Suspected Hypoxia at 9,800 ft Cabin Altitude
A two-pilot crew is cruising at FL410 on a night sector. The cabin altitude has been steady at 9,800 ft for 30 minutes after a pack fault; the cabin altitude warning has not activated. The pilot flying has been awake for 19 hours, has a head cold, and took a sedating antihistamine 6 hours earlier. He reports tingling in his fingers, light-headedness, a narrowed visual field, and a feeling of warmth and euphoria. He is breathing at 28 breaths/min and speaking rapidly. He refuses the oxygen mask, saying ‘I’m fine, it’s just the pressure.’ The other pilot is on oxygen and has normal readings. The aircraft is 40 minutes from a suitable airport; MSA is 14,000 ft.
1. Assess the likely cause of the pilot’s symptoms and explain why they must not be dismissed as ‘just pressure’.
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The symptoms are consistent with mild hypoxia at a cabin altitude of 9,800 ft: tingling, light-headedness, visual field narrowing, warmth and euphoria are classic early signs. The pilot’s judgement is already suspect because euphoria and denial are themselves symptoms. Hyperventilation can produce similar tingling and light-headedness through hypocapnia, especially with rapid breathing, but the safe assumption is hypoxia until proven otherwise. The head cold, 19 hours awake, and sedating antihistamine increase susceptibility and impair performance. The absence of the cabin altitude warning does not make the cabin altitude safe; 9,800 ft is below the warning threshold but still causes reduced oxygen saturation, particularly at night.
2. State the immediate actions and the correct sequence of treatment.
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The other pilot should take control, put the affected pilot on 100% oxygen immediately, and check the mask, hose and oxygen supply. The affected pilot should sit down, stop speaking, and breathe normally. The crew should descend to a lower cabin altitude if possible and plan to land at the nearest suitable airport. If symptoms resolve rapidly with oxygen, hypoxia is confirmed; if they persist, hyperventilation may be considered, but oxygen should not be removed to ‘test’ the diagnosis. The affected pilot must not act as pilot flying or make command decisions until fully recovered. The crew should declare a PAN or MAYDAY as appropriate and advise ATC.
3. What fitness-to-fly and medication issues arise, and what should happen after landing?
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The sedating antihistamine is a significant impairment, even if taken 6 hours earlier; drowsiness and slowed reaction can persist. The head cold may affect ears, sinuses and general wellbeing, and the 19-hour wakefulness adds fatigue. The pilot should be removed from duty after landing and reviewed by a medical examiner. He should not self-medicate and fly; the medication and cold should be declared. Return to flying should wait until symptoms have resolved, the medication has been stopped for the required period, and a medical assessment clears him. The operator should also review why a pilot with a cold and sedating medication was allowed to report for a night flight.