Night VFR Return with a Failing Cabin Altitude Warning
You are pilot flying a twin turboprop at FL200 on a night VFR sector, 40 minutes from destination. The cabin altitude warning horn sounds intermittently; the cabin altimeter reads 9,500 ft and climbing slowly. The outflow valve position indicator shows no movement when you cycle the switch. The first officer reports mild tingling in the fingertips and says the cockpit seems dimmer than before. You feel slightly euphoric and have just noticed you have been reading the same checklist line twice. Outside air temperature is -25 C, terrain along the route rises to 8,000 ft, and the nearest suitable alternate is 25 minutes away. You have 20 minutes of oxygen in the crew masks at present flow.
1. Assess the physiological situation and decide the first three actions in order, justifying each.
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The picture is a slow cabin pressurisation failure with early hypoxia at about 9,500 ft cabin altitude, plus a probable hyperventilation component from stress. Tingling, dimming vision and euphoria are classic hypoxia signs; re-reading the checklist line is a reliable self-check for cognitive impairment. At 9,500 ft the time of useful consciousness is long (hours), but impairment begins well before unconsciousness, so the priority is to stop the deterioration, not to wait for symptoms to become dramatic.
Actions in order:
- Don oxygen masks and establish 100 percent oxygen, both crew, and confirm flow. This reverses hypoxia within seconds and restores judgement.
- Level off or descend to a cabin-safe altitude, and run the pressurisation failure checklist. Descending to 10,000 ft or below (terrain permitting) removes the need for supplemental oxygen once masks are off.
- Declare the situation to ATC and revise the plan. Night, terrain at 8,000 ft and 20 minutes of oxygen mean you must treat the destination as unattainable unless descent is rapid. The 25-minute alternate is beyond your oxygen endurance at present flow, so either descend first or select a closer field.
Key judgement: the euphoria and repetitive reading mean you cannot trust your own assessment until oxygen is on. Do not troubleshoot the valve before masks are on.
2. The first officer wants to continue to destination because the cabin altitude is rising slowly and the weather is better there. How do you evaluate this proposal?
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Reject it, and explain why in operational terms rather than by rank. Three factors combine against continuation:
- Trend, not level: the cabin altitude is climbing and the valve is not responding, so the rate may accelerate. A slow leak at FL200 can become rapid if the seal fails further.
- Oxygen endurance: 20 minutes at present flow is a hard limit. Destination is 40 minutes away, so continuation guarantees you run out before landing unless you descend below 10,000 ft, which itself ends the cruise plan.
- Degraded crew: one pilot has symptoms and both have been breathing hypoxic air. Night VFR into rising terrain with impaired cognition is a high-risk combination even if weather is good.
The correct decision is to descend to a safe altitude, land at the nearest suitable aerodrome, and have the aircraft inspected. The first officer's preference is understandable but is anchored on weather rather than on the limiting resource, which is oxygen and crew performance. Frame the decision around the limiting factor so it is a shared conclusion, not an order.