What happened
On June 22, 2001, a scheduled passenger flight operated by a commercial airline from Munich to Hamburg experienced a sudden loss of cabin pressure while cruising at flight level 350. The Boeing 737-31S experienced a rapid decompression at a rate exceeding 2,000 feet per minute, causing the cabin altitude to rise to 10,000 feet. In response to the depressurization, oxygen masks deployed automatically, and the crew performed an emergency descent to flight level 100. The aircraft continued the flight to Hamburg, landing without further incident. There were no injuries among the 126 passengers or the 5 crew members on board.
To investigate the cause, the operator conducted a ferry flight at flight level 90 without pressurization to Munich. During the troubleshooting process, technicians identified defects in two pressure seal clamps located outside the cabin, which seal the APU air duct penetrations. One clamp at the aft pressure bulkhead had slipped from its sealing sleeve, and a second clamp in the main wheel well area was found to be broken.
The investigation
The BFU investigation focused on the failure of the pressure seals. Following the incident, the airline inspected its entire Boeing 7 37 fleet. This inspection revealed that a majority of the aircraft had clamps that were insufficiently tightened. Two specific aircraft were found to have the exact defects that caused the decompression event on June 22.
Findings
- The rapid decompression was caused by the failure of two pressure seal clamps used to seal the APU air duct penetrations.
- The investigation determined that the defects likely originated from manufacturing issues at the aircraft manufacturer.
- A critical contributing factor was that the specific locations of these clamps were not included in the manufacturer's Maintenance Planning Document, meaning they were not subject to routine inspections.
Safety action
Following the investigation, the BFU issued a safety recommendation to the LBA (Luftfahrt-Bundesamt). The recommendation requested that all operators of Boeing 737 aircraft within the authority's jurisdiction be instructed to perform detailed visual inspections of the pressure seal clamps located at the APU air duct penetrations in both the aft pressure bulkhead and the main wheel well area.