What happened
On 5 March 2018, an Etihad Airways Airbus A320-232, registration A6-EIF, was operating a scheduled passenger flight from Abu Dhabi to Karachi. While descending through 28,300 feet, the flight crew observed that the cabin altitude was rising at a rate of approximately 300 feet per minute. After attempting to level off at 27,000 feet without success, the commander initiated an emergency descent and declared a PAN-PAN to air traffic control.
As the aircraft descended through 19,200 feet, an 'EXCESS CAB ALT' warning appeared on the electronic centralized aircraft monitoring system. When the cabin altitude exceeded 10,000 feet, the crew manually deployed the passenger oxygen masks. The aircraft eventually descended below 10,000 feet, at which point the crew cancelled the emergency status and proceeded to Karachi International Airport, landing without further incident.
The investigation
The investigation, conducted by the UAE Air Accident Investigation Sector, focused on the technical failure of the cabin pressurization control system. Investigators examined the behavior of the two cabin pressure controllers (CPC1 and CPC2) and analyzed the flight data and maintenance records. The inquiry specifically looked into why the system failed to maintain the required pressure differential and why the primary controller failed to trigger appropriate warnings during the initial altitude climb.
Findings
- The primary cause of the incident was a slow cabin depressurization resulting from the first cabin pressure controller (CPC1) processing incorrect landing field elevation data.
- This data corruption was likely caused by a bit corruption in the memory cell of the CPC1 digital electronic system, potentially due to a single event upset (SEUC) or fatigued solder joints on the main board's integrated circuits.
- While the first controller failed to trigger an excessive cabin altitude warning during the initial pressure loss, the second controller (CPC2) correctly triggered the warning once the threshold was reached, as it was using the correct landing field elevation of 96 feet.
Safety action
- The AAIS issued a safety recommendation to the operator regarding system monitoring.
- A safety recommendation was also issued to the aircraft manufacturer regarding the integrity of the cabin pressure controller's digital memory and hardware components.