Incident Overview
On 5 March 2018, an Etihad Airways Airbus A320-232, registration A6-EIF, was operating scheduled passenger flight EY200 from Abu Dhabi International Airport to Karachi International Airport. The aircraft carried 82 persons: 76 passengers, two flight crew, and four cabin crew.
Flight Sequence
After takeoff and climb, the aircraft cruised at FL370. During descent, as the aircraft passed 28,300 feet pressure altitude, the flight crew observed the cabin altitude increasing at approximately 300 feet per minute. The crew leveled off at FL270 to assess the situation, but the cabin altitude continued to rise. The commander initiated an emergency descent, and the co-pilot declared a PAN-PAN to Karachi area control center (ACC), reporting a cabin pressure failure.
At 19,200 feet pressure altitude, an 'EXCESS CAB ALT' warning appeared on the ECAM. During the emergency descent, cabin altitude exceeded 10,000 feet, prompting the flight crew to manually deploy passenger oxygen masks at approximately 16,400 feet. The commander instructed passengers to use the masks, and cabin crew ensured compliance.
After descending below 10,000 feet pressure altitude, the emergency descent was completed, the PAN-PAN was cancelled, and passengers were informed they could remove their masks. The flight continued to Karachi International Airport, where an ILS approach to runway 25L was performed, and the aircraft landed uneventfully.
Investigation Findings
The investigation, conducted by the UAE Air Accident Investigation Sector (AAIS), determined that the slow cabin depressurization resulted from the number one cabin pressure controller (CPC1) processing corrupt landing field elevation data. The corruption likely originated from a bit corruption in the memory cell of CPC1's digital electronic system, possibly due to a single event upset (SEU) or fatigued solder joints on integrated circuits of the CPC1 main board.
Contributing Factors
Two contributing factors were identified:
- CPC1, which was controlling the cabin pressure control system, did not trigger an excessive cabin pressure warning while controlling the cabin toward a higher altitude based on the corrupt landing field elevation. The cabin altitude rate was 300 to 400 feet per minute. The excessive cabin altitude warning was triggered by CPC2 when the cabin altitude reached the threshold; CPC2 used a landing field elevation of 96 feet, approximately the actual elevation of Karachi International Airport.
- The corrupted memory was most probably caused by either an SEU in one memory cell of CPC1 or erroneous data from fatigued solder joints on the ICs of the CPC1 main board.
Safety Recommendations
The AAIS issued two safety recommendations: one to the operator, Etihad Airways, and one to the aircraft manufacturer, Airbus. The report did not elaborate on the specifics of these recommendations.
Damage and Injuries
No injuries to persons were reported. The aircraft sustained no damage as a result of the incident.