Cabin Altitude Excursion at FL250: Masks, Descent or Diagnosis?
You are PIC of a twin-engine turboprop cruising at FL250 with nine passengers and one cabin crew. The pressurization controller is in AUTO and the cabin has been stable at 7,500 ft with a differential of 5.4 psi. Eight minutes after entering continuous light chop, the cabin vertical-speed indicator shows 900 ft/min climbing, then 1,500 ft/min. The CABIN ALT warning illuminates, the cabin altitude passes 9,800 ft and keeps rising. The outflow valve position indicator shows FULL OPEN although the selector is still in AUTO; holding the switch in MANUAL CLOSED for 20 seconds changes neither the indication nor the cabin rate. Bleed-air pressure, duct temperature, the controller circuit breaker and both generator loads are normal. Crew quick-donning masks are stowed in their boxes. Passenger masks are the drop-down type that deploy automatically at a 14,000 ft cabin altitude. Route MSA is 11,200 ft. The nearest suitable airport is 65 nm ahead; ATC offers FL180 immediately or any lower level you need, and reports no other traffic below.
1. State your first actions in order and explain why each has to come before system diagnosis.
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Oxygen first, then descend, then diagnose. At 25,000 ft the time of useful consciousness is roughly 3 to 5 minutes, and the cabin is already at 9,800 ft and climbing at 1,500 ft/min, so the crew's own ability to think and fly is the first thing at risk. Both pilots don quick-donning masks, select 100% oxygen, establish interphone communication and confirm the other is lucid before touching anything else. Then power to the descent attitude, idle, speedbrake and start down: the memory drill is masks, communicate, descend. Diagnosis (valve position, controller, circuit breakers, door seals) is done while descending, not before. Advise ATC of an emergency descent and the level you need; tell the cabin crew to secure the cabin and prepare the passengers. Only once the descent is established do you run the checklist and consider whether the indication itself might be faulty.
2. Using the figures given, work out how long you have before the passenger masks deploy and describe the descent you would fly, including the altitude you would level at.
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With the cabin at 9,800 ft and rising at 1,500 ft/min, masks deploy in about (14,000 - 9,800) / 1,500, which is roughly 2.8 minutes: under three minutes. A descent at 2,000 ft/min removes only 5,600 ft in that time and the cabin would keep climbing, so the descent must be at the highest rate the airframe allows. Idle power, Vmo/Mmo and speedbrake out give 4,000 to 5,000 ft/min, which takes the aircraft from FL250 to about FL120 in three minutes and pulls the cabin down before it reaches 14,000 ft. I would not level at 10,000 ft: MSA is 11,200 ft, so I would level at 11,200 ft, or the higher of 10,000 ft and MSA, until clear of terrain. If the cabin still goes above 14,000 ft, the masks drop automatically; I would also consider selecting passenger oxygen manually at about 12,500 to 13,000 ft to create margin rather than relying on the automatic trigger.
3. Manual closure had no effect. Give the likely causes, the checks you would make, and what you would tell ATC and the cabin crew.
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Manual closure having no effect points to something downstream of the selector: an outflow valve stuck open or its actuator/motor failed, a controller that has failed with the valve driven open (many valves are fail-safe open), a safety or negative-pressure relief valve stuck off its seat, or a door or skin leak large enough to outrun the valve. An indication fault is possible, but the cabin rate and the warning corroborate a real loss of pressure. Checks: cabin pressure gauge against cabin altitude, door annunciators, the DUMP switch, pull and reset the controller circuit breaker, then try MANUAL again once. Do not sit there recycling the switch. Decide now to land at the nearest suitable airport and tell ATC: emergency descent, pressurization failure, request priority and an expedited approach. Tell the cabin crew to expect the mask-deployment noise, to sit down and to secure the cabin, and ask for medical services on arrival if anyone shows signs of hypoxia.