What happened
On 7 October 2000, a Boeing 737-800, registration EI-CSC, was performing a scheduled passenger flight from Stansted Airport to Knock International Airport. Shortly after takeoff, the crew was notified of a potential bird strike. While climbing through 10,000 feet, the flight deck received a "Master Caution" alert. Shortly thereafter, the passenger oxygen masks automatically deployed in the cabin.
As the aircraft continued its climb to 14,300 feet, the crew realized the pressurization system was not functioning correctly. The aircraft was subsequently descended and leveled at 10,000 feet. Upon investigation, the flight crew discovered that the engine bleed air switches had not been properly set to the "On" position, which prevented the aircraft from maintaining cabin pressure. The aircraft returned to Stansted and landed safely with no injuries to the 177 passengers or 6 crew members.
The investigation
The investigation focused on why the pressurization system failed to maintain cabin altitude and why the crew was unaware of the deteriorating conditions. Investigators examined the aircraft's technical systems, including the bleed air and pressurization components. A post-incident inspection confirmed that the pressurization and air conditioning systems were fully serviceable.
Data from the flight recorders revealed that the engine bleed air switches were left in the "Off" position. Furthermore, investigators found that the circuit breaker for the cabin altitude aural warning horn had been left in the "Open" position, which prevented the audible alarm from alerting the pilots to the rising cabin altitude. The investigation also looked into the crew's adherence to Standard Operating Procedures (S2) and the manufacturer's known issues regarding oxygen mask deployment altitudes.
Findings
- The primary cause of the incident was the failure of the flight crew to adhere to the operator's SOPs regarding the completion and verification of checklists.
- The engine bleed air switches were not correctly configured for flight, which resulted in the aircraft operating without pressurization or air conditioning.
- The cabin altitude aural warning failed to alert the crew because the associated circuit breaker was in the "Open" position.
- The flight crew failed to verify the status of the pressurization system as the aircraft climbed through 10,000 feet.
- The crew did not don their oxygen masks despite the cabin altitude exceeding 10,000 feet, which was contrary to established non-normal procedures.
Safety action
Following the investigation, several safety recommendations were made to the operator, including the need to reinforce the requirement for crew to don oxygen masks when cabin altitude exceeds 10,000 feet or when pressurization integrity is uncertain. Additionally, the investigation noted that the manufacturer, Boeing, subsequently revised its non-normal procedures to include specific steps for selecting engine bleed and packs to "On" to prevent similar occurrences.