What happened
On the night of 8 January 2016, a CL-600-2B19 aircraft, registered as SE-DUX and operated by West Atlantic Sweden AB, crashed in the Oajevágge region of Norrbotten County, Sweden. During the flight, the crew encountered a sudden and critical malfunction involving the aircraft's Inertial Reference Units (IRUs). As the aircraft's attitude deviated, the cockpit voice recorder captured a period of intense distress, with the crew attempting to manage severe bank angle warnings and overspeed conditions. The aircraft ultimately impacted the terrain, resulting in two fatalities.
The investigation
The Swedish Accident Investigation Authority (SHK) examined the wreckage, flight data, and cockpit voice recordings to reconstruct the sequence of events. The investigation focused on the functionality of the aircraft's flight instrument systems and the crew's ability to respond to the sudden-onset emergency. Investigators also reviewed the performance of the regional rescue services, noting the challenges of conducting search and rescue operations in a mountainous environment during winter conditions. The study included simulator reference flights to understand how the aircraft's Primary Flight Display (PFD) reacted to the malfunction.
Findings
Investigators determined that the accident was caused by insufficient operational prerequisites for the management of a failure in a redundant system. Specifically, the malfunction of one IRU created a critical situation that the crew was not adequately prepared to handle.
Several contributing factors were identified:
- The flight instrument system failed to provide sufficient guidance or clarity regarding the nature of the ongoing malfunctions.
- The aircraft's PFD units utilized a 'declutter' function that removed pertinent cautions during unusual attitudes, potentially misleading the crew.
- An initial maneuver involving negative G-loads likely impaired the pilots' ability to maintain rational decision-making.
- There was a lack of an effective, standardized communication system for managing abnormal and emergency situations.
Safety action
The SHK issued several safety recommendations to international aviation bodies, including ICAO, EASA, and the FAA. These recommendations urge the implementation of standardized callouts for unexpected situations and improvements to PFD design criteria to ensure that critical warnings are not removed during declutter modes. Additionally, recommendations were made to the Swedish authorities to improve coordination between air traffic control and rescue centers, and to enhance specialized training for mountain rescue crews.