Aviation Safety Letter 4-2003
Slush on the Runway and What it Does to Aircraft Performance
By Gerard van Es, Senior Research Engineer, Flight Testing & Safety Department, National Aerospace Laboratory NLR, The Netherlands
Analysis of accidents that occurred in the last 20 years has shown that the risk of overrunning the end of a slush- or water-covered runway is about eight times higher than on a dry runway. The hazardous effect of slush on aircraft field performance was first brought into prominence after an accident involving a BEA Airspeed Ambassador aircraft, in which 23 people were killed, in Munich in 1958. The introduction of tricycle undercarriages and higher operating speeds of modern aircraft in the late 1950's were associated with this new hazard to aircraft operations. In the early 1960's, investigations on the effects of slush were carried out in the United States, the United Kingdom and France. Tests were conducted using catapult-driven test carriages as well as full-scale aircraft. These early tests gave a clear picture of what slush does to an aircraft that takes off or lands. It was found that the acceleration during takeoff was reduced due to an increase in total drag acting on the aircraft. This increase in drag was caused by the tires displacing the slush and the impingement (interference, intrusion) of the spray of slush on the airframe thrown up by the tires. It was shown that the additional drag increased with increasing slush depth. It was also discovered that there was a considerable possibility of loss of engine power, system malfunctions and structural damage due to spray ingestion or impingement. Furthermore, the problem of very low braking friction between the tires and surface was identified in which aquaplaning of the tires plays an important role. The problem of slush is more acute for aircraft with turbine engines than for aircraft with piston engines because of the higher operating speeds and increased susceptibility to ingestion and impingement due to geometrical characteristics of aircraft with turbine engines.
Let us have a look at some typical numbers with respect to the effect of slush on take-off performance. Just 13 mm (0.5 in.) of slush can subject a large jumbo jet to a drag that is equal to approximately 35% of the thrust of all its four engines. This number increases to 65% for 25 mm (1 in.) of slush, making it impossible to take off. In general, for a multi-engine transport aircraft, just 13 mm (0.5 in.) of slush can increase the take-off distance by some 30-70%.
Slush can have an adverse effect on the landing performance. Braking friction can be low because aquaplaning is likely to occur on slush-covered runways. This will increase the landing distance compared to a dry runway. Although it sounds strange, a thicker layer of slush can be better than a thin layer because the drag from the slush helps stop the aircraft. The more slush you have on the runway, the higher the drag on the aircraft. This also applies to rejected takeoffs and can lead to strange performance restrictions when taking off from slush-covered runways. For instance, more slush can give lower take-off weight penalties.
What about regulations for operating on slush-covered runways? In 1992, the Moshansky Commission of Inquiry into the Air Ontario crash at Dryden, made several recommendations regarding operations on contaminated runways. The commission recommended that Transport Canada should require that Aircraft Flight Manuals (AFMs) contain guidance material for operating on wet and contaminated runways and that operators provide adequate training to their crews with respect to the effects of contaminated runways on aircraft performance. At the present time, Canadian Operational Regulations do not provide for a Canadian operator of turbo-jet aircraft to have any information in the manuals for operating on contaminated runways. But on the other hand, effective August 1992, an AFM associated with a new type approval must have performance advisory material that deals with operations on contaminated runways. What is this situation elsewhere in world?
In Europe, any commercial operator whose principal place of business is in a Joint Aviation Authorities (JAA) Member State, must comply with the operational regulations, JAR-OPS 1, which formalize requirements for operational performance information. JAR-OPS 1 requires that an operator account for the effect of contaminated runways on take-off and landing performance. Several non-European countries have adopted JAR-OPS 1. At present the regulations in the United States do not address performance on contaminated runways. The Flight Test Harmonization Working Group will address harmonization of this issue with the JAA in the future. However, this is awaiting harmonization of the associated operating rules by the Airplane Performance Harmonization Working Group. Transport Canada and Canadian operators are some of the members of this last working group.
There are more problems caused by slush than described here. For instance loss of directional control when operating in crosswind and the accumulation of slush in the main landing gear bay areas that could freeze and interfere with the landing gear, just to name a few.
Remember that slush on the runway today is as big a risk to aircraft operations as it was 40 years ago. Fly safely in all upcoming winters!
You can read about some real-life occurrences in which slush was a factor in Transportation Safety Board of Canada (TSB) accident reports with numbers A98O0034, A96A0047 A96A0050, A96C0232 (see http://www.tsb.gc.ca/). Additional reading material can be found in NLR-TP-2001-216: "Safety aspects of aircraft performance on wet and contaminated runways" (see http://www.nlr.nl/public/
2001-216-dcs.php) and NLR-TP-2001-003: "Safety aspects of tailwind operations" (see http://www.nlr.nl/public/en/index.php?url=http://
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