Tingling fingers at 12,500 ft with a cold and an antihistamine
You are crossing a ridge at 12,500 ft in a non-pressurised PA-28 on a two-hour ferry flight, expecting to be above 10,000 ft for about 40 minutes. You slept five hours, have a heavy cold, and took a sedating antihistamine tablet at 07:00 before an 08:00 departure. Twenty minutes after levelling off you notice tingling in your fingers and around your mouth, a warm flushed feeling and a mild sense of well-being. The chart seems harder to read than usual and you have to concentrate on holding a heading. Your passenger asks a question; you answer slowly, then realise you have drifted 10 degrees off heading. The OAT is -9 C, the cabin heat is on, and there is no oxygen on board. Below you the terrain drops away into a valley at 4,000 ft, and there is a suitable airfield 15 minutes ahead.
1. What is happening, and how do the altitude, the cold and the medication interact?
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This is hypoxia, most likely hypoxic hypoxia from the reduced oxygen partial pressure at 12,500 ft, made worse by three compounding factors.
- Altitude: at 12,500 ft the available oxygen pressure is roughly 40 % lower than at sea level. Useful consciousness is measured in tens of minutes, and judgement is impaired well before consciousness is threatened.
- The medication: a sedating antihistamine depresses the central nervous system, slows reaction time and adds its own drowsiness. Combined with mild hypoxia the effects are additive, not simply one plus one - each masks the other.
- The cold and fatigue: a blocked nose from the cold reduces comfortable breathing and sleep loss lowers the threshold at which hypoxia symptoms appear.
- Cabin heat: if the heat exchanger is leaking, carbon monoxide produces a very similar picture - headache, flushing, confusion - and is a genuine possibility in an ageing light aircraft with the heat on.
The dangerous feature is the euphoria and the loss of self-critique. The tingling and warm feeling are often mistaken for hyperventilation; in fact hyperventilation is frequently a response to hypoxia, and at this altitude hypoxia must be assumed first.
2. State the oxygen requirement that applies to this flight and whether it is being met.
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For a non-pressurised aeroplane the flight crew must use supplementary oxygen:
- when the cabin pressure altitude exceeds 10,000 ft for more than 30 minutes, and
- continuously when it exceeds 13,000 ft.
You plan about 40 minutes above 10,000 ft, so the first condition is triggered and you must be using oxygen from shortly after passing 10,000 ft. There is no oxygen on board, so the flight cannot be conducted as planned.
The practical implications:
- The 30-minute allowance is not a licence to spend 30 minutes getting hypoxic before donning a mask; it is the planning limit within which the requirement starts.
- Climbing to 12,500 ft and staying there without oxygen is a planning error made on the ground, not an in-flight emergency to be managed.
- The correct plan is a lower crossing - below 10,000 ft - or a route with a shorter time above 10,000 ft, or an aircraft with a serviceable oxygen system.
3. Set out your immediate actions and the follow-up before you fly again.
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In flight, in this order:
- Descend. Initiate a descent to below 10,000 ft, ideally 8,000 ft or lower, using a normal rate. Descent is the only treatment available with no oxygen on board, and it works quickly.
- Take over the navigation task deliberately. Confirm the heading, the position and the terrain clearance aloud; the passenger can hold the chart and read figures back, which compensates for your degraded scanning.
- Turn the cabin heat off and open a fresh-air vent until CO poisoning is excluded - the symptoms overlap too closely to ignore it.
- Land at the airfield 15 minutes ahead and stop. Do not continue to destination once hypoxic symptoms have appeared; residual effects and fatigue persist after descent.
On the ground:
- Do not fly again with a sedating antihistamine in your system; the correct route is to consult an aeromedical examiner about an alternative that is compatible with flying, and to accept being grounded while the cold lasts.
- Re-plan the route at a level that respects the oxygen rules.
- Note the human-factors lesson: three small errors - short sleep, a cold, a tablet - combined into one impaired pilot, and the impairment was detected only by a heading drift.