Aeronautics and Space Administration Continue to Occur (NASA) and the U.S. Federal Aviation Admin- istration (FAA) conducted specialized tests on A recent Boeing Company study reported that tire spin-up speeds after touchdown rather than during 30 years of jet transport service there spin-down speeds in rollout that confirm that have been 48 runway overrun accidents with hydroplaning occurs at substantially lower more than 400 fatalities resulting from RTOs speeds than noted previously. and another 28 incidents which were poten- tially serious. The study also noted that these Significant advances have been made in opti- overrun accidents continue to occur with no mization of aircraft traction performance on apparent improvement in the rate at which wet runways. Runway grooving and textur- they occur. Thus, while the probability of ex- ing with improved drainage have improved periencing an RTO may appear rather remote FLIGHT SAFETY FOUNDATION • FLIGHT SAFETY DIGEST • MARCH 1993 1 in light of the statistics, individual pilots can Hydroplaning Challenges expect to perform one or more high-speed RTOs Technology during their flying careers; short-haul pilots will be more vulnerable to an RTO because they are exposed to a greater number of Factors involved in hydroplaning have only takeoffs. been known for some three decades, and be- fore then, wet runway accidents were blamed Jet transport accident statistics confirm that on the pilot. It seemed obvious that the pilot one in every 3,000 takeoffs is rejected and ap- simply attempted to land at a much greater proximately one-third of the RTOs are unsuc- speed than that specified in the flight manual. cessful, resulting in serious overrun accidents or incidents. With some 15 million takeoffs The first serious research in hydroplaning was per year (a rate of approximately 34 per minute conducted in the United Kingdom in 1956. each day) this historical rate can be expected That research referred to the uncontrollable to continue producing at least five RTO over- skidding incidents on wet runways as “aqua- run accidents or incidents annually. planing.” Perhaps the most significant research breakthrough was made by NASA when it Studies indicate that about 80 percent of past discovered that variable hydroplaning speed RTO overrun accidents and incidents could was directly associated with tire inflation pres- have been avoided. Making a decision to abort sure rather than the type of tire tread. NASA a takeoff in the rather hectic environment of tests concluded that the higher the tire infla- the flight deck while rapidly accelerating on tion pressure the higher would be the actual the runway is not nearly as reliable as later ground speed before hydroplaning developed. contemplating the go/no-go factors while study- More recent tests revealed that landing on water- ing details of the situation around a confer- covered runways can result in hydroplaning ence table! at some 15 percent lower speeds than had been accepted as standard. The Boeing study revealed that approximately 74 percent of the RTOs were non-engine re- Manufacturers have produced higher pressure lated, although engine anomalies have been tires, coupled with other developments such used traditionally to abort a takeoff in simula- as anti-skid braking devices and thrust reversers tor training. Only 26 percent of the RTOs in- to improve the technology of all-weather high- volved engine anomalies, followed by tire/ speed jet landings. wheel failure (24 percent) and aircraft con- figuration (13 percent). The remainder of the Phenomenon Defined RTOs were attributed to bird strikes, crew co- ordination and other factors. It was noted that transferring control from first officer to cap- Hydroplaning has been defined in technical tain played an important role in determining terms in lengthy reports and reduced to for- the outcome of an RTO. mulas, and is being investigated continually. Simply stated, it is the condition under which After concluding that the great majority of pneumatic tires of aircraft [or highway ve- RTO overrun accidents were preventable, the hicles] roll over fluid-, ice- or slush-covered study noted that 58 percent of the events were pavements and hydrodynamic pressure begins initiated from speeds above V1, the speed from to build between the tire footprint and the which flight is normally continued. At this pavement. As the ground speed increases, the high speed, deceleration and stopping on the pressure grows larger, and at a critical speed remaining runway is questionable even under hydrodynamic lift, resulting from the pres- ideal weather conditions; in one-third of the sure under the tire, will equal the weight of accidents wet or slippery runways were a ma- the vehicle riding on the tire. When this oc- jor factor. curs, hydroplaning speed has been reached. Any increase in speed above this critical value 2 FLIGHT SAFETY FOUNDATION • FLIGHT SAFETY DIGEST • MARCH 1993 will lift the tire completely off the pavement, An example of spin-up hydroplaning on a leaving it supported by the fluid alone; this is flooded runway involved a Gates Learjet 35 total-tire hydroplaning (also called the “wa- and was reported in the June 1992 Flight Safety ter-skiing” speed) at or above which tires slide Digest. The accident occurred during a second over fluid on the runway, making directional night-landing attempt during a thunderstorm. control difficult, nose wheel steering ineffec- The pilot was unable to stop or maintain di- tive and braking or traction nonexistent. rectional control because of hydroplaning and the aircraft went off the runway, resulting in Tire underinflation lowers normal hydroplan- substantial damage to the aircraft; however, ing speed by one knot for each two to three there were no injuries. A water-covered run- pounds below proper pressure. Underinflation way, inadequate hazard notification, rain and can result from rapid descent from much colder a cross wind were factors that contributed to altitudes as well as from careless servicing the accident. and insufficient preflight inspection. NASA Distinguishes New Formula Sets Reduced Speed Three Types of Hydroplaning at Which Hydroplaning Occurs NASA has distinguished three types of hydro- The velocity at which hydroplaning occurs is planing: (1) Dynamic hydroplaning, which occurs predictable and has been expressed in two math- when there is standing water on the surface; ematical formulas. The tire-ground contact pres- (2) Viscous hydroplaning, which occurs when sure can be approximated by the tire inflation the runway surface is damp or wet; and, (3) pressure. Using this in hydrodynamic formula reverted-rubber hydroplaning, which occurs leads to formula definitions for two separate when the rubber of a tire becomes sticky and situations. The first formula involves the spin- tacky (reverting to a condition similar to down speed of rotating tires and defines the its original uncured state) caused by steam hydroplaning speed to be equal to nine times generated by friction between the tire foot- the square root of the tire inflation pressure. print and wet runway surface. This formula has been used for many years in 1 computing probable hydroplaning speeds of Dynamic hydroplaning occurs when /10 inch various aircraft, based upon specific tire infla- or more of water on the runway acts to lift the tion pressure, and is still applicable in aborted tire off the runway and the tire is supported takeoff situations. by a film of water. The second formula involves a more realistic Viscous hydroplaning occurs when a very thin 1 situation of landing or touching down on a film of fluid, /1000 inch or so, cannot be pen- wet runway where the water depth is greater etrated by the tire, and the tire rolls on top of than the tire-groove depth. In this situation the film. This can occur at much lower speeds with non-rotating tires, the spin-up speed when than dynamic hydroplaning, but requires a hydroplaning occurs is only 7.7 times the square smooth or smooth-acting surface. root of the tire inflation pressure. Reverted-rubber hydroplaning requires a pro- With the first formula for example, using a longed locked-wheel skid, reverted rubber and tire inflation pressure of 100 pounds, the hy- a wet runway surface. Reverted rubber acts as droplaning speed would be 90 knots; with the a seal between the tire and the runway and second formula, the hydroplaning speed would delays water exit from the footprint area.