While in cruise at flight level 230, on a flight from Darwin to
Gove, the pilot of an aeromedical Beech Super King Air 200 aircraft
noticed that the cabin altitude gauge was indicating just below
10,000ft and that the cabin differential pressure gauge was
indicating 4.2 pounds per square inch. Normal pressurisation
schedule figures for the aircraft at that altitude were, 6,500ft to
7,000ft cabin altitude and 5.7 pounds per square inch
differential.
The pilot checked for correct selection of the aircraft's
pressurisation controller and informed the flight nurse of the
situation. Shortly after, the CABIN ALT WARN annunciator
illuminated and the passenger oxygen masks deployed. That action
was designed to occur at a cabin altitude of 12,500ft. The pilot
donned a crew oxygen mask before descending the aircraft.
During the descent, the pilot attempted to isolate the problem
by selecting the engine bleed air, for one engine at a time, "off"
then "on". The air for the cabin pressurisation is sourced from the
engine bleed air supply. Each time a system was isolated there was
a corresponding rise in the indicated cabin altitude. Both bleed
air systems appeared to be operating. The pilot levelled the
aircraft at 10,000ft, where the pressurisation system appeared to
operate normally. The pilot returned the aircraft to Darwin.
An initial maintenance investigation, carried out by the
operator, could not replicate the problem. However, subsequent
system testing found that the right environmental bleed air flow
control valve was intermittently regulating at an incorrect
pressure. A replacement valve was fitted. The left flow control
valve remote pneumostat unit was also found to be intermittently
sticking in operation and was removed, cleaned and re-fitted.
A ground pressurisation check of the aircraft identified several
small pressurisation leaks. As a result of that check, the outflow
and safety valves were replaced due to leaks at the valve sealing
surfaces, and several minor airframe pressurisation leaks were also
repaired. Subsequent testing indicated that the system functioned
normally.
The aircraft has since returned to service and the problem has
not re-occurred.