A PIA Pakistan International Airlines Airbus A320-200, registration AP-BLV performing flight PK-369 from Islamabad to Karachi (Pakistan), was enroute at FL320 about 45nm east of Multan (Pakistan) when the crew received an ECAM indication of excessive cabin altitude (above 5300 feet) and worked the related checklists. The cabin altitude however continued to increase and reached 14,000 feet at which point the passenger oxygen masks automatically deployed. The crew initiated an emergency descent, managed to control the cabin pressure manually and levelled off at FL240. The aircraft contined to Karachi for a safe landing about one hour after leaving FL320.
Pakistan's SIB rated the occurrence a serious incident and opened an investigation.
Pakistan's SIB released their final report concluding the probable cause of the serious incident was:
Cabin Pressure Controller No 2 (CPC #2) failed to regulate the cabin pressure at the desired value and failed to automatically switch over the system to CPC #1. However, Cockpit Crews delayed recognition of the situation as well as wrong actions aggravated the situation till automatic dropping of Oxygen masks (as designed).
The investigation found:
On PK-369, a problem in auto pressurization control started 30 mintes after flight, at around FL310. Three Excess cabin altitude warnings came within a period of4 minutes and finally all emergency cabin oxygen masks were dropped. The cabin altitude had actually reached, more than 14,000 ft limit, where these masks deploy automatically. 17 minutes later, the saftey valve also operated to avoid structural collapse, as cabin ∆P (differential pressure) had crossed the upper limit of 8.5 psi.
During this time CPC # 2 was in control of auto pressurization. This CPC had faulted during previous (most recent) flight legs. It is believed that this CPC malfuntioned in controlling the required cabin verticle speed, as a result cabin altitude kept climbing and crossed its limits. It is also evident, that unlike previous flight legs where after fault, CPC #2 declared itself faulty and switched over the system to CPC #1 automatically, but for this flight the faulty CPC #2 remained in control even after its failure to control the cabin pressure.
The crew was unable to promptly monitor and correctly analyse the
situation. The crew made a reseting attempt contrary to the laid procedure
given in Quick Reference Handbook (QRH), didnt reduce the altitude
below FL 100 and didnt discontinue the flight especially after dropping of the oxygen masks (by landing at Lahore). Instead the crew initiated wrong selection, opted to maintain altitude and conveyed incorrect / incomplete situation to the situation room. Had the cockpit crew reacted promptly as per their laid down procedures, the incident of dropping of the masks was avoidable and pressurization was controllable either by the CPC #1 or manually by crew.