What happened
On September 29, 2019, a Boeing 737-800, registration SP-ENL, departed from Madrid (LEMD) and reached an altitude of approximately 2,000 ft. During the climb, two cabin crew members heard a loud whistling sound originating from the forward galley area and experienced a rapid drop in temperature. The crew notified the captain, who noted that there were no cockpit indications suggesting the aircraft doors were unlatched.
As the aircraft continued to climb, the crew determined that cabin pressurization could not be maintained in any mode. Consequently, the climb was halted at flight level 100, and the crew executed the "AUTO FAIL or Unscheduled Pressurization Change" procedure. The captain decided to return to Madrid for an emergency landing. Upon arrival, the lead flight attendant discovered a significant gap at the 1L door. Although the door handle was in the horizontal position and the door appeared closed, maintenance personnel confirmed the door had been improperly secured. No ground personnel had reported any issues with the door prior to departure, and the crew had not observed any visible gaps after arming the door.
The investigation
The PKBWL investigation focused on why the door failed to remain sealed despite appearing closed. Maintenance crews performed a visual inspection of the door area, which revealed no obvious defects. A subsequent cabin pressure test performed after re-closing the door yielded a positive result. The investigation examined the door locking mechanism and the sensor configuration.
Findings
- The primary cause of the depressurization was the improperly closed door.
- A foreign object may have prevented the door from locking correctly; this object was likely sucked out of the aircraft during the depressurization event, explaining why it was not found during the post-flight inspection.
- The cockpit failed to receive a warning because the door closure sensor is installed only on the right side of the door frame, leaving the left side unmonitored.