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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.

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