Pressurisation Controller Fault After Takeoff
A twin-turboprop is climbing through FL180 with the cabin altitude holding at 6,500 ft and a differential pressure of 5.2 psi. The outflow valve is electrically driven and controlled by a single automatic controller with a manual backup. The crew notices the cabin altitude begins climbing at 800 ft/min while the aircraft itself climbs at 1,200 ft/min, so the differential pressure stays roughly constant. No master warning is present, and the outflow valve position indicator shows the valve partially open and not responding to the controller's demand. Engine bleed pressure and air-conditioning pack flow indications remain normal. The aircraft's maximum differential is 6.6 psi, and the structural damage tolerance limits at this pressure are unknown to the crew. The destination is 40 minutes away at FL250.
1. Diagnose the most probable fault and explain why the indications rule out a bleed or pack failure.
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The key clue is that differential pressure is holding steady while cabin altitude rises at almost the same rate as aircraft climb. If a pack or bleed had failed, cabin altitude would rise faster than aircraft altitude and differential pressure would fall, because less conditioned air would be entering the cabin and the outflow valve would be trying to hold a pressure it cannot achieve. Here the valve is partially open and not responding to controller demand, which points to a controller, valve actuator, or valve position feedback fault rather than an air supply failure. The controller is commanding a valve position that the valve does not achieve, so the cabin leaks at a rate that tracks aircraft climb. A manual backup exists, so the crew should select manual control and close the valve progressively while monitoring cabin altitude and differential pressure. This is a judgement call: the crew must decide whether to continue to destination or divert based on whether manual control restores a stable cabin altitude.
2. State the operational decision and the monitoring parameters that would justify continuing to FL250.
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If manual control can hold cabin altitude at or below 8,000 ft and differential pressure below the 6.6 psi limit, continuing at FL250 is reasonable with continuous monitoring. The crew should watch cabin altitude, differential pressure, outflow valve position, and cabin rate of climb, and should brief the cabin crew on a possible rapid descent. If manual control cannot hold cabin altitude below 10,000 ft, the crew should descend to a level where cabin altitude can be maintained without exceeding differential limits, and should consider diversion. The decision must not rely on the absence of a master warning, because a single controller fault may not trigger one. The crew should also consider that a partially open valve at high differential pressure stresses the structure, so reducing differential pressure by descending is the conservative option if control is uncertain.