What happened
On the night of February 23, 2001, an Embraer EMB-120ER, registration SE-LKB, was operating a non-scheduled charter flight from Sundsvall/Härnood airport to Jönköping. The flight was carrying 27 passengers and a crew of three. Approximately 17 minutes into the climb, passing through Flight Level 150, the crew detected a growing burning odor accompanied by a light haze in the cockpit.
In response to the developing situation, the commander deactivated both air conditioning packs, which were being supplied by the APU. The pilot declared an emergency with Sundsvall air traffic control and requested radar vectors for an immediate return to the departure airport. Although air traffic control suggested Hudiksvall airport as a closer alternative, the crew opted to return to Sundsvall due to the shorter runway length at the alternative site. The flight proceeded with a depressurized cabin, and while the haze dissipated, a persistent, sticky burning smell remained. The aircraft landed safely at Sundsvall/Härnösand without further incident.
The investigation
The Swedish Accident Investigation Authority (SHK) examined the aircraft's electrical systems and the crew's emergency response. Post-flight inspections of the aircraft revealed that a bolt on the connection terminal board for the right windshield heating element had become loose. This loose connection had caused electrical arcing, which led to localized overheating and damage to the cable insulation and the terminal board itself.
Investigators also reviewed the crew's adherence to the Quick Reference Handbook (QRH). While the crew followed procedures for smoke evacuation, they did not follow the initial steps of the checklist, which required donning oxygen masks. Furthermore, the investigation looked into Crew Resource Management (CRM) and the communication of the emergency status to the cabin attendant.
Findings
- The primary cause of the incident was electrical arcing resulting from a loose bolt on a connection terminal board.
- This arcing caused significant overheating, which produced the burning smell and haze observed by the crew.
- The pilots failed to don their oxygen masks as prescribed by the emergency checklist.
- There was a lack of communication regarding the severity of the situation, as the cabin attendant was not informed that an emergency had been declared.