Ice and altitude

The testimonies of the weather forecasters at Capodichino indicate that conditions were most favorable at or above 15,000 feet altitude. This contrasts with the actual recorded Flight Plan altitudes of around 7,500 feet, but this alone is not highly significant, since pilots would be seeking the best altitude under the prevailing conditions, particularly in an area of thunderstorms. What is a problem is the fact that the clearance altitude for Mont Blanc is around 18,000 feet or higher, and that the 12,000 feet impact altitude on the Aiguille des Glaciers is demonstrably too low to ensure safe passage. So was this altitude their “chosen” one, or were they being forced lower by external influences, for example severe downdraughts or ice buildup on the plane itself? According to the weather forecast, icing could be expected above 7,500 feet. Swiss sources indicate that in the early morning of November 1st 1946 it was raining in Geneva and early morning ground temperatures had been as low as 1.5°C. At altitude, and with the rising air currents driven up and across the Alps by the depression extending right across the Rhone Valley, it would be reasonable to suspect icing conditions even as high as 20,000 feet.

Ice buildup on an aircraft has two main effects. It disturbs the aerodynamic profile, particularly of the wings which provide the lift, and it adds weight. Both of these negatively affect the ability to climb and can even force an aircraft lower. A B-17 is a powerful aircraft. It has four large turbocharged engines normally capable of lifting it to 25,000 feet But icing can cause difficulties for even such a plane. If we take icing into consideration, at least two possible scenarios exist: (a) by the time they reached 12,000 feet, they had accumulated a significant ice buildup on much of the plane’s surface and the extra weight prevented them from going any higher. 12,000 feet would be enough to get them across the western edge of the Maritime Alps in safety, but unfortunately, they were now further east than they believed themselves to be; or (b) they climbed happily through 18,000 feet taking the wind from the West into account, aiming to pass to the West of Mont Blanc, but ran into stronger winds and worsening icing conditions at altitude. They were possibly forced lower by ice or perhaps even became caught in the notorious downdraughts over Mont Blanc. Either way, the unequal struggle ended shortly after 4 am local time when a blinding flash illuminated the night some 500 feet from the top of the Aiguille des Glaciers as the B-17 impacted.

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