Cabin Altitude Warning at FL350
A twin-engine business jet with two pilots and six passengers is cruising at FL350 when the cabin altitude aural warning sounds. The cabin is at 9 200 ft and climbing steadily at about 1 100 ft/min. The outflow valve position indicator shows the valve open about one-third, and in manual mode the valve does not respond to the switch. Two resets of the pressurisation controller have had no effect. Maximum differential pressure is 9.0 psi. The crew can achieve 6 000 ft/min descent with idle thrust and speedbrakes, scheduling M 0.80 above 10 000 ft and 250 KIAS below. Crew quick-donning masks are available with 45 minutes of oxygen at 100 percent demand; passenger drop-down masks are chemical generators with a 12-minute duration. The nearest suitable aerodrome is 110 NM ahead. The operator's dispatcher suggests continuing to destination at FL250 to save fuel, because the passengers have masks.
1. Using the numbers given, compare the time available before the cabin reaches 14 000 ft with the time needed to reach FL100, and state the immediate action.
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Time available. From 9 200 ft to 14 000 ft is 4 800 ft at 1 100 ft/min = 4.4 minutes, assuming descent starts instantly.
Descent time. FL350 to FL100 is 25 000 ft at 6 000 ft/min = 4.2 minutes. On paper that is marginal; but nothing happens instantly. Masks, the checklist and the radio call cost 30–45 seconds, during which the cabin gains another 550–800 ft. Every 30 seconds of delay takes roughly 550 ft off the margin.
More precise treatment. Because the valve is jammed partly open, the cabin tracks towards the aircraft altitude. Solve 35 000 − 6 000t = 9 200 + 1 100t, giving t ≈ 3.6 minutes and a cabin peak near 13 200 ft - only 800 ft below the limit. Add 45 seconds of delay and the peak is about 13 900 ft.
Immediate action. Crew masks on and 100 percent oxygen, establish communication, then descend immediately - do not complete the non-normal checklist first. Descend to 10 000 ft or below, where the cabin altitude cannot exceed 10 000 ft and no oxygen is required; leave the thrust and speedbrakes set until the cabin stops climbing. Passengers will not be on generators until the cabin passes about 14 000 ft, and the generators give only 12 minutes, so the descent must be flown to completion, not started and then levelled.
2. What does the unresponsive manual mode tell you about the failure, and what pressure limits must the crew protect during the descent?
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What it tells you. Manual mode is the intended fallback when the automatic controller fails: the switch drives the valve directly. If the valve does not move in manual either, the fault is downstream of the controller - the valve actuator or the valve itself is jammed. Repeated resets cannot fix a mechanical jam, and forcing or cycling the switch wastes the time the oxygen is buying.
Which failure is safer. A valve jammed open is the benign case: the cabin climbs, but it cannot over-pressurise, and the differential will fall as the aircraft descends. A valve jammed closed is far worse: no outflow, and the crew must then control the cabin by careful descent or by using the other outflow path, or the cabin pressure will exceed the differential limit.
Limits to protect. Maximum differential is 9.0 psi. With the valve open, the danger is not positive differential but the crew's instinct to close it: if the valve is subsequently driven shut at low altitude, the cabin can become over-pressurised, and a rapid descent with a closed valve can drive the cabin negative relative to ambient, which damages the pressure hull and doors. Descend smoothly, monitor the differential gauge and cabin rate, and if the valve later responds, control the cabin rate rather than chasing a number.
Crew discipline. One pilot flies and manages the descent and the pressure gauges; the other manages oxygen, passengers and ATC.
3. Assess the dispatcher's proposal to continue at FL250.
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The proposal should be rejected. With the outflow valve jammed open, the aircraft is effectively unpressurised: the cabin altitude will equal the aircraft altitude. At FL250 that means all eight occupants at a cabin altitude of 25 000 ft.
- Above roughly 10 000 ft cabin altitude, supplementary oxygen must be available to every occupant. The passenger chemical generators last 12 minutes; that is a descent/escape supply, not a cruise supply, and it cannot be reset or replenished in flight.
- Crew quick-donning masks give 45 minutes at 100 percent demand - not enough for a 110 NM cruise at FL250 plus an approach, and the crew would be degraded by then anyway.
- The decompression is not the only risk: at FL250 with an open valve there is no barrier left if anything else goes wrong (engine failure, further leak, smoke), and the whole point of the abnormal configuration is to minimise time at altitude.
Correct plan. Descend to 10 000 ft or below (where no pressurisation is needed), then either land at the nearest suitable aerodrome - 110 NM ahead is well within range at low level - or, if the operator's performance analysis shows terrain and fuel permit, continue at or below 10 000 ft to destination. The fuel saving does not buy back the loss of the oxygen barrier.
The commander, not the dispatcher, decides; the dispatcher has no visibility of the cabin altitude trend.