What happened
On 16 January 2001, a B737-50 and registration EI-CDF, operating a scheduled flight from Dublin to Dusseldorf, experienced a significant loss of cabin pressure control during its climb. While ascending to 37,000 feet, the aircraft's pressure differential reached a critical level, causing the cabin altitude to climb at rates exceeding 4,000 feet per minute.
As the cabin altitude rose, the crew observed the automatic system fail and transition to standby mode. Despite attempts to regain control using both standby and manual modes, the crew was unable to stabilize the pressure. The situation escalated to the point where oxygen masks deployed, and a burning smell was noted in the cockpit and cabin. After initially being denied a descent request due to air traffic, the pilot declared a PAN emergency, which prompted immediate clearance to descend. The aircraft eventually regained pressurization control as it passed through 28,000 feet, allowing the crew to return to Dublin, where the aircraft landed safely with no reported injuries.
The investigation
The AAIU investigation examined the aircraft's maintenance history, noting that a major check had been completed just days prior. During the inspection of the aircraft following the incident, several components were replaced, including the cabin pressure controller (CPC), the outflow valve, and a ruptured supply duct.
Subsequent testing by component manufacturers revealed significant technical issues. The manufacturer of the outflow valve identified a short circuit in the DC motor, which would have rendered both the standby and manual DC modes inoperative. Additionally, the manufacturer of the CPC found temperature sensitivity anomalies in the autofault board. While the ruptured duct was identified, investigators determined it was likely not the primary cause of the extreme cabin altitude climb.
Findings
- The loss of pressurization control occurred upon reaching 37,000 feet and lasted approximately four minutes.
- A combination of faults in the cabin pressure controller and the outflow valve actuator prevented the crew from effectively managing the pressure during the emergency.
- The aircraft's rapid ascent at a light weight may have triggered the safety relief valves, contributing to the pressure instability.
- A ruptured air supply duct was also present, which may have further complicated the recovery of cabin pressure.
Safety action
Following the investigation, several safety recommendations were issued:
- Establishing a standardized set of modifications for the CPC across the operator's fleet.
- Improving shop testing procedures for outflow valves at maintenance organizations.
- Updating flight crew training manuals to alert pilots that rapid ascents at low gross weights can potentially trigger the activation of pressure relief valves.