The onset of the European winter signals a trying and testing time for aviation. Longer periods of darkness and cold, poor visibility, rain, ice and snow all mean potential trouble for airlines. Recent accidents underline the need for flightcrews to be trained properly in the potential hazards of snow and ice on the performance of the aircraft they are flying. FAA and JAA rules are very clear that no one may take off with frost, snow or ice adhering to any propeller or powerplant or with snow or ice on the wings or control surfaces.

This winter may see a new partnership between airports and aircraft operators to ensure continuing safe operations. After a number of accidents and near misses directly attributed to cold weather conditions and deicing procedures, the UK CAA was sufficiently concerned to issue in 2001 a flight operations communication dedicated to winter operations. It identified a number of safety-related issues requiring operators to establish procedures for deicing and anti-icing to ensure that aircraft are free from ice and snow. Certain manuals and procedures for de-/anti-icing and winter ground operations were found inadequate, as was the training of flight, ground handling and engineering staff. Flight safety programs failed to highlight winter issues, quality systems needed to be improved, ground handling contracts did not contain sufficient operator or aircraft-specific information, and the transfer of responsibility for snow and ice precautions between ground staff and flightcrew needed to be clarified, the communication said.

In 2003, CAA issued a notice to airport operators indicating that it is their responsibility to ensure that all parts of the movement area in use, including taxiways, have adequate surface friction levels to be used safely by aircraft. Operational priorities for the treatment and clearance of snow were reviewed. Across Europe, the major hubs are well equipped for and experienced in dealing with winter conditions. However, airline crews from warmer climates may be less familiar with winter conditions and procedures than they should be. Airports in more temperate climates may have difficulty coping with unexpected snowfalls and it is in these conditions that operators should be especially vigilant. There are geographical, climate and terrain differences in the intensity and severity of winter operations, but even warm climates can experience ice on wings during turnarounds and the importance of inspection and possible deicing is just as evident in these regions.

Zurich and Munich are good examples of major European airports well experienced and well equipped to deal with the bleakest winter conditions.

Zurich, with 17.5 million passengers per year, counts the winter readiness period from Nov. 1 to March 31 during which it statistically measures 24 days of snow with an average depth of 1.5-2 cm. The maximum snowfall in 24 hr. was 20 cm. in 2004 and there are an average of 27 days involving deicing activities. The winter organization has 112 employees56 in each of two shiftsplus a further 70 subcontract workers per shift. Snow clearance is carried out first on the three runways and taxiways, then the apron areas, followed by service roads, maintenance areas and lastly the general aviation area.

The airport employs some 100 snow-clearing vehicles including Boschung plows, Schorling airblast sweepers and Rolba and Bucher rotary snow blowers. There are six Kupper-Weisser deicer vehicles. Black top runway condition is achieved in 45 min. after a moderate snowfall. The airport consumes around 820,000 liters annually of Depatinol pavement deicer fluid, and tests are being carried out with acetates and formiates. The stands at the aircraft docking area are heated.

As an indication of the importance of environmental considerations, Zurich spent Sfr2.9 million ($2.5 million) to treat wastewater from deicing operations last year. A new irrigation system with 700 sprinkler heads sprays the treated water over a 20-hectare area of greenery to reduce carbon concentrations. However, significant quantities of deicing agents still are being released in untreated form, a problem that is being addressed in a new drainage plan. Munich Flughafen, with about 28 million annual passengers, also is well equipped to clear two parallel runways and a total surface of 4 million sq. m. Some 102 employees and 288 subcontractors stand ready to tackle winter. Mechanical equipment includes 40 D-GPS-equipped sprayers, sweeper blowers, deicers and friction testers plus 133 various types of winter support vehicles. D-GPS allows documentation of the spread pattern of the deicer vehicles, the results of friction testing and the snow clearance process, particularly avoidance of runway incursions. Munich, too, is highly environmentally conscious, using mats alongside taxiways and runways to prevent unpurified melted snow and ice from seeping into the ground. The mats direct waste to a gravel purification zone where ground bacteria degrade the glycol.

Last year the deicing/anti-icing group of the Assn. of European Airlines published an extremely comprehensive 130-page booklet entitled Training Recommendations and Background Information for Deicing/Anti-icing of Aircraft on the Ground. The booklet, which assembles a massive amount of experience on the subject, is only available on the Internet at www.aea.be.

WORST-CASE ICING

After an informative review of the conditions and areas for ice buildup, the publication recounts a worst-case scenario of ice on the wing breaking off during takeoff due to flexing of the wing and being ingested by aft-fuselage-mounted engines, leading to surge, vibration and complete thrust loss. However, this does not mean that wing-mounted engines are exempt from ice problems.

The need for deicing and anti-icing usually is determined well in advance by trained and qualified ground or flight crews. Operations have been performed in a variety of ways in the past, including the use of paste on the wings, wing covers, ropes, brushes and a variety of fluids. Forced-air and infrared technology are new elements. The AEA experts point out that there is no single correct way of performing a deicing operation as it must be tailored for each airport and airline along with local conditions. Anti-icing has its own requirements of fluids to be used. Mechanical means cannot be considered as an anti-icing procedure because the surfaces must be protected from refreezing.

Contamination should be removed from underneath wings and tail surfaces, but frost may be allowed in some areas, usually around fuel tanks, depending upon manufacturer requirements and airline procedures. Frost does not change the basic aerodynamic shape of the wing but the roughness of its surface spoils the smooth flow of air, causing a slowing of the airflow and early flow separation and leading to lift loss. The horizontal stabilizer always should be clean on both surfaces. The AEA experts underline that deicing/anti-icing fluids can be misunderstood to be a contaminant on the wing similar to slush or sleet. Fluids are tested to perform so that they shear off the wing during takeoff, leaving only a marginal film that does not produce a noticeable aerodynamic effect. Some fluids are thicker than others and a less viscous fluid drains off more easily. The wing form leaves a different amount of fluid in different areas.

So-called holdover times begin when the application of anti-icing fluid commences and expire when the fluid loses its effectiveness. Holdover times, published on the AEA website, have been determined from standard SAE tests. Each fluid type has a particular holdover time that is dependent upon weather factors. The fluid can withstand a certain amount of water or snow or slush but then becomes so diluted that it no longer provides protection against freezing or fulfills aerodynamic criteria.

AEA stresses that winter operations in harsh climates are bound to affect the punctuality of any airline. There is no shortcut to a safe deicing/anti-icing procedure. Flights often are restricted to a certain takeoff time, causing excessive pressure to complete the deicing process, but under no circumstances should this cause a diversion from established procedures. Airlines that do not operate on a regular basis to regions with harsh conditions might not be as aware of the importance of having good deicing/anti-icing procedures.

Flight Safety Foundation President and CEO Stuart Matthews sums up the winter situation: Accidents involving contaminated aircraft surfaces are easily avoidable. This is a human-factors issue, not an aircraft-design issue. If the aircraft is not clean, dont go.