What happened
On 1 July 2016, a BAe Avro 146-RJ85, registration EI-RJW, was performing a scheduled passenger flight from Paris Charles De Gaulle Airport to Newcastle, United Kingdom. During the initial climb, the flight crew noted weak airflow from the cockpit vents. As the aircraft climbed through flight level 160, the crew observed a rapid increase in both the cabin rate of climb and cabin altitude, accompanied by a pressurization advisory light.
As the aircraft reached flight level 200, a more serious pressurization system failure light illuminated. Despite the crew attempting to manage the situation via the manual mode, the cabin altitude continued to rise sharply, eventually reaching 9,500 feet. This triggered a cabin high altitude warning. The crew declared a Mayday, donned oxygen masks, and initiated an emergency descent to flight level 100. During the descent, the crew identified that a ram air switch on the overhead panel was in the OPEN position. By switching this to SHUT, they were able to restore normal pressurization control and complete the flight to Newcastle without further incident. There were no injuries to the 89 passengers or 4 crew members on board.
The investigation
The AAIU investigation focused on why the pressurization system failed to maintain the cabin altitude. While the flight crew could not explain how the ram air switch moved to the OPEN position, the investigation uncovered a significant mechanical error. It was discovered that a non-return valve within the aircraft's ram air system had been installed in the wrong orientation.
Investigators found that the maintenance task description for a previous inspection was inaccurate, which likely led a technician to install the valve incorrectly. Furthermore, the aircraft manufacturer's maintenance manual did not require a post-installation test for this specific valve, and no post-manufacturing test existed to verify its correct orientation.