ICE is by far the most noire of all the betes to haunt an airline pilot in his daily work. When it adheres to the exterior surfaces of an aircraft, it affects performance in two ways.
In the first place, like water, ice is a very heavy substance; if it builds up on the wings in any quantity the extra weight may make it difficult for the aircraft to maintain its height, particularly if this occurs when fully laden at the time of takeoff.
But the second reason is even more important. The wings and other external surfaces of a modern aircraft have a shape which is carefully chosen to allow them to do their job effectively - to keep the aircraft in the air, and to make it easy to control. If ice forms on these surfaces it changes their shape, and this has an adverse effect - sometimes a disastrous effect on their aerodynamic efficiency.
One is reminded of these matters this morning by the fate of a twin engined Air Florida Boeing 737 which took off from National Airport, Washington DC, 15 years ago today.
There had been snow with sub zero temperatures all day, and almost as soon as it left the ground, the aeroplane began to stall; after only minutes in the air it lost height rapidly sliced through the evening traffic and crashed into a bridge across the frozen Potomac river in downtown Washington.
Ice was the villain of the piece, and, as Alexis de Tocqueville said of the French Revolution: "Never was an event so inevitable, yet so utterly unforeseen." Although measures had been taken earlier to remove the ice from the airframe, a thin layer of hoar frost had formed on the aircraft's wings in the 35 minutes that intervened between treatment and take off; the coating was as thin as a sheet of fine sand paper, but it was enough to cause a tragedy in which 78 people lost their lives.
Modern aircraft, of course, are equipped with very efficient ways of ridding themselves of this hazard. Mechanical devices are sometimes used to dislodge the ice; more usually a liquid with a low freezing point is sprayed on the vulnerable parts of the airframe, which not only clears away any ice already on the aircraft but also provides a protective film to prevent a further build up. But as the incident in Washington has shown, such measures in severe conditions must be timely to be effective. Luckily, they nearly always are.