Boeing 737-400 Cabin Pressure Incident Near Warsaw (EI-STL)

Incident Overview and Immediate Response On 13 September 2021, the crew of a Boeing 737-400-42C, registration EI-STL, operated by ASL Airlines Ireland, was conducting a flight from EDDT to EPKT. Approximately six minutes after reaching cruising level FL370 and passing the Polish border, the crew observed an increased cabin altitude above 10,000 ft. The pilots identified the symptom as a decompression event and donned oxygen masks. While the First Officer was completing the related memory items from the Quick Reference Handbook (QRH) for a Cabin Altitude Warning, the Captain initiated an emergency descent in accordance with the QRH. ## Descent, Stabilization, and Decision-Making During the descent, the crew increased speed to 330 knots. As indicated airspeed increased, the aircraft pitched down, reaching a vertical speed of up to 7,000 ft/min. Both the Captain and First Officer experienced changes in ear pressure, and the First Officer reported a stomach issue. The First Officer later stated that he was able to manually control cabin pressurization. The crew declared a MAYDAY. As the descent stabilized and the aircraft eventually leveled at FL100, the cabin altitude was under control. The pilots then analyzed the situation using the TDODAR decision-making model, considering remaining fuel, aircraft status, and meteorological conditions. They decided to cancel the emergency situation and continue the flight to the destination airport, EPKT. The aircraft landed at EPKT without further issues in CAT II conditions. ## Findings and Contributing Factors The State Commission on Aircraft Accident Investigation determined that the loss of cabin pressure was caused by a failed cabin pressure control panel. The Commission also noted that, before the incident, there had been several pressurization system malfunctions, as confirmed by related technical log entries. ## Preventive Measures The Commission accepted the following preventive measures carried out by the operator: - The Director of Engineering completed a base maintenance check and reminded personnel of the importance of completing work to an acceptable standard, following implemented procedures, and reporting or rectifying any findings. - The Director of Engineering reminded those involved in Weekly or Daily Inspections of the importance of completing each step correctly, reporting any issues or findings, and correctly identifying required rectification. - The Director of Engineering implemented a program to inspect all door edges for nicks or deformations that may impede or prohibit pressurization, inspect the cargo door seal for abrasions or cuts, and perform a pressurization leak check in accordance with AMM 05-51-91, recording the associated leakage rate or figures. The Commission advised no additional safety recommendations.