What happened
On May 6, 1988, a Widerøe Airways de Havilland Canada DHC-7-103 (registration LN-WFN) crashed into the Torghatten mountain near Brønnøysund, Norway, during an approach to Brønnøysund Airport. The flight, operating as route WF 710 from Namsos, was flying through clouds when the collision occurred. The accident resulted in 36 fatalities, including all 33 passengers and the 3 crew members on board.
An original 1989 investigation determined that the descent was initiated approximately 4 nautical miles too early, causing the aircraft to descend below the safe terrain clearance altitude. While the primary cause was identified as an improper descent profile, new information surfaced decades later regarding the presence of mobile devices on the aircraft.
The investigation
In 2013, the Norwegian Safety Investigation Authority (NSIA) launched a supplemental inquiry following a witness report that a passenger was carrying a mobile phone during the flight. The investigation sought to determine if electromagnetic interference (EMI) from these devices could have disrupted the aircraft's instruments or autopilot systems.
Investigators confirmed that two mobile phones, specifically Mobira models using the NMT-450 network, were present on the aircraft. To test the potential impact, the NSIA analyzed flight recorder data, cockpit voice transcripts, and consulted experts from the UK's Air Accidents Investigation Branch (AAIB) and various aerospace manufacturers. The scope included examining the DME (Distance Measuring Equipment), the autopilot system, and the altimeters for any signs of signal disruption or uncommanded behavior.
Findings
After a thorough review, the NSIA found no evidence that mobile phones influenced the accident. Key findings included:
- Flight Recorder Analysis: Data from the flight recorder showed no irregularities in the navigation or altitude parameters that would suggest interference. A previously noted fluctuation in engine torque on a prior leg was attributed to a simple bit error in the data stream rather than EMI.
- Autopilot and Instruments: The autopilot functioned as programmed, leveling the aircraft at pre-selected altitudes of 1,500 feet and 550 feet. The altimeters and DME readings were consistent with the actual terrain and flight path.
- Crew Activity: Cockpit voice recordings showed no indication that the crew noticed any instrument malfunctions or unusual electronic interference. Furthermore, the descent appeared to be a deliberate manual action, as evidenced by the crew's adjustment of engine power, flaps, and landing gear.
Ultimately, the investigation concluded that the premature descent remained the primary cause, and the presence of mobile phones played no role in the tragedy.