Perth and Kinross

On vacating FL140, the aircraft's climb rate was so erratic at 140 KIAS that it seems likely that by then, the aircraft had already gathered sufficient ice to seriously affect its performance. If all the
turbocharger inlets had become partially blocked, then manually selecting both engines to alternate air induction should have introduced warmer air into the turbochargers and restored power. The description of engine operation in the Superstar manual states:
'If manifold pressure continues to decrease after opening the manual alternate air, it is an indication that turbocharger inlets are still restricted and the engine may become normally aspirated through the automatic alternate air door located below the induction air filter'.
Normal aspiration reduces the manifold pressure to ambient or less and at FL140 the ambient pressure is about 17.6 inches which is less than half the climb rated manifold pressure. That might explain the inability to climb above FL 160 but it would also have deprived the pilot of pressurisation. There was no change in his voice consistent with donning an oxygen mask so he may not have lost pressurisation completely. Nevertheless, since he lost control at around FL160 and 110 KIAS, and because the aircraft initially turned to the right, a combination of airframe icing and asymmetric power loss seem the most likely explanation for the sustained loss of control. The split in the EDP diaphragm which almost certainly occurred during this flight may have contributed to an asymmetric power problem. Alternatively, the pilot might have become mildly hypoxic and decided to begin an emergency descent. If so, he did not declare an emergency at the time he started to descend, although he did utter the word "SCOTTISH" after control was lost, so he was conscious even if his mental abilities had been impaired by hypoxia. On balance, the tone of his voice and his initial failure to respond to ATC messages suggested that the descent was begun through loss of control rather than a deliberate act followed by loss of control. At the time of the accident the aircraft had been flying below the freezing level (8,000 feet) for about five minutes and much of the airframe and induction system ice may have melted. The would-be rescuers would have taken at least five minutes to reach the crash site and so the fact that none of them reported seeing or treading on any ice was not surprising. Witness and propeller evidence indicated that power had been restored on at least one engine but there seemed to be insufficient power to climb out of Glen Lyon. The aircraft was out of control when it crashed at low speed from a sharp turning manoeuvre. Before this manoeuvre the pilot may have had partial control, albeit with a power problem which prevented him from climbing, and he finally lost control totally when he attempted to turn around within the confines of the Glen. He had no choice but to attempt the turn since, had he not turned, he would have flown into the side of the hill above the crash site.

There were no eyewitnesses to the final moments of the flight, and the aircraft was not fitted with an Accident Data Recorder or Cockpit Voice Recorder. The Board of Inquiry's investigation therefore remains conjecture; notwithstanding that specialist advice from various sources, including the Air Accident Investigation Branch of the Department of Transport and the aircraft manufacturer supports their findings. The investigation found that the cause of the accident was consistent with the aircraft having stalled at an altitude from which recovery was impossible.

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.

The ballast weights slid forward, causing the aircraft to dive and crash.

Failure of the right engine in flight.

Controlled flight into terrain.

Engine failure.