What happened
On March 28, 2016, a Boeing 737-76N, registration PR-GOV, operated by GOL Airlines, experienced a serious pressurization incident while cruising at 38,000 feet. During the flight from Belém to Manaus, the crew received 'LH BLEED TRIP OFF' and 'RH BLEED TRIP OFF' warnings.
In an attempt to stabilize the cabin pressure, the crew manually closed the outflow valve. Despite these efforts, the cabin altitude began to rise at a rate of approximately 2,000 feet per minute. The crew initiated an emergency descent to 10,000 feet. As the aircraft descended through 30,000 feet, the passenger cabin oxygen masks deployed automatically. Once the aircraft reached 10,000 feet, the cabin pressure stabilized, allowing the flight to continue to its destination. The aircraft sustained no damage, and all 133 passengers and 5 crewmembers were unharmed.
The investigation
CENIPA investigators examined the aircraft's maintenance records, flight data, and the specific components involved in the pressurization system. The investigation focused on the Precooler Control Valves (PCCV) installed on the engines. Laboratory analysis of the valves from PR-GOV revealed internal clearance in the butterfly plate, along with clogging and leaks in the internal holes. Data from the Non-Volable Memory (NVM) of the valves confirmed a cabin pressure rate variation of 1,200 feet per minute, which was consistent with insufficient air inflow into the cabin.
Findings
- The primary cause of the depressurization was the failure of the Precooler Control Valves (PCCV).
- The specific valve model (PN 3289562-5) had a documented history of low reliability and premature failure due to design and material issues.
- The investigation identified a design flaw in the PCCV series, where previous attempts to update the part (models -6 and -7) also suffered from high failure rates, leading Boeing to revert to the older -5 model for certain production lines.
- The operator had been monitoring the system via Pneumatic Duct Pressure Surveys, and the components were operating within established manufacturer limits prior to the failure.