What happened
On 22 March 2007, an Avro RJ 100, registration SE-DSP, was performing a scheduled commercial flight from Stockholm/Bromma to Gothenburg/Landvetor. During the climb, the crew activated the aircraft's de-icing systems due to changing weather. Because the aircraft's performance was limited, the pilots had planned to keep the air conditioning pack switches off during takeoff to maximize engine power, intending to activate them later.
As the aircraft climbed, an "Avionics fan off" warning light illuminated in the cockpit. The crew began following the emergency checklist for this specific warning, but the aircraft continued to climb without cabin pressurization. At approximately 18,000 feet, the cabin crew notified the pilots that oxygen masks had dropped in the passenger cabin. The pilots realized the cabin was unpressurized and immediately initiated a descent. During the descent, the high cabin altitude warning failed to activate as intended.
The investigation
The Swedish Accident Investigation Board (SHK) examined the aircraft's systems and the crew's procedures. The investigation focused on why the cabin remained unpressurized despite the warning light and why several oxygen mask hatches failed to deploy. Investigators also reviewed the maintenance records regarding the recent repacking of the oxygen mask hatches.
Findings
- The climb checklist was insufficiently detailed, lacking specific instructions to verify the status of the air conditioning pack switches during the climb.
- The "Avionics fan off" warning light did not include a note indicating that the fault could be caused by low cabin pressure.
- A damaged pressure sensor prevented the high cabin altitude warning from activating at the required threshold.
- Deficient quality control during the recent repacking of the oxygen mask hatches by external personnel resulted in 20 of the 43 hatches failing to open.
- The cabin crew attempted to move passengers to the right side of the aircraft to avoid the non-functional masks, but the chief cabin attendant performed these actions without first ensuring their own oxygen supply.
- The manual tool required to open the oxygen hatches was difficult to locate and use under stress.