The Beech Super King Air 200 aircraft was on an aeromedical flight from Darwin to Gove. The flight had been planned to maintain a cruise level of flight level (FL) 150. The crew consisted of the pilot and a flight nurse. An off-duty company pilot was also on board for the flight back to Gove and occupied the co-pilot's seat. During the climb the pilot carried out the normal transition altitude checklist actions. The actions included checking the pressurisation system for normal operation and that the engine bleed air switches were selected to "ON". The crew oxygen mask in-line indicators were both noted to be "green", and the position of the ceiling mounted oxygen control was also visually checked.
At FL130, the pilot contacted the air traffic controller and amended the planned cruise level to FL270. As the aircraft passed through FL254 he noticed that the left and right "Master Warning" captions, positioned on the instrument coaming, had begun to flash. The cabin "ALT WARN" annunciator was also illuminated, indicating the cabin altitude rising above 12,500 ft, and that the cabin ceiling mounted passenger oxygen masks had automatically deployed. The cabin pressure altitude alert system was set to activate at 12,500 ft.
The pilot, the passenger and the flight nurse immediately donned oxygen masks, with the pilot carrying out the applicable "Phase One" actions for a loss of cabin pressurisation. The pilot then contacted the air traffic controller and gained clearance for an immediate descent to FL210.
During the descent the air traffic controller made several unsuccessful attempts to contact the aircraft and, when the aircraft was observed to descend through FL200, an "Alert Phase" was declared. As the aircraft neared FL190, contact was re-established with the pilot. The aircraft was then climbed back to FL210, as the cabin pressurisation had stabilised and was maintaining a cabin pressure altitude of between 8,000 and 9,000 ft. The flight was continued to Gove.
Prior to landing, the pilot decided to again climb the aircraft to assess at what altitude the pressurisation operation began to malfunction. The pilot contacted the air traffic controller, and was given a clearance to operate "not above FL150" while the problem was assessed. It was found that by FL140 the cabin pressure was less than normal, and after less than a minute of maintaining level flight at that altitude, the cabin altitude began to climb. The aircraft was then descended and a normal landing carried out. The alert phase was cancelled.
An initial examination of the aircraft by maintenance personnel found numerous small pressurisation leaks from the airframe. The aircraft was then flown back to Darwin for a more thorough examination. During that examination several other leaks were evident, including a split main cabin door seal. All leaks were repaired and the door seal replaced. The left bleed air flow pack also had a leaking seal joint on the venturi body and the left flow pack pneumostat was faulty. The flow pack and pneumostat were replaced and the aircraft returned to service.
The pilot indicated that the reason he did not level the aircraft at FL210, as originally cleared by air traffic control, was that he had been fully occupied with the pressurisation problem and with going through the "Phase Check" manual during the descent. Due to that action he had forgotten to set the altitude select function "ALT SEL" on the autopilot as he normally did. That omission had prevented the autopilot "auto capturing" the assigned altitude during the descent, as he expected, and the aircraft was allowed to descend lower than intended. He reported that he had set the aural altitude alert system correctly, but due to his concentration on the pressurisation situation he did not notice the alarm.
|Date:||05 June 2001||Investigation status:||Completed|
|Time:||2325 hours CST|
|Location:||170 km E Darwin, (NDB)|
|Release date:||20 March 2002||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Beech Aircraft Corp|
|Type of operation||Aerial Work|
|Damage to aircraft||Nil|
|Departure point||Darwin, NT|
|Departure time||2253 hours CST|
|Role||Class of licence||Hours on type||Hours total|