Cabin Differential Falling While the Outflow Valve Says CLOSED
A twin-turboprop commuter aircraft with a maximum cabin differential of 7.8 psi is climbing through FL200 with 19 passengers on board. The cabin altimeter reads 8,200 ft and is rising at about 700 fpm; the cabin vertical speed indicator agrees. Cabin differential pressure is 6.9 psi and slowly decreasing. Both engine bleed valves indicate open, both packs are delivering air with outlet temperatures of 18 °C and 20 °C, and the outflow valve position indicator shows CLOSED. The controller is in AUTO with the rate selector at the index mark, and no warning has appeared yet; the cabin altitude warning is set at 10,000 ft. Outside air temperature is −22 °C and the aircraft is cleared to FL240. The captain wants to keep climbing because the destination is forecast below alternate minima and fuel is tight.
1. The outflow valve indicates CLOSED. What does the falling differential pressure tell you, and what leak paths would you suspect, in order?
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With both packs delivering and no other demand on the air, a genuinely closed outflow valve would let cabin pressure build towards the 7.8 psi limit during the climb. A differential that falls while the aircraft climbs means outflow exceeds inflow - the hull is leaking faster than the packs can supply. Because the indicator says CLOSED, the first suspicion is that the indication is not the valve: a slipped rigging or torque tube, a failed position transmitter, or a valve seated on a damaged seal. Next, the safety/relief valve off its seat - a stuck negative-pressure relief valve is a classic slow leak. Then door and window seals (a passenger door not fully latched on the last turn), and only then a skin penetration or the aft pressure bulkhead, which is unlikely without a prior event. The size of the leak can be judged from the cabin rate at a known pack flow.
2. What should the crew do now, and why is continuing to FL240 not an option?
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Cabin altitude is 8,200 ft at 700 fpm; the 10,000 ft warning is roughly two and a half minutes away, and the cabin would pass 14,000 ft about eight minutes after that - hypoxic territory with 19 passengers. Stop climbing and descend. A descent reduces the differential the packs must hold, so the cabin rate should reverse within a few thousand feet; a level around FL140–FL160 will usually hold a cabin near 8,000 ft with a moderate leak. Advise ATC, request an immediate lower level and a diversion, and treat it as an urgency. Crew oxygen masks at 10,000 ft cabin, passenger masks if the cabin passes 14,000 ft. Continuing to FL240 increases the differential required and shortens the time available - it converts a controllable leak into an emergency.
3. On the ground, what test would you run to isolate the fault, and what would you sign in the log?
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Run a leak check, not a guess. Per the AMM: blank or cap the safety valve, pressurise with a ground cart or APU bleed, and measure the time for the differential to decay between two set values; compare with the limit. If it fails, isolate with soap solution on the outflow valve seat, the door and window seals and the safety valve, taping and testing each door in turn. Check outflow valve rigging and position transmitter calibration against the AMM, and inspect the seal and torque tube for wear. Then repeat the leak check to within limits, check the controller in manual and auto, and test the 10,000 ft warning circuit. The log entry must identify the defect, the corrective action and the AMM reference; the aircraft should not be released on a 'monitor' basis.