The Slow Leak: A Decompression Hidden Behind a Passenger 'Hyperventilating'
A wide-body is cruising at FL370 from Europe to Mauritius, about three hours from destination. The cabin altitude log shows 6,500 ft at 06:00Z, 7,400 ft at 06:15Z and 8,200 ft at 06:20Z, with the outflow valve indicating a partial seal failure and a steady rise of about 90 ft/min. For the last 25 minutes the crew has been dealing with a passenger who complained of nausea, dizziness and tingling in the hands and around the mouth, and whom the cabin crew believe is hyperventilating; the passenger was given a paper bag to breathe into. At 06:35Z the cabin altitude warning sounds at 10,000 ft. The first officer reports a mild headache and tingling in the fingers. The captain feels well.
1. Explain why a 90 ft/min leak was not recognised, and what the rate implies about the time available.
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A 90 ft/min rise is roughly 1.5 ft per second. There is no sudden noise, no fog, no ear discomfort, no obvious rush of air - the classic cues of an explosive decompression are all absent. The only indication is a slowly climbing cabin altimeter and an outflow valve position that a busy crew reads as 'a bit out of the ordinary'. The crew's attention was fully occupied by the medical case, which is exactly the distraction that allows a slow leak to run.
Time available
From 8,200 ft at 90 ft/min to 10,000 ft is 1,800 / 90 = 20 minutes. From 6,500 ft the crew had about 39 minutes before the warning. That is a long time - which is the trap: it feels like there is no urgency, when in fact the aircraft has been at an effective cabin altitude of 6,500–9,000 ft for most of an hour.
Why the threshold matters
At 8,000 ft the crew should have treated the rise as a pressurisation malfunction: confirm both cabin altimeters, check the outflow valve position and the bleed configuration, run the checklist, and plan the descent while there is still a large margin. A slow leak is not dangerous because it is fast; it is dangerous because it is slow enough to be ignored until the warning does the recognition for the crew.
2. Interpret the passenger's and the first officer's symptoms. Why does hypoxia mimic hyperventilation?
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The physiology
Hypoxia at a cabin altitude of 8,000–10,000 ft produces exactly the symptom cluster described: headache, dizziness, nausea, tingling and numbness in the fingers and around the mouth (paraesthesia), and a feeling of breathlessness that leads to increased breathing rate. The increased breathing rate washes out carbon dioxide, which produces the tingling and carpopedal spasm of hypocapnia - a picture indistinguishable from hyperventilation. Breathing into a paper bag treats hypocapnia; it does nothing for hypoxia and may worsen it by adding rebreached CO2 to an already hypoxic person.
The clues that should have redirected the crew
- The passenger did not improve on the bag.
- A second person (the first officer) developed the same tingling and a headache at the same time - a medical cause does not usually affect a passenger and a crew member simultaneously.
- The cabin altitude trend was already on the panel.
Crew performance
At 8,000–10,000 ft cabin altitude, healthy crew members lose night vision first, then judgement and coordination; the first officer's symptoms indicate that the effective cabin altitude had been degrading performance for some time. The captain 'feeling well' is not evidence: hypoxia impairs the very faculty used to assess it, and the person least affected is the least able to notice.
3. State the actions required now that the warning has sounded, and justify the descent decision.
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Immediate actions (memory items)
- Both pilots don oxygen masks and establish communication; select 100% oxygen with the emergency selector as required.
- Confirm the cabin altitude and the rate of climb, and check the mask seal and the regulator supply.
- Run the decompression drill: confirm the pressurisation mode, check the outflow valve, select the appropriate bleed configuration, and advise the cabin crew to don their portable oxygen and check the passenger oxygen masks.
- Advise ATC and obtain a clearance for the descent; if no clearance is available, declare an emergency.
The descent decision
With a cabin altitude that will keep rising at 90 ft/min, the aircraft must descend. The target is the highest altitude that will keep the cabin at or below about 8,000 ft, or the minimum safe altitude for the route - over the Indian Ocean, terrain is not the constraint, so this will typically be a level in the 8,000–10,000 ft range rather than a rush to 10,000 ft. Do not descend so low that fuel and the subsequent drift become a new problem, but do not delay the descent waiting for a smooth clearance either.
Why the descent is not the whole answer
At a 10,000 ft cabin altitude there is no time-of-useful-consciousness problem - the crew has minutes and hours, not seconds. That is the paradox of the slow leak: the emergency descent is required to stop the deterioration, but the greater risk in this scenario was the 55 minutes in which nobody recognised the leak at all. The post-flight report and the maintenance investigation of the outflow valve follow from that, not from the descent.