Cabin Heater Symptoms at Altitude
You are cruising at 10,000 ft in a 1978 single-engine aircraft. Outside air temperature is -5°C. You have had the cabin heat on for 40 minutes. You develop a dull headache, feel nauseated, and your passenger has become quiet and drowsy. You initially attribute this to fatigue and mild turbulence. The engine instruments are normal. There is no smell of exhaust in the cabin. You have a pulse oximeter: your SpO2 reads 88%, the passenger’s 84%. You have 1.5 hours to destination and 20 minutes to a suitable airport. You have oxygen on board but no CO detector. The heater uses a muff around the exhaust.
1. What is the most likely diagnosis, and why do normal engine instruments not exclude it?
Read the worked answer
The likely cause is carbon monoxide from the cabin heater muff. CO is colourless and may have no exhaust smell; normal engine instruments do not exclude a cracked muff or leaking heat exchanger. Headache, nausea and drowsiness in both occupants after 40 minutes of heater use are classic. The low SpO2 supports hypoxia, but a standard pulse oximeter can be misleading in CO poisoning; do not rely on a single reading. The combination of heater use, symptoms and altitude makes CO the first suspect.
2. What immediate actions should you take, and why is opening a window not enough?
Read the worked answer
Immediately shut off the cabin heat, open fresh-air vents, and descend to a lower altitude. Do not continue the flight. Divert to the airport 20 minutes away, declare an emergency if needed, and land. Put both occupants on 100% oxygen if available, even during descent. After landing, seek medical evaluation; CO can cause delayed effects. Do not just open a window and continue; the source may still be feeding CO into the cabin, and the exposure continues.
3. Why does 100% oxygen help but not solve the problem?
Read the worked answer
CO binds haemoglobin with much higher affinity than oxygen, so it reduces oxygen delivery even when oxygen is available. 100% oxygen accelerates elimination, but it does not repair the source. The only sure action is to stop the exposure and land. Normal engine instruments and no exhaust smell do not rule out CO; the heater muff can leak exhaust into the cabin air. A CO detector would help, but absence of one does not justify continuing.