What happened
On November 20, 2017, a Boeing 737-500, registration OB-2140-P, was performing a scheduled flight from Jauja (SPJJ) to Lima (SPJC) carrying 68 passengers and 6 crew members. The aircraft departed from Francisco Carlé Airport, an airfield situated at an elevation of 11,034 feet.
During the climb through 17,000 feet, the first officer noticed that the cabin altitude had exceeded 14,000 feet. This was accompanied by a Master Caution alert and an 'AUTO FAIL' light for the cabin pressure controller. As the cabin altitude continued to rise, the oxygen masks deployed at approximately 14,800 feet.
The captain attempted to regain control of the pressurization by switching to standby and manual modes, including closing the outflow valve, but was unable to stabilize the cabin pressure. The aircraft climbed to approximately 18,000 feet, which was the minimum safe altitude for the area, before the crew initiated an emergency descent. The aircraft returned to Jauja and landed safely without injuries or aircraft damage.
The investigation
The CIAA investigation focused on the aircraft's air conditioning and pressurization systems, specifically regarding operations at high-altitude airfields. The investigation examined the crew's use of the 'No Engine Bleed Air Takeoff' procedure, which requires specific configurations for the APU and engine bleed valves.
Investigators also reviewed the maintenance records, noting that while the aircraft was airworthy, there were discrepancies in the digital flight data recorder (DFDR) regarding certain parameters. The investigation also looked into the deployment of the oxygen masks and the crew's adherence to the Flight Crew Operating Manual (FCOM) during the emergency.
Findings
- The primary cause was the inadequate configuration of the air conditioning and cabin pressurization systems during takeoff and climb from a high-altitude airfield, combined with an incomplete response to pressurization alerts.
- The crew failed to precisely follow the sequence of steps in the FCOM, specifically regarding the stabilization of the cabin rate before reactivating engine bleed air.
- The crew reactivated the engine bleed air at 1,500 feet AGL, rather than the prescribed 1,000 feet AGL.
- The aircraft's climb rates exceeded established limits, which overloaded the pressurization system.
- There was evidence of a possible failure in the maintenance or reliability of the passenger service units (PSU) regarding the deployment of oxygen masks for the cabin crew.