What happened
On August 16, 2006, a Piper PA42-720 was conducting an instrument flight rules (IFR) training flight from Karlsruhe/Baden-Baden to Wunstorf, with a planned continuation to Bremen. The crew consisted of a flight instructor and two flight students, one of whom was occupying the front left pilot seat.
While cruising at FL 220 near the Lichtenau VOR, a student seated behind the pilots noticed that the oxygen masks had fallen out of their holders in the cabin. Shortly thereafter, the cabin warning light illuminated, followed by a master caution alert and an audible warning horn. At this stage, the cabin altitude was approximately 11,000 feet with a pressure differential of about 3 PSI. The crew requested an immediate descent to below FL 100 from Air Traffic Control. During this period, a loud noise was heard from the rear of the aircraft, and the pressure differential dropped abruptly to zero.
The crew initiated an emergency descent procedure, donned oxygen masks, and descended to FL 70. Once the aircraft reached the assigned altitude and the masks could be removed, the crew proceeded to Bremen, where they completed a normal landing via an ILS approach.
The investigation
The BFU investigation focused on the cause of the sudden depressurization. The aircraft was operating in an area with significant cloud cover and moderate icing conditions, with air temperatures between -20 and -22 °C. The investigation examined the aircraft's maintenance history and the specific configuration of the cabin door seal system.
Findings
- The loss of cabin pressure was caused by ice formation within the pneumatic seal of the rear left cabin door.
- A T-shaped fitting with a drain hole had been previously installed in the "Main Cabin Door Seal" air line as part of an engineering instruction to address issues with door seals during temperature fluctuations.
- Due to the positioning of the hose under the cabin floor, the drain hole became blocked.
- Accumulated water in the line froze, obstructing the airflow to the door seal and preventing it from maintaining an adequate seal to hold cabin pressure.