Vickers Viscount

Aircraft structure factor - tail unit, involving the failure in flight of the tailplane spar, so that the left tailplane and elevator became detached and control of the aircraft was lost. An examination of the aircraft wreckage revealed that the upper flange of the left tailplane spar had sustained a fatigue fracture, two distinct areas being visible, namely the area of progressive fatigue and the area of instantaneous failure. This evidence obviously explained the accident, since the already weakened structure was not able to withstand the loads imposed when the aircraft entered an area of turbulence. Laboratory tests subsequently carried out confirmed the failure.

Improper operation of the fuel system followed by failure to extend the undercarriage.

The aircraft struck a mountain peak whilst flying over snow covered high terrain in marginal visual meteorological conditions. Failure to maintain a safe altitude and insufficient attention to navigational procedures were contributory factors. The following factors were reported:
- Although the Captain obtained some weather information during his visit to the meteorological office he did not seek a briefing from the Duty Forecaster. This may have deprived him of information about the strong winds at his proposed flight level,
- The minimum sector altitude for the area was 4,400 feet and BEA's minimum safe altitude was 5,000 feet; nevertheless the decision to fly at FL 40 (3,800 feet amsl) was permissible for a VFR flight,
- The decision to operate under VFR in the prevailing weather conditions was questionable but probably explicable in the light of the nature of the flight,
- Map reading over snow covered terrain in the prevailing weather conditions would have presented obvious difficulties. The possibility of error may have been increased by temporary distractions resulting from preoccupation with the flight engineering test programme,
- The exact circumstances of the accident are not known, but it probably occurred whilst the aircraft was flying in 'whiteout' conditions associated with a snow shower.

According to the investigating committee, the accident was the direct result of a collision with the terrain, which occurred at night in stormy conditions in the vicinity of a cold front, while the aircraft was following a seemingly normal approach procedure but deviated to the east about 30 km. The genesis of the accident has two anomalies. The first consists in the fact that the crew was convinced to be really vertical of Clermont-Ferrand whereas this beacon was, in fact, more than 30 km away. It can only be explained by a frank rotation of the radiocompass whose crew did not detect the aberrant character. It must also be admitted, moreover, that the radiocompass indications subsequently showed consistent variations with the successive positions of the aircraft during the double descent circuit and with the second overflight of the Clermont-Ferrand beacon. The second anomaly is characterized by the announcement of the aircraft flying over the beacon, with more than three minutes ahead of an estimated eight minutes. This difference may have been neglected by the crew because of their reliance on the radiocompass indication. We can also think that this difference went unnoticed by the crew either by forgetting to check the time, or by a reading error, the crew having to cope with a difficult steering because of the turbulence and the presence of an instructor who could also be a factor of concern and distraction at this time. The commission's work has made it possible to rule out the assumption of a pirate transmitter, the erroneous indication of the radiocompass could have come from either a fault in the installation of the on-board antenna system associated with certain conditions of the electric field , or more likely very localized precipitation that accompanied the cold front in the area of ​​the accident and that could constitute a kind of transmitter (series of micro discharges) powerful enough to be during all this phase of flight detected by the radiocompass of the plane while stifling the field of the beacon of Clermont-Ferrand. The interception of the ILS alignment plan probably reinforced the crew's conviction. In addition, it is not impossible that the luminous halo of the city of Thiers, perhaps visible at certain times, could constitute an additional factor of motivation. Although the instructor pilot had the reputation of attaching great importance to a cross-checking of positions, which was possible in particular by the radial of Moulins, it seems that the crew did not do anything about it. The commission ultimately accepts the possibility of an aberrant indication of a radiocompass, but can not explain the reasons which prevented the crew from properly controlling the point from which the descent was started and continued. In conclusion, it is understood that the accident was the consequence of a controlled flight into terrain.