Norwegian re-investigation of a fatal de Havilland Canada Dash 7 crash 25 years ago has turned up no evidence that mobile phone interference contributed to the accident.
The aircraft, operated by Wideroe, collided with the Torghatten mountain during its approach to Bronnoysund airport on 6 May 1988. None of the 33 passengers and three crew members survived.
Investigators at the time determined that the aircraft had prematurely descended from 1,500ft while still 8nm from the airport, instead of the required 4nm. But the reason for the early descent could not be established.
The inquiry was re-opened in July this year after Norwegian investigation authority SHT received information from a witness who had disembarked at Namsos, before the flight continued to Bronnoysund.
This witness claimed a passenger, seen sitting in a cockpit observation seat, had a mobile phone – something which was not mentioned during the original probe. SHT adds that two mobile phones were discovered at the site of the crash.
It has gathered supplementary information from several sources including Viking Air, which holds the type certificate for the Dash 7, and operators of the type such as Air Greenland.
The UK’s Air Accidents Investigation Branch also examined flight-recorder data from the crashed aircraft for indications of electromagnetic interference.
Cockpit recordings show no sign that the passenger was talking on the phone during the flight, although SHT adds that it cannot establish whether either of the two phones was switched on. But conversations between the two pilots have not revealed any evidence of instrument abnormalities.
The probe focused particularly on the autopilot as well as the VOR-DME navigation systems. Avionics specialist Honeywell told the inquiry that it was “not aware” of any autopilot disconnect incidents arising from external radio interference, and that such an event “would likely not have occurred” on the Wideroe flight.
SHT says that proving the presence of interference would be “virtually impossible”, and instead looked at a hypothetical worst-case scenario in which the phones were active and transmitting. It then compared theoretical effects on the aircraft with the factual data from the accident.
But the crew appeared to have continuous control of the aircraft and there was no misleading instrument data that made the Dash 7 difficult to handle.
If the aircraft had precisely followed its VOR radial, it would have passed east of the mountain. But the horizontal course deviation was “within tolerance limits”, says SHT, even if there had been an interference effect.
Vertical navigation was the critical aspect of the crash, and the inquiry considered whether the DME system had given misleading distance information or whether altimeter was showing the correct height.
But the crew had reported to Bronnoysund tower, as they initiated the descent, that they were 8nm from the airport. Use of trim and adjustment of engine power indicates that the aircraft started its descent under deliberate control, and the aircraft’s levelling at a pre-selected height of 550ft suggests the autopilot was functioning normally and the altimeters showed no discrepancies.
As a result of its analysis SHT has concluded that the accident sequence was “not affected” by electromagnetic radiation from mobile phones on board. The reasons for the fatal descent remain unexplained and the original final report into the crash has not been amended.