What happened
On December 22, 2022, a De Havilland Aircraft of Canada Limited DHC-8-103, registration LN-WIP, was conducting a commercial flight from Bodø to Svolvær Airport Helle. The flight, operated by Widerøe, encountered weather conditions that required the aircraft to enter a holding pattern due to necessary snow removal on the runway.
During the approach to runway 01, the crew failed to update the local barometric pressure (QNH) on the aircraft's altimeter. Because the altimeter was set to standard pressure rather than the local setting, the instruments provided an inaccurate altitude reading. The aircraft was actually flying approximately 700 feet lower than the crew believed. The situation was only identified when the Enhanced Ground Proximity Warning System (EGPWS) issued a 'Too Low terrain' alert while the aircraft was 3.1 nautical miles from the runway threshold. The crew immediately aborted the approach and diverted to Bodø, where they landed safely. There were no injuries among the 29 passengers or the 3 crew members.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined several data sources, including cockpit voice recordings, radar data, and flight recorder information. The investigation focused on why the crew failed to recognize the altitude discrepancy and why the existing safety barriers failed to prevent the error. Investigators looked into the technical capabilities of the aircraft's transponder and the procedures used by the operator for verifying altimeter settings.
Findings
- The primary cause of the altitude discrepancy was that the crew forgot to adjust the barometric altimeter to the local QNH.
- Existing checklists and procedures were found to be insufficient, as they contained dependencies that made them less effective as safety barriers.
- The aircraft's transponder was not capable of transmitting QNH values to air traffic services, meaning controllers could not detect the pressure setting error.
- There is currently no automated system in Norwegian airspace capable of detecting deviations between reported and actual aircraft pressure settings.
Safety action
- The NSIA recommended that the Norwegian Civil Aviation Authority perform a risk assessment regarding incorrectly set altimeters.
- A recommendation was made to Widerøe to simplify and clarify procedures for setting and verifying QNH, ensuring they are performed every time regardless of the operational context.
- The NSIA also recommended that Widerøe upgrade the transponders on its fleet to enable the transmission of pressure settings, allowing for better monitoring by air traffic services.