Cabin Depressurization During IFR Training Flight in a Piper PA-42-720

Incident Overview and Flight Details On 16 August 2006, a Piper Aircraft Corp. PA42-720 was conducting an instrument flight rules (IFR) training flight from Karlsruhe/Baden-Baden to Wunstorf, with a planned continuation to Bremen. The aircraft was crewed by a flight instructor and two students, one of whom occupied the left front pilot seat. The flight was en route at flight level 220 when a cabin pressure loss occurred. ## Sequence of Events Near the Lichtenau NDB, on the segment from Fulda VOR (FUL) to waypoint ROBEG, a student seated behind the pilots reported that oxygen masks had deployed in the cabin. Approximately two seconds later, the red cabin warning light illuminated, followed three seconds later by the master caution light and warning horn. The cabin altimeter indicated a cabin altitude of 11,000 feet, with a differential pressure of approximately 3 psi. Air traffic control granted clearance for an immediate descent below FL100 due to a technical problem. During this period, a loud noise was heard in the rear of the aircraft, and the differential pressure dropped abruptly to zero. The flight instructor initiated the emergency descent procedure. Air traffic control cleared the descent to FL70 with a 10-degree course change. The crew donned oxygen masks and commenced the immediate descent. ## Landing and Personnel After reaching the assigned flight level and removing the masks, the crew decided to proceed directly to Bremen, where they landed normally following an ILS approach. The 42-year-old flight instructor held an ATPL and had approximately 4,800 total flight hours. The 27-year-old student pilot held a private pilot license and had approximately 200 hours as pilot-in-command, including 16 hours on the PA42. ## Aircraft and Maintenance Findings The 1987 Piper PA-42-720 Cheyenne, serial number 42-5501042, was powered by two Pratt & Whitney PT6A-61 turboprop engines and had a total operating time of 18,432 hours at the time of the incident. The last airworthiness certificate was issued on 22 December 2005 at 18,032 hours. The aircraft was not equipped with a flight data recorder or cockpit voice recorder, as these were not required. ## Meteorological Conditions According to the German Weather Service (DWD), the flight path was on the leading edge of a deep low-pressure system centered over the Bay of Biscay. The aircraft likely flew in clouds between FUL and ROBEG at FL220. The freezing level was between 2,500 and 3,000 meters above sea level (approximately FL100), with moderate icing within the clouds. The air temperature was approximately -20 to -22 °C. ## Probable Cause The maintenance organization determined that the cabin pressure loss was caused by icing within the pneumatic seal of the rear left entry door. A T-fitting with a drain hole had been installed in the main cabin door seal air line to address issues with the door seal during large temperature fluctuations. A loose hose with the T-fitting under the cabin floor blocked the drain hole, preventing accumulated water from draining. When the water froze, the ice blocked the air supply to the door seal, preventing it from maintaining an adequate seal and cabin pressure.