Cabin Altitude Rising at FL370 With a Partly Open Outflow Valve
A twin-jet is climbing through FL370 with 180 passengers. The cabin altitude is 6,500 ft and rising at 700 ft/min. Differential pressure is 7.4 psi and decreasing. Both engine bleed pressures are normal, pack outlet temperatures are normal, and there is no smoke or engine abnormality. The outflow valve position indicator shows about 60% open; in a normal climb it should be modulating near the closed position. The cabin pressure controller is in AUTO, with MANUAL available. Maximum structural differential is 8.6 psi. The cabin altitude warning activates at 10,000 ft, and passenger oxygen masks deploy automatically at 14,000 ft. The captain wants to continue to the destination, 55 minutes away, because ‘the pressurisation is only a little slow.’ The first officer suggests descending. The aircraft is over mountainous terrain with an MSA of 16,000 ft; the nearest suitable airport is 20 minutes away at FL250.
1. Identify the most likely fault and explain why a bleed or pack failure is less likely.
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The most likely fault is an outflow valve or pressurisation controller fault causing excessive outflow. The outflow valve is about 60% open in a climb, when it should be nearly closed, and that directly explains the falling differential pressure and rising cabin altitude. Both bleed pressures are normal and pack outlet temperatures are normal, with no smoke or engine abnormality; a bleed or pack failure would normally show abnormal bleed pressure, pack temperature, or flow indications. A pack failure can reduce inflow, but the abnormally open outflow valve is the positive indication here. The crew should not assume the problem is ‘slow pressurisation’ and continue; the cabin altitude is rising at 700 ft/min and will reach 10,000 ft in about 5 minutes.
2. State the immediate actions and the altitude strategy, taking account of the MSA.
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The crew should don oxygen masks, establish interphone communication, and check mask and hose connections. Select MANUAL on the pressurisation controller and attempt to close the outflow valve; if the valve responds, control cabin altitude and differential pressure within limits. If it does not respond, declare an emergency and descend. The descent should be to the lowest safe altitude, but the MSA is 16,000 ft, so a descent to 10,000 ft is not available over this terrain. The practical target is FL250 initially, then the MSA or the nearest suitable airport as soon as possible. Continuing at FL370 is unsafe because the cabin altitude will exceed 10,000 ft and may reach 14,000 ft, triggering passenger masks and exposing everyone to hypoxia.
3. If the outflow valve cannot be controlled and cabin altitude reaches 14,000 ft over the mountains, what should the crew do?
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At 14,000 ft cabin altitude, passenger masks will deploy and the crew must be on 100% oxygen. The crew should establish crew communication, confirm oxygen flow, and make an emergency descent toward the MSA and the nearest suitable airport. The aircraft cannot descend to 10,000 ft because the MSA is 16,000 ft; the crew must accept a cabin altitude near the aircraft altitude if the valve is stuck open, but must limit exposure time and land as soon as possible. They should declare MAYDAY, advise ATC of the cabin altitude and intended diversion, and prepare for a possible emergency evacuation if the cabin altitude cannot be controlled. The captain’s original plan to continue for 55 minutes is not acceptable.