The Bottom Line

It is possible that a similar Board of Enquiry today would concentrate on ice buildup being the principal factor in the accident, with a conscious route deviation due to thunderstorms on the original route as the instrument which brought the plane into these extremely hazardous weather conditions. Icing was a known phenomenon, but the speed with which severe icing can occur was probably not appreciated by the majority of pilots at the time. The catastrophic effects of near-instantaneous icing were to remain largely unknown for a number of years beyond this accident.

To quote from the March 18, 1965 issue of the well-known and respected magazine “Flight International”:

“The fact that severe icing can be experienced at very low temperatures is stressed in the latest Ministry of Aviation pink Civil Aviation Information Circular (22/1965). The situation - of regular occurrence in the region of the Alps in winter - can occur when a well-developed warm front, associated with vigorous air circulation, crosses high ground. It seems that a deep layer of saturated air can be lifted so that large water droplets move up through, say, 5,000ft.
The circular uses a seven-year-old incident as a classic illustration of this phenomenon—as encountered by a turbine-powered aircraft en route from Rome to Britain on January 6, 1958. All went well until, when flying in cloud near Mont Blanc at an indicated airspeed of about 200kt at 20,500ft with an air temperature of minus 27°C, there was a sudden loss of power accompanied by heavy ice accretion. Before it had been cleared off the leading edges by the de-icers, it was seen to be very lumpy in formation and projecting forward. After full power had been recovered, it was possible to regain the 600ft lost in the incident and to maintain the original flight level—although at an IAS of only 125kt.
In his report the pilot wrote that "it was not until after some minutes in this condition that we passed into a layer of clear air and the outside air temperature was seen to have increased to minus 16°C. The indicated airspeed then increased very slowly over a further period of 10min until 170kt was reached; normal cruising power was then resumed and the IAS gradually reached the original cruising figure of 200kt and the remainder of the flight was quite normal.
"It was seen," the report continued, "that ice, which had melted on the leading edges, had run back and re-frozen in a lumpy formation on the upper surface of the mainplanes. The ice formation around the windshield could be examined closely and was in the form of numerous rough opaque pillars of rime ice projecting forward some 6in to 8in. Assuming that this existed on other surfaces unprotected by de-icers, it would, of course, greatly augment the drag of the ice on the mainplanes and the tailplane.
"Approaching the Alps we were flying just on top of stratiform cloud with a wind [direction and] velocity at our altitude of 300° and 90kt. About 4min before passing Mont Blanc, we entered cloud of lenticular formation, the top being approximately 22,000ft and, in order to maintain altitude, it became necessary to reduce the IAS from 200kt to 160kt.
"At this time no appreciable ice accretion had occurred and it was obvious that we were in the down-flow side of a standing wave. After about 3min, and just before reaching Mont Blanc, the indicated airspeed of 200kt was regained and this speed was being held at the time of the power loss. No turbulence was experienced and, upon passing into the clear layer after the incident, it became apparent that we had passed through the top of a wave-formation cloud lying over that part of the Alps and projecting through the general layer of cloud at about 20,000ft - the heavy ice accretion occurring as we entered the up-flow side of the standing wave.
"As stated previously, the ice was of rime type and formed so rapidly that the whole mass appeared to be deposited almost instantaneously upon the aircraft, although the ambient air temperature was well below the expected icing range for this type of cloud."
The circular includes a Meteorological Office analysis of the synoptic situation, which was such that "the upper-air situation was favourable for the development of a wave-like flow in the airstream, such conditions being a stable layer of air sandwiched between two layers of lesser stability. When such waves occur, the effect of the underlying terrain in producing or accentuating vertical motion of the air does not simply decrease with height in the atmosphere. Vertical motion of the air can be at a maximum at some level in the middle of the troposphere and the regions of marked uplift are not necessarily located directly over the mountain ranges. "The temperature at Payerne (46°49'N 06°57'E) at 450mbs (ICAO 20,800ft approximately) at midday was minus 21°C, but temperature was somewhat lower to the south of the Alps. The air mass was saturated throughout a deep layer and it seems highly likely that orographic effects, general uplift of the air mass over the Alps together with a wave-like flow which conceivably reached its maximum amplitude at the time of the incident, could have brought large water droplets, originally at around the 15,000ft- 18,000ft level, up to 20,500ft with temperatures in the range reported by the pilot."
The circular warns pilots to be on the alert when this situation is evident on the synoptic chart. Those flying aircraft without adequate modern de-icing equipment are advised to consider an alternative routeing or a postponement of the flight; others are advised to turn on the de-icing system in good time so as to obtain maximum protection. (Readers may like to be reminded that these circulars - both pink and white - are available free from the Aeronautical Information Service, Tolcarne Drive, Pinner, Middlesex.”)

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