How Ice and Rain Affect Aircraft Operations and Flight Safety

Introduction: Weather as a Flight Hazard

There is a clear interrelation between safe and satisfactory air travel and weather. Most airplane accidents occur due to adverse weather, which is also one of the leading contributing factors in flight delays. All flight operations are affected by unfavorable weather conditions, which may prevent normal handling of a flight entirely or only partially. The expenses incurred due to delays and route changes caused by adverse weather are very high. Both passengers and the aviation industry bear the brunt of these situations — passengers lose time and face hefty hotel charges, while the industry endures increased fuel consumption, additional servicing costs, equipment changes, and crew reassignments, all of which make flying more expensive than planned. Fogs, thunderstorms, freezing rain, snowstorms, crosswinds, poor visibility, icing, and en-route turbulence are among the weather types that make delays and route changes unavoidable.

According to available reports, the day the Singapore Airlines 747 jetliner crashed, there was heavy rain in the city of Taipei. Pilots are not automatically prevented from flying in such conditions. Each airline company has its own guidelines regarding acceptable weather conditions for takeoff and landing, but the final decision whether to proceed rests with the pilot. Normally, aircraft — especially the 747 — fly even in heavy rain and strong winds. However, as the jetliner accelerated down the runway, the pilot's visibility was likely impaired by the relentless rain. The pilot reportedly mentioned that the aircraft struck an object on the runway during the takeoff roll. A pilot's vision is essential for keeping the nose of the aircraft aimed straight while on the ground, and when heavy rain is striking the windows, it severely limits that visibility. It is also possible that the engine fire was extinguished by heavy rain entering the turbo engine. An engine fire can be extinguished if the rate of rain striking the engine exceeds two inches per hour. A turbo jet engine takes in approximately 200 pounds of air, and if more than 10% of that intake is water, the fire in the combustion chamber may be extinguished.

Aerodynamic Effects of Frost and Ice on Aircraft

All aerofoils — including the propeller — are adversely affected by ice, snow, and frost, which also add to the overall weight of the aircraft. Even a light layer of frost can significantly reduce an aircraft's lift and become hazardous when it adheres to airfoils such as wings, control surfaces, propeller blades, and rotor blades. Frost forms when the surface temperature of the airplane drops below the dewpoint (below 32°F) or below the freezing point. Although frost typically forms in the early morning hours, it usually melts with sunrise as heat and sunlight increase throughout the day. However, a pilot must take extra precautions to clear frost before takeoff if the surface temperature remains below freezing and skies are overcast, preventing any warming from sunlight.

When some pilots claim there is not much frost on the wings, or that a simple visual inspection shows no change in shape, they overlook the fact that the crystal structure of frost creates surface roughness that can substantially disrupt airflow across the airfoil. This affects lift and makes takeoff and landing at low speeds risky (Bernstein, Omeron, McDonough, & Politovich, 1997).

While frost reduces or destroys lift, aircraft icing adds weight in addition to degrading lift. These combined factors can make it impossible for the pilot to maintain the required altitude, or may prevent the aircraft from staying above the minimum safe altitude for sustained flight. When thunderstorms accompany icing conditions, the situation becomes even more dangerous. Every year, a number of accidents are attributable to icing. Structural icing — also known as airframe icing — accounts for 40% of these accidents (Lankford, 2000), while carburetor and induction system icing, or engine instrument icing, accounts for the remaining 60%.

Aircraft may frequently be required to fly through atmospheric conditions with high levels of structural icing. Most aircraft, including even the Aerosonde unmanned aircraft, are not equipped with adequate de-icing systems for flight in such extreme conditions. Even where de-icing equipment is fitted, all aircraft should avoid conditions where ice formation is high or very high — that is, where ice can accumulate at a rate greater than 2 cm per minute (Lankford, 2000).

Incremental ice accumulation causes problems such as reduced lift and airspeed, as well as increased drag. Ice buildup on main rotor and tail rotor blades suddenly increases drag and degrades lift, requiring additional power to drive the blades. Ice on the fuselage also demands more engine power, as it increases overall weight. Glassy ice formation on windscreens reduces or completely blocks visibility, forcing the pilot to rely on side windows or sliding panels. Vibration imbalances occur when ice falls from the blades at uneven intervals. The engine air intake system can become blocked by ice, and chunks or slabs of ice falling from the fuselage can damage rotors or obstruct normal airflow in the intake system. Freezing water can enter areas where it is not expected. Normal operation and control movements become difficult when ice accumulates on flight controls such as bell cranks, rod ends, and pitch horns. Different texts attribute icing to different temperature ranges; it may occur at temperatures of 0°C or below and may vary depending on a number of additional factors.

References

Bernstein, B.C., Omeron, T.A., McDonough, F., & Politovich, M.K. (1997). The relationship between aircraft icing and synoptic-scale weather conditions. Weather & Forecasting, 12, 742–762.

Lankford, T.T. (2000). Aircraft icing: A pilot's guide to supercooled drizzle droplets, icing accident case studies and cold weather techniques. Practical Flying Series. McGraw-Hill Companies, Inc.

Rasmussen, R., Politovich, M., Marwitz, J., Sand, W.R., McGinley, J., Smart, J., Pielke, R., Rutledge, S., Wesley, D., Stossmeister, G., Bernstein, B., Elmore, K., Powell, N., Westwater, E., Stankov, B.B., & Burrows, D. (1992). Winter Icing and Storms Project (WISP). Bulletin of the American Meteorological Society, 73, 951–974.

Sand, W.R., Cooper, W.A., Politovich, M.K., & Veal, D.L. (1984). Icing conditions encountered by a research aircraft. Journal of Climate and Applied Meteorology, 23, 1427–1440.

Symons, L. (1996, December). Weather, 51, 419–425.

Thompson, G., Bruintjes, R.T., Brown, B.G., & Hage, F. (1997). Intercomparison of in-flight icing algorithms — Part I: WISP94 real-time icing prediction and evaluation program. Weather & Forecasting, 12, 878–889.

Author Unknown. (n.d.). Air traffic accident on 27 December 1991 at Gottrora, AB. Report C. 1993:57, Case L-124/91.