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.

How Ice Affects Drag, Lift, and Wing Stall

Drag — the resistance to an aircraft's forward motion through the air — is greatly increased by ice contamination on the aircraft wing. There are two types of drag: parasite drag and induced drag. Induced drag is produced as a byproduct of lift and increases as the angle of attack increases. Aircraft with ice-contaminated wings must fly at a higher angle of attack than clean-wing aircraft to generate the same lift at a given airspeed, and therefore experience greater induced drag. Moreover, the dirty surface created by ice causes airflow to separate earlier from the upper wing surface, further elevating induced drag at any given angle of attack. The increase in induced drag due to ice contamination is proportionally greater than the increase in parasite drag. On a contaminated swept wing, the normal stall progression is also altered — an effect that is directly influenced by the surface roughness of the ice.

With a contaminated wing, the normal nose-down pitching moment that accompanies a stall and aids in stall recovery is reduced. The degraded pitching moment quality produces varying responses with changes in angle of attack from an out-of-trim condition, which can result in abnormal reactions to control column inputs. The leading edge portion of the wing is especially sensitive to ice contamination. The farther forward the contamination extends from the leading edge, the more pronounced its effects. At temperatures just below the freezing point, ice accumulates slowly yet causes severe degradation of aircraft handling. The major cause of takeoff accidents in jet transport aircraft is widely attributed to the damaging effects of ice accumulation and associated surface roughness on wing lift and drag.

Accident Case Studies Caused by Airframe Icing

Several documented aircraft accidents illustrate the deadly consequences of airframe icing. The crash of icing-related aviation accidents such as these share a consistent pattern: failure to de-ice before takeoff, the presence of freezing precipitation, and the absence of leading-edge or de-icing devices.

On 27 December 1968, Ozark Airlines Flight 982, a Douglas DC-9-15, crashed on takeoff from Sioux City Airport, Sioux City, Iowa. The cause was attributed to a stall near the upper limits of ground effect and subsequent loss of control, resulting from aerodynamic degradation and increased weight due to airfoil icing. The aircraft had not been de-iced prior to takeoff.

On 27 November 1978, Trans World Airways Flight 505, a Douglas DC-9-10, crashed on takeoff from Newark International Airport, Newark, New Jersey. At approximately 65 feet above ground level and a speed of 154 knots, the aircraft lost control shortly after liftoff. Again, failure to de-ice before takeoff and airframe icing were identified as the causes.

On 5 February 1985, an Airborne Express Douglas DC-9-15 crashed on takeoff from Philadelphia International Airport, Philadelphia, Pennsylvania. Here, too, failure to de-ice and airfoil icing were determined to be the cause. Common factors across all three accidents include: (1) the aircraft stalled at a below-normal angle of attack shortly after takeoff; (2) freezing rain and/or snow was present; (3) the aircraft had not been de-iced before departure; and (4) none of the aircraft were equipped with leading-edge or de-icing devices.

On 27 December 1991, a Scandinavian Airlines System (SAS) McDonnell Douglas MD-81, registered OY-KHO, was departing Stockholm (ESSA) for Copenhagen (EKCH) when both engines failed immediately after takeoff, forcing a landing in a field. The impact broke the aircraft into three sections. It is well established that when atmospheric humidity is high or rain is present and the wings are chilled, ice can form on the upper wing surfaces, and that such ice tends to break off at liftoff due to the swift flexing movement of the wings. During the inbound flight from Zurich, the fuel had become very cold. At landing, each wing tank contained 2,550 kg of fuel — approximately 60% of tank volume — sufficient to chill the upper wing surfaces. Weather conditions were such that ice formation was possible. A flight technician noticed ice on the wings during a nighttime inspection. Passengers observed that de-icing indicator tufts were not moving during the de-icing treatment, and on takeoff, ice was seen peeling from the wings. Section 2.3.2 of the accident report showed that engine damage began with "soft" objects being ingested. Taking all of these findings into account, the Board of Accident Investigation concluded that ice peeled off the wings on takeoff and was ingested by the engines, causing the damage. It had been known for years that this engine type was susceptible to ice ingestion. In 1985, a DC-9-51 had suffered similar engine damage. This risk is even greater on the MD-80 series due to its wing tank configuration and the larger air intake area of its engines (Air Traffic Accident on 27 December 1991 at Gottrora).

Japan Airlines Flight 8054, a Douglas DC-8-62-F, crashed on takeoff from Anchorage International Airport, Anchorage, Alaska. The aircraft stalled at approximately 60 feet above ground level at or shortly after reaching V2. Airframe icing was identified as the cause, and once again the aircraft had not been de-iced before departure. As the aircraft approached, icing conditions were such that ice had accumulated on the wings.

The United States introduced regulations prohibiting takeoff when frost, snow, or ice is adhering to the wings, propellers, or control surfaces of an aircraft. These regulations remain in force today under Federal Aviation Regulations (FAR) 121.629, 135.227, and 91.209. Known as the "clean aircraft concept," these rules are based on the established fact that any type of ice formation inevitably alters an aircraft's flight characteristics and degrades its performance.

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.