What happened
On 19 April 2012, a BAe Avro RJ100, operated by Brussels Airlines SA/NV, departed Brussels for a scheduled passenger service to Stockholm, Broma. Approximately 15 minutes into the flight, while climbing through the Amsterdam Upper Control Area, the flight crew observed master caution warnings regarding the aircraft's pressurization system. Following established procedures, the pilots transitioned to manual pressurization control.
As the aircraft continued its climb, the cabin altitude rose past 9,000 feet. The crew requested a descent from Maastricht UAC, but after initial difficulties obtaining the necessary clearances, the pilots were forced to declare a mayday. The aircraft subsequently descended to FL100, at which point a high-altitude cabin warning horn activated. The crew donned oxygen masks and initiated an emergency descent. The aircraft eventually leveled off at 10,000 feet and returned to Brussels, where all 84 persons on board were safely disembarked.
The investigation
Following the incident, the aircraft was moved to a hangar for maintenance and underwent a pressurized leak test. This inspection revealed a significant 1089 mm crack in the left-hand aft fuselage skin. The investigation further identified additional cracks located in frames 41X and 42.
Findings
Technical analysis determined that the primary cause of the depressurization was a large crack in the left-hand aft lower skin panel, specifically within the lap joint area situated below the aft toilet servicing door. This fracture originated from multiple fatigue crack propagation that began within a chemically etched channel on the external surface of the skin panel.