Incident Overview
On June 13, 2019, the flight crew of a Boeing 737-800 (registration SP-RSQ) was operating a scheduled service from LIME to Warsaw Chopin Airport (EPKK). The aircraft was descending at Flight Level 300 when the "CABIN ALTITUDE WARNING" light illuminated, accompanied by an audible alert. The cabin pressure panel indicated a cabin altitude of 10,000 feet, exceeding the normal limit of 8,000 feet.
Crew Response and Actions
Upon receiving the warning, the pilots donned oxygen masks and verified their ability to communicate. They performed memory items for the "CABIN ALT WARNING" scenario. Without consulting the appropriate checklist, the crew switched the pressurization system from "AUTO" to "MANUAL" mode and manually closed the outflow valve, which was already nearly closed. This action suggested the automatic pressure regulation mechanism may have been functioning correctly.
Subsequently, the crew tested the system in "MANUAL" mode, then switched to "ALNT (ALTERNATIVE)", and finally returned to "AUTO". By Flight Level 250, the warning light extinguished, indicating cabin pressure had normalized. At Flight Level 200, the pilots removed their oxygen masks and completed the checklists for "CABIN ALTITUDE WARNING / RAPID DEPRESSURISATION" and "AUTO FAIL / UNSCHEDULED PRESSURIZATION / DESCEND".
Investigation Findings
The investigation determined that the incident was caused by transient incorrect indications of cabin pressurization status despite the system being operational. A contributing factor was the crew's execution of memory items without reading the required procedures, which mandated remaining in "MANUAL" mode until landing if selected during flight. The crew’s deviation from this procedure constituted an error.
Outcome and Recommendations
The aircraft landed safely at EPKK with no injuries or damage. Post-incident technical inspections found no faults in the pressurization system. The operator conducted simulator training for the crew regarding proper response to cabin altitude warnings and administered a practical line check. Additionally, the operator’s training department integrated this case into its standard curriculum. No safety recommendations were issued by the commission.