What happened
On a scheduled flight from Toronto, Ontario, to Fredericton, New Brunswick, an Air Canada Jazz Canadair CL600-2B19, registered C-FSJJ, experienced a significant loss of cabin pressure while cruising at flight level 3/30. The flight crew first noticed the cabin altitude rising at a rate of roughly 1,000 feet per minute.
In an attempt to manage the situation, the pilots requested a descent to flight level 250. However, even after leveling off at the lower altitude, the cabin altitude continued to climb. As the cabin altitude surpassed 10,000 feet, the crew donned their oxygen masks. The ascent within the cabin continued until it reached 14,000 feet, at which point the passenger oxygen masks automatically deployed. The crew eventually stabilized the aircraft at 9,000 feet and landed in Fredericton without any injuries to the 50 passengers or 3 crew members.
The investigation
Investigators examined the aircraft's air conditioning and pressurization systems to determine why the cabin altitude could not be maintained. The inspection revealed several simultaneous mechanical failures.
On the right side, the pressure regulating shut-off valve (PRSOV) sense line had become detached, causing the right air conditioning unit (ACU) pack to go offline. On the left side, a supply duct had detached from its flange at the water separator joint, allowing air to leak into the unpressurized tail section. Additionally, investigators found that the return springs for both the left and right bulkhead check valves were missing. This absence of springs allowed cabin air to backflow through the systems.
Findings
- The primary trigger for the pressure loss was the detached PRSOV sense line on the right system, which caused the right ACU pack to shut down.
- When the right pack failed, the left pack automatically switched to high-flow mode to compensate.
- The increased pressure from the left pack's high-flow mode likely caused the already weakened left system duct flange to fail.
- The missing return springs on the bulkhead check valves prevented the valves from sealing, allowing air to escape from the cabin into the ducting.
- The simultaneous failure of the left duct flange and the right system's inactivity meant the remaining air supply was insufficient to maintain cabin pressure.