What happened
On July 13, 2003, a McDonnell Douglas MD-82, registration LN-RML, operated by SAS Scandinavian Airlines System as flight SK 682, experienced a significant cabin altitude climb while cruising at FL 320. The aircraft was en route from Rome to Copenhagen when the crew noticed a loss of cabin pressure and an unusual ear pressure change.
As the cabin altitude rose at a rate of approximately 1000 feet per minute, the flight crew executed emergency checklists and declared an emergency. During the descent, as the aircraft passed through 17,000 feet, the passenger oxygen masks deployed automatically. The crew diverted the aircraft to Zurich, performing an uneventful approach and landing on runway 14. While there were no fatalities or serious injuries, one passenger required medical attention for ear pain caused by barotrauma.
The investigation
Investigators examined the technical failure of the air-conditioning systems and the electrical signals that triggered the shutdown. The investigation focused on the ACARS Management Unit (MU) DLM-900 and its interaction with the aircraft's air-conditioning auto shutdown relays.
Technical analysis revealed that the left and right air-condition systems shut down because they received an incorrect ground discrete signal. This signal was generated by a failure within the ACARS MU. Further inspection of the replacement unit revealed an internal short-circuit on a power supply circuit board. The investigation also noted that the communication between Air Traffic Control and the airport ground authorities was not optimal, leading to a delay in passenger care upon arrival.
Findings
- The simultaneous shutdown of both left and right air-conditioning systems was triggered by an incorrect ground discrete signal sent to the auto shutdown relays.
- This erroneous signal originated from an internal short-circuit within the ACARS MU DLM-900.
- The electrical failure was specifically caused by the failure of capacitor C20 in the power supply section due to voltage transients, which damaged circuit A4U28.
- The flight crew's decision to perform an emergency descent and divert to Zurich was appropriate and necessary due to fuel constraints for a low-level flight.