They were navigating with what today would be considered fairly primitive Low/ Medium Frequency "Radio Range" equipment, plus the usual maps, cockpit instruments and stopwatches. It is reasonable to surmise that they had already established how much sideways drift was taking place and were compensating for this. By using dead reckoning (position calculation using speed, time, heading, drift angle and track) they also had a fairly good idea of their position. With hindsight we could say that their fatal mistake lay in not being able to confirm exactly where they were! We suspect they would start the climb through the clouds towards the altitude they would need to cross the Alps and rapidly find themselves in a new set of conditions. As they got closer to the Alps in near-zero visibility, the westerly wind began to push them eastwards into worsening icing conditions. Temperature drops by about 2 degrees C per thousand feet of altitude and eventually humidity begins to freeze onto the sub-zero metal skin of the aircraft. Evidence exists that this can also occur almost instantaneously under certain conditions, causing tremendous drag and a significant decrease in speed. It is possible that a stronger wind at altitude increased the plane’s drift eastwards, and the wind compensation angle (the angle the plane points into the wind to counteract the effect of drift) was no longer sufficient to keep them on their intended track but, with no references other than the audio signal from Pisa’s radio range, possibly deflected by the mountains, this was most likely not very evident to the crew. So far as we are aware, the (few) ground stations along the route were operative, although the frequency band of the Radio Range system in use at the time was subject to disturbance by static discharges and deflection by ground profile.

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