Pressurisation Failure and Oxygen Planning at FL350
You are cruising at FL350 in a pressurised twin-jet with 180 passengers and 6 crew. The cabin altitude is normally 8000 ft. Suddenly the cabin altitude warning sounds and the cabin climbs at 1500 ft/min. The captain initiates an emergency descent to FL100. The aircraft carries 12 minutes of supplemental oxygen for the crew at 100% and passenger chemical generators rated for 15 minutes. The nearest suitable airport is 85 NM away; descent to FL100 takes 6 minutes at the planned rate, and the subsequent cruise to the airport at FL100 takes 14 minutes. The cabin reaches 14000 ft 4 minutes after the warning.
1. Determine the time available before the cabin exceeds 14000 ft and assess whether the crew oxygen supply is sufficient for the descent and diversion. State the regulatory basis for your reasoning.
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The cabin starts at 8000 ft and climbs at 1500 ft/min. To reach 14000 ft requires (14000 − 8000)/1500 = 4 minutes. That matches the stated 4 minutes. Above 14000 ft, crew must be on oxygen; but the crew should don masks immediately at the warning, not wait.
Crew oxygen: 12 minutes at 100%. The emergency descent to FL100 takes 6 minutes, and the diversion at FL100 takes 14 minutes - total 20 minutes. However, once the cabin is below 10000 ft (or the aircraft is at FL100 with a safe cabin altitude), supplemental oxygen is no longer required for the crew. The descent to FL100 takes 6 minutes, so the crew needs oxygen for only about 6 minutes (plus a prudent margin). 12 minutes is sufficient for the descent with a 6-minute reserve.
Passenger oxygen: chemical generators give 15 minutes. Passengers need oxygen while the cabin is above 14000 ft, which is from 4 minutes after the warning until the cabin descends back through 14000 ft. During the emergency descent at, say, 6000 ft/min, the cabin follows down; the time above 14000 ft after the peak is short - likely under 3 minutes. 15 minutes is ample. The key regulatory point: 14 CFR/EASA-style rules require 10 minutes of passenger oxygen above 14000 ft, and crew oxygen sufficient for the descent profile; both are satisfied. The judgement is to don crew masks first, then start the descent, and not to delay the descent to troubleshoot.
2. During the descent, a passenger becomes unconscious. What is your priority order, and how does this change your diversion decision?
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Priority order: aviate, navigate, communicate - the descent must continue because the cabin altitude is the root cause. Do not level off or slow the descent to attend to the passenger. Once at FL100 and the cabin is safe, the unconscious passenger should be managed with the onboard medical kit and, if a doctor is aboard, their assistance; consider whether the passenger needs oxygen from a portable bottle.
The diversion decision: the nearest suitable airport at 85 NM is already your destination. An unconscious passenger is a medical emergency but not necessarily a reason to change to a different airport unless a closer one with better medical facilities exists. If the current diversion airport has medical services, continue; if not, and a closer airport with a hospital exists within your fuel and performance limits, consider it. Communicate with ATC (PAN PAN or MAYDAY as appropriate), request priority and medical services on arrival. Do not compromise the emergency descent for the passenger - the entire cabin is at risk until the cabin altitude is reduced.