What happened
On September 13, 2021, a Boeing 737-400-42C, registration EI-STL, operated by ASL Airlines Ireland, was performing a flight from Tallinn (EDDT) to Krakow (EPKT). Approximately six minutes after reaching a cruising altitude of FL370 and crossing the Polish border, the crew received a cabin altitude warning indicating the pressure had risen above 10,000 feet.
Recognizing the symptoms of decompression, both pilots donned oxygen masks. While the first officer performed memory items from the Quick Reference Handbook (QRH), the commander initiated an emergency descent. During this maneuver, the crew increased speed to 330 knots, resulting in a significant pitch-down attitude and a vertical descent rate reaching 7,000 feet per minute. During the descent, both pilots experienced ear pressure changes, and the first officer reported stomach discomfort.
After the aircraft stabilized at FL100 and the cabin altitude was controlled, the crew utilized the TDODAR decision-making model to evaluate their options. Given the aircraft's fuel status and weather conditions, the crew decided to cancel the Mayday declaration and proceed to the destination. The aircraft subsequently landed at EPKT under CAT II conditions without further incident.
The investigation
The investigation focused on the mechanical failure that led to the loss of pressure and the subsequent flight dynamics during the emergency descent. The commission reviewed technical logs and maintenance records to determine the origin of the depressurization.
Findings
- The primary cause of the loss of cabin pressure was a failed cabin pressure control panel.
- The incident was exacerbated by the fact that several previous pressurization system malfunctions had been recorded in the aircraft's technical logs prior to this event.
Safety action
Following the investigation, the operator's Director of Engineering performed a base maintenance check and reinforced the necessity of performing all inspections to an acceptable standard. Specific instructions were issued regarding the accuracy of daily and weekly inspections. Additionally, a new inspection program was implemented to check all door edges for deformations, inspect cargo door seals for abrasions, and perform pressurized leak checks in accordance with the Aircraft Maintenance Manual (AMM).