What happened
On 20 May 2023, an Etihad Airways Airbus A320, registration A6-EIX, was operating flight EY535 from Abu Dhabi to Beirut. The flight was carrying 114 passengers and 8 crew members. During the takeoff roll, an ECAM alert for a "VENT SKIN VALVE FAULT" triggered at 47 knots. Although the crew proceeded with the takeoff, the aircraft failed to pressurize normally as it climbed.
Upon reaching 11,400 feet, the crew received a "Cabin Excess Altitude" warning. To maintain safety, the pilots leveled off at 11,600 feet and initiated a descent to 10,000 feet. While the pilot monitoring briefly declared a "mayday," the declaration was cancelled as the cabin altitude remained below 10,000 feet. The aircraft entered a holding pattern near Abu Dhabi to coordinate with maintenance and fuel-burn for landing weight, eventually returning to Abu Dhabi safely.
The investigation
Investigators from the GCAA AAIS examined the aircraft's maintenance history and the configuration of the avionics ventilation system. The investigation focused on the skin air outlet valve (SAOV), which is used to evacuate hot air from the avionics compartment. The aircraft was operating under a Minimum Equipment List (MEL) deferral, which allowed the SAOV to be inoperative under specific conditions.
Post-flight inspection by maintenance personnel revealed that the SAOV was in a fully open position. This contradicted the required configuration for the MEL deferral, which mandates that the valve be set to a partially open position (with the main flap closed and the internal flap at 45 degrees) to ensure proper cabin pressurization.
Findings
- The primary cause of the cabin pressurization failure was an air leak through the skin air outlet valve because the valve was left in a fully open position.
- The maintenance deactivation of the SAOV was likely not performed in compliance with the required AMM task procedures.
- The aircraft was released for flight with a deferred maintenance item (MEL 21-26-04A) that was not correctly configured.
- Company supervision regarding the verification of MMEL task completion was found to be ineffective.
- The flight crew correctly followed established abnormal procedures during the flight.