Cabin Altitude Climb After One Pack Trips
At FL250 in a pressurised twin turboprop, the cabin altitude is 8,000 ft and rising at 350 ft/min. The left engine bleed pressure is low and the left pack valve has closed automatically. The right pack is in high flow and the right bleed pressure is normal. The outflow valve indication is 45% open. The cabin pressure controller is in AUTO. The differential pressure is 5.2 psi, below the 7.5 psi limit. Outside temperature is -35°C. There are 2 crew and 9 passengers. The passenger oxygen system uses chemical generators rated at 22 minutes; crew masks are supplied from a 15-minute cylinder at 100% demand. The nearest suitable airport is 18 minutes away at FL200; destination is 55 minutes away. The aircraft is stable, but the cabin altitude rate has increased to 500 ft/min over the last 2 minutes. No smoke or abnormal engine indications are present.
1. Interpret the pressurisation problem. What is the most likely reason the cabin altitude is climbing, and what checks would you make before selecting manual outflow-valve control?
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The left pack has dropped offline, but the right pack in high flow should still hold the cabin at FL250 if the outflow valve closes. A valve stuck at 45% open, or a controller driving it open, is more likely than simply one pack failed. Before manual control, confirm that the outflow valve indication actually moves when commanded, and check cabin differential pressure, cabin rate, both bleed pressures, pack outlet temperatures, and door or seal leakage. Then select manual and close the valve slowly while monitoring differential pressure; do not exceed 7.5 psi. If the cabin rate does not improve, treat it as a fuselage leak and descend. A 500 ft/min climb at altitude is significant and should be acted on before the cabin reaches 10,000 ft.
2. At what cabin altitude would you don oxygen masks, and what is your plan? Include a simple oxygen-endurance check.
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Put crew masks on at 10,000 ft cabin altitude, or earlier per SOP; at 500 ft/min from 8,000 ft that is about 4 minutes. Passenger generators give 22 minutes, so the plan must be a controlled emergency descent, not a continued cruise. From FL250 at 2,000 ft/min to 10,000 ft takes about 7.5 minutes. The nearest airport is 18 minutes away, which fits the passenger oxygen, but the crew cylinder is only 15 minutes at 100% demand, so crew endurance is the limiting factor. Divert to the 18-minute airport, descend at the maximum practical rate, and land. Do not press on for 55 minutes to destination.