Headache and Drowsiness with Cabin Heat On
Pilot and passenger in a C172, 2.5 h into a 3.5 h flight. OAT -2°C, cabin heat fully on. Pilot has a throbbing headache, dizziness, and mild nausea; passenger is flushed and nauseated. Pilot initially attributes symptoms to fatigue and turbulence. There is no CO detector. The pilot opens a window and the symptoms lessen slightly but do not disappear. The engine runs normally, fuel is 1.2 h, and a suitable aerodrome is 15 nm away. The pilot is at 5,500 ft, VMC, and the autopilot is not fitted. The pilot feels capable of continuing but is slower to respond and has difficulty concentrating. The pilot has not taken medication or alcohol.
1. What is the most likely diagnosis, and why did opening the window not resolve it?
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Carbon monoxide poisoning is the most likely diagnosis. Cabin heat in a light aircraft can carry exhaust gases if the heater muff or ducting leaks. CO binds to haemoglobin with high affinity and reduces oxygen delivery; symptoms include headache, dizziness, nausea, flushing, and impaired judgement. Opening a window reduces further exposure but does not rapidly remove CO already bound to haemoglobin. The slight improvement is consistent with reduced concentration, not with recovery. The pilot's slower responses and poor concentration are red flags. Fatigue alone would not explain the passenger's flushed appearance and nausea in a cold cabin, and the temporal link with cabin heat is strong.
2. What immediate actions should the pilot take?
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Turn cabin heat off, open fresh-air vents, and descend to a lower altitude if terrain allows. Declare an emergency and divert to the aerodrome 15 nm away. If oxygen is available, use 100% oxygen. Land as soon as practicable; do not continue the flight. After landing, both occupants should seek medical assessment, and the aircraft should be inspected for exhaust and heater leaks before further flight. The pilot should not fly again until cleared and should not attempt to self-diagnose as fatigue. The priority is to remove the source and get on the ground, not to complete the trip.
3. The pilot feels much better after 10 minutes on the ground with the heater off. Is it acceptable to continue the flight?
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No. CO has a long half-life on room air, and impairment can persist even when the headache improves. The pilot has already shown degraded concentration, and the passenger was symptomatic. Continuing would expose both to renewed CO if the leak remains and would also raise fitness-to-fly concerns. The aircraft should not be flown until the heater and exhaust system are inspected and the pilot is medically cleared. The pilot should arrange alternative transport or a later flight with a different aircraft. The decision should be based on the risk of recurrence and residual impairment, not on the relief of symptoms.