Night, Heat On, Headache: Recognising CO in the Cockpit

Most likely: carbon monoxide from the cabin heat system. The classic presentation is exactly this - headache, nausea, tingling, irritability and visual disturbance, appearing insidiously in a cold cabin with the heater full on, where the heat muff surrounds the exhaust. CO binds haemoglobin with about 200–250 times the affinity of oxygen and shifts the dissociation curve to the left, so tissue delivery falls even though the pulse oximeter may read normally. The retina has one of the highest oxygen demands per gram of tissue in the body, so night vision degrades before day vision.

Competing explanations:

• Hypoxic hypoxia from altitude - 9,500 ft in a non-smoker gives mild impairment only, and it would improve on descent. It does not explain the tingling or the nausea.
• Hyperventilation - explains tingling and dizziness, but not usually a progressive headache with a heat source.
• Fatigue and dehydration - real contributors on a 9-hour day, but they do not produce fingertip paraesthesia.

The correct reasoning is not to pick one and stop: treat for the worst plausible cause. Actions: heater fully OFF (not turned down), fresh-air vents and a window open, accept the cold; descend to the lowest safe level; if oxygen were available, 100% at 10–15 L/min; turn towards the nearest suitable aerodrome and land. Do not fly for 20 minutes to see whether it settles.

Go to the nearer airport.

1. Exposure is dose × time. The carboxyhaemoglobin level is still rising. Every extra minute at 9,500 ft with the heater on adds to the impairment; 45 minutes is more than twice the exposure of 20 minutes.
2. Your own judgement is the instrument that is failing. Irritability and poor night vision are early signs of cognitive impairment, and impaired judgement reliably chooses to press on. That is precisely why the decision must be made on a written rule (nearest suitable aerodrome) rather than on how you feel.
3. The nearer airport is easier to land at - 2,000 m, lit, regional. A degraded pilot flying a longer leg at night into an unfamiliar destination is stacking risk on top of an incapacitation risk.
4. Incapacitation is progressive. Symptoms at this level can deteriorate to vomiting or loss of consciousness, and there is no autopilot to save the situation.

Declare an urgency call (PAN PAN) stating the suspected CO exposure, the diverting airport and the number of persons on board, and request medical services on arrival. A PAN PAN is appropriate: the flight is not in immediate danger of crashing, but it needs priority and assistance.

Aircraft:

• The heat muff/exhaust shroud is the single most common source of CO in light aircraft; cracks and corrosion are easy to miss and are found on inspection, not in flight.
• No CO detector. A cheap chemical spot detector or an electronic sensor converts this from a diagnosis into an alert.
• No portable oxygen, so the only treatment available is descent and ventilation.

Flight planning and human factors:

• 9 hours on duty with one sandwich is a fatigue and hydration problem in its own right, and fatigue reduces the margin before symptoms become dangerous. It also makes the pilot more likely to misattribute the symptoms to tiredness.
• Three legs in a cold cabin with the heater constantly on is exactly the exposure profile that produces CO rather than a sudden leak.
• Night VFR over featureless terrain with no autopilot leaves no spare capacity to monitor yourself.

Changes: fit a CO detector, have the exhaust and heat muff inspected, carry portable oxygen on night flights above 8,000 ft, plan a proper meal and rest break, and pre-brief a rule for any headache in flight: heater off, descend, land. If CO is confirmed after landing, the pilot should not fly again that day and should be assessed medically.