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.