Aircraft Icing

Introduction

Plane crashes occur for a number of reasons. There seems to be a consensus with the general public that flying is dangerous, engines fail and planes crash. That is true some times, although the majority of plane crashes occur largely due to a combination of human error and mechanical failure. In much of aviations accidents mechanical failure has been a contributing factor. It is impossible however to blame plane crashes on one reason since events leading up to an accident are so varied. Reasoning for plane crashes can be placed in a broad number of categories.

Environmental conditions play a vital part in aviation as a whole. Much planning goes into a flight based on the current and forecast weather conditions for safety reasons. Accidents have occurred due to flying in bad weather such as thunderstorms with low level wind sheer, lightning, hale, icing conditions and poor visibility. Poor weather especially icing can be very dangerous to flight but most accidents can be avoided if the right precautions are taken to avoid potential bad weather situations. I will take a closer look at icing conditions on aircraft and give examples of icing related accidents

Body

Icing, or ice buildup on the wings, is a particular problem for aircraft. When ice builds up on wings, it can disrupt airflow, robbing an airplane of lift and can decrease its angle of attack, which keeps it in the air.

Wind tunnel and flight tests have shown that frost, snow, and ice accumulations (on the leading edge or upper surface of the wing) no thicker or rougher than a piece of coarse sandpaper can reduce lift by 30 percent and increase drag up to 40 percent. Larger accretions can reduce lift even more and can increase drag by 80 percent or more. (AOPA, 2002, 2).

Deicing equipment is installed on most transports today, however much is still to be learned about icing. Nature can create conditions, which exceed those test conditions, and even aircraft certified to operate in icing conditions are not guaranteed to be able to cope with all conditions. There have been several accidents due to the build up of ice on the wings. Air Florida's Flight 90 and American Eagle Flight 4184 offer prime examples of icing related crashes.

On January 13, 1982 an Air Florida 737 crashed into the Potomac River in Washington D.C. when the crew forgot to turn on the deicing equipment. In this accident both procedure and weather were the main contributing factors. This accident could have been avoided if proper procedure was followed in deicing.

The NTSB investigation concluded that the combination of the crew's use of thrust reverse on the ground for push back, and their failure to active the engine anti-ice system caused the crash. By failing to activate the engine anti-ice system, large amounts of snow and ice that were sucked into the engines during reverse thrust were allowed to remain there. Ice built up on the compressor inlet pressure probe, which measures engine power. As a result instrument indications in the cockpit showed an Engine Pressure Ratio of 2.04, while the power plants were in reality only producing 1.70 EPR, or about 70% of available power. The combination of the ice covered wings and low power caused an immediate stall on takeoff killing 74 people. (Kilroy, 9).

In Americans Eagle Flight 4184 October 31, 1994, heavy air traffic and poor weather postponed the arrival of this flight at Chicago's O'Hare International Airport, where it was to have landed en route from Indianapolis, Indiana.

The ATR-72, a twin-engine turboprop carrying 68 people, entered a holding pattern 65 miles southeast of O'Hare, which it maintained for over an hour in freezing rain. As the plane circled, a ridge of ice formed on the upper surface of its wings, eventually causing the aircraft's autopilot to suddenly disconnect and the pilots to lose control. The ATR disintegrated on impact with a field below, killing everyone aboard. Following an NTSB investigation, the FAA required that all ATR aircraft be fitted with expanded de-icing equipment. It also issued 18 airworthiness directives for all pilots operating small commuter aircraft, instructing them on how to recognize and respond to dangerous icing conditions. (Krock, 2004, 4).

As a step to resolve icing accident investigations the FAA has issued airworthiness directives for aviation operators such as:

Monitoring ice build-up, and getting out of icing conditions if the buildup is unusually high, on anti-icing equipment sooner rather than later, avoid abrupt and excessive maneuvering that may exacerbate control difficulties, do not extend flaps when holding in icing conditions (which can reduce angle of attack), If flaps are extended, do not retract until the airframe is clear of ice, and report icing conditions to ATC. (Holzapfel, 2000, 9).

Aircraft icing is closely studied in wind tunnels, although it is very difficult for them to simulate different icing conditions. Such icing research is conducted at John H. Glenn Research Center using the Icing Research Tunnel, the largest refrigerated icing tunnel in the world, and the Twin Otter icing research aircraft. New techniques are also under investigation