Aviation Safety Letter 1/2003

Just a Bit of Slush.

by William Ives, Technical Inspector, Aerodrome Safety, Transport Canada

Just a bit of slush can seriously affect aircraft control
Just a bit of slush can seriously affect aircraft control.

The most important obligation that all pilots have is for the safety of their passengers during all phases of flight. Like clockwork, we again find ourselves having to deal with periods of rapidly changing winter weather and runway conditions that most people within the Canadian aviation industry have come to dread.

Weather and its direct impact on aircraft movement areas is one of the most significant factors in aviation safety. Historically, weather and weather-related phenomena have made up a major part of the significant percentage of aviation accidents. If you were to conduct a review of all fatal aviation accidents in North America over the past ten years, you would discover that approximately one-third, or 33%, of such accidents are directly attributable to the prevailing weather conditions at or near the airport. Mother Nature is an extremely dynamic force and still to date is very unpredictable. Winter hazards cause everyone in the aviation community to experience significant safety concerns relating to low ceilings, airframe-icing conditions, poor visibility, and runway contamination by ice, snow, water and in particular slush.

Flying in Canada during the winter months requires that all airport operators, air traffic control units, airline dispatch agencies and pilots in particular, pay extra attention not only to the most current weather conditions at departure, destination, and alternate airports, but to the most current Airport Movement Surface Condition Reports (AMSCRs) for each. Principally, under changing severe weather conditions it is of the utmost importance for pilots to receive the most accurate, complete and current runway surface condition (RSC) and Canadian Runway Friction Index (CRFI) reports from the appropriate air traffic service units, and/or NOTAMs, (in some cases, from information provided by airline dispatchers).

It is well known that snow, ice and specifically slush on aircraft movement surfaces can degrade the coefficient of friction and reduce aircraft braking and directional control. With this in mind, a large part of any airport's annual operational and maintenance budget is spent directly in dealing with the seasonal impact of winter weather conditions on airport manoeuvring areas.

Of all runway contaminates, the effects of slush has been, and continues to be, greatly underestimated by airport operators, air traffic control units, airlines and pilots as witnessed every winter by the number of runway excursions being reported. Slush means partly melted snow or ice, with a high water content, from which water can readily flow. It usually occurs when the outside temperature hovers around the freezing mark. Departing and landing on slush-covered runways continues to be a considerable test of pilot knowledge, ability and skills. Snow and ice control operations greatly improve the friction levels on such contaminated runways and once complete, friction testing to obtain CRFI values can be successfully conducted. Timeliness of RSC reports, along with associated CRFI, is but a "snap-shot of a single moment" thus subject to rapid deterioration as time goes on. It must be fully understood by all parties in the pilot's decision-making tree (go/no-go decision-making) that the actual effective time of any RSC or CRFI report should and must be of paramount concern.

To date, Transport Canada, in partnership with the National Research Council Canada (NRC), the Federal Aviation Administration (FAA), the National Aeronautics and Space Administration (NASA) along with participation from the European aviation community as well as manufacturers of both aircraft and friction testing equipment, has undertaken an extensive research program. This multi-year effort is known as the Joint Winter Runway Friction Measurement Program (JWRFMP). To date, (1996-2001) data on over 400 test runs with aircraft and more than 15 000 runs with 44 ground friction measuring vehicles have been collected on various winter contaminated surfaces.

Guidance information, respecting the CRFI and the Recommended Landing Distance Tables, currently published in the Canada Flight Supplement (CFS) and the A.I.P. Canada has been developed for pilot use and is compiled for certain winter contaminants based on actual field test results from instrumented test aircraft and the corresponding CRFI data.

At this time of the year, all pilots, airport operators, airlines, and air traffic service units are strongly urged to refresh their knowledge of winter maintenance operations and reporting procedures in a concerted effort to reassess their own decision-making awareness.  This can be accomplished by paying particular attention to the information published in the A.I.P Canada, AIR Section, Part 1.6, dealing with CRFI and AMSCR prior to encountering poor winter weather and runway conditions.

Canadian airports have made vast improvements over the years, from providing state-of-the art standards in equipment to new techniques for dealing with winter runway contaminates such as snow, ice and slush. Airport operators are also keenly aware of their responsibilities for the provision of effective and safe facilities under current Canadian Aviation Regulations (CARs), with attention to the timeliness, completeness and accuracy of information gathered and reported via AFTN/ADIS and NOTAM. Current and accurate RSC reports can be of significant benefit to pilots in their decision-making (go/no go) process only if they allow the time required for maintenance crews to properly deal with any contaminates and report remaining conditions in a satisfactory manner prior to any aircraft movement.

Finally, the responsibility for the decision to take off or land, based on information supplied from various parties and knowledge of the aircraft, ultimately rests with the pilots. Obviously, these decisions can be critical and pilot requirements for effective and consistent evaluation of runway conditions, along with a reliable means for relating those conditions to the aircraft's capabilities, cannot be overstressed. Inconsistent, or untimely reporting of runway conditions, such as the presence of slush on the active runway, can be a contributing factor to aircraft ground handling incidents. In spite of advances in technology and operational procedures, safe winter operations remain a challenge for all stakeholders in the aviation industry, especially for all concerned who must coordinate their efforts under rapidly changing weather conditions.

Be prepared, brush up on your decision-making process and take appropriate actions to attain the highest degree of aviation safety that can be achieved this and every winter season.

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