THE topic of yesterday's Weather Eye, you may recall, was the Air Florida Boeing 737 that crashed because of airframe icing after taking of from Washington on January 13th, 1982. That spot on the calendar is an unfortunate one, in this context on the same day three years earlier, a similar accident had occurred at Biggin Hill airfield in England. The official report described the incident as follows:
"At 10.45 GMT on January 13th, 1979, the Cessna 172 had ice and snow removed from its wings with a broom, because airfield stocks of de-icing fluid were exhausted. Observers noted some ice or snow still adhering to upper wing surfaces while the aircraft was taxiing out to take off. The air temperature was 5C. The climb was very slow, and then sinking was observed the aircraft rolled to the left, then to the right, and then crashed almost vertically. All four occupants were killed."
The event was a further illustration of the potentially lethal effect of ice on aircraft. As we saw yesterday, it affects performance in two ways if it adheres to the aircraft in any quantity the extra weight may make it difficult for the aircraft to maintain height and even more importantly, a build up of ice affects the aerodynamic efficiency of the wings and other surfaces by altering their shape.
Ice is sometimes deposited directly on the airframe in the same way as the white crystalline hoar frost forms on grass on a very cold morning. This may happen on the ground when the temperature is very low, as was probably the case at Biggin Hill, or it may happen during flight if for example, the aircraft is very cold compared with the air around it, having just descended from a higher altitude. But a much more dangerous kind of icing occurs if the airplane comes into contact with supercooled water droplets - drops of water, either of cloud or rain, which are still in the liquid state even though the surrounding temperature is well below zero.
The tiny droplets in a cloud are very pure in composition, and it is the absence of impurities that allows them to remain in the liquid state even at very low sub zero temperatures. The supercooled state, however, is very unstable a supercooled water droplet quickly freezes when it comes into contact with a solid object such as an aircraft - but not before the liquid has streamed back over the airframe. The result is a hard transparent coating of ice with a glassy appearance which greatly reduces the aerodynamic efficiency of the wings and other surfaces.