Cabin Altitude Warning at FL350
A transport-category aircraft is cruising at FL350 with the cabin pressurized to a differential pressure of 8.6 psi. The maximum certified differential pressure is 8.8 psi. The cabin-altitude warning activates at 10,000 ft cabin altitude, and the cabin rate-of-climb indicator shows 2,000 ft/min and increasing. The outflow valve position indicator shows the valve fully open. The crew is unable to close the valve manually. The first officer wants to continue to a destination 25 minutes away because the aircraft is above the minimum terrain-clearance altitude. The captain elects an emergency descent. After landing, maintenance finds that the outflow valve actuator linkage is disconnected.
1. Explain why the emergency descent is the correct decision and what structural limits are relevant.
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The emergency descent is correct because the cabin altitude is rising rapidly and the outflow valve cannot be controlled. At FL350, the time of useful consciousness after a rapid decompression is short, and a cabin altitude above 10,000 ft exposes occupants to hypoxia. Continuing for 25 minutes is not acceptable when the pressurization system has failed open. The crew should don oxygen masks, establish communications, and descend to 10,000 ft or the minimum safe altitude, whichever is higher, at an appropriate rate while monitoring structural limits.
The structural limit is the maximum differential pressure of 8.8 psi. The indicated 8.6 psi is close to the limit, and an uncontrolled outflow valve can produce pressure fluctuations. The crew should avoid rapid thrust changes and abrupt manoeuvres that could increase differential pressure or impose additional loads on the pressure bulkheads. The objective is to reduce cabin altitude without exceeding the positive or negative differential-pressure limits.
2. What post-flight structural inspection should be performed after the event?
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After the event, inspect the pressure hull and associated structure for signs of overpressure or negative-pressure damage. Check the forward and aft pressure bulkheads, door and window seals, and fuselage skin around the outflow valve for deformation, popped rivets, cracked sealant, or skin buckling. Inspect the outflow valve and its actuator linkage for the disconnected condition and for secondary damage. Verify that the cabin-pressure controller and relief valves are functioning. If the differential pressure exceeded the limit, a structural inspection per the maintenance manual is required, possibly including eddy-current or visual inspection of high-stress areas. Do not return the aircraft to service until the linkage is reconnected, the system is leak-tested, and the pressure-control system is functionally checked.
3. Assess the maintenance finding and the actions needed before the next flight.
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A disconnected outflow valve actuator linkage is a direct cause of the uncontrolled cabin altitude and is a serious airworthiness defect. The linkage must be reconnected with the correct hardware, rigged per the maintenance manual, and checked for full travel and correct valve indication. A functional test of the pressurization system should include automatic and manual modes, cabin-altitude warning, and relief-valve operation. The cause of the disconnection should be investigated: missing lockwire, incorrect installation, or worn rod-end bearings. If the linkage can disconnect again, the aircraft is not airworthy. After the repair, perform a leak check and verify that the cabin altitude can be controlled. Record the event and the repair. The aircraft should not be released until the system operates normally and the structural inspection is complete.