Carbon Monoxide (CO) — Insidious Poisoning
Carbon monoxide (CO) is a colourless, odourless, tasteless gas produced by incomplete combustion. In light aircraft, the most common source is a defective heat muff on the exhaust manifold — heater air spreads through the cockpit.
Source: FAA AC 20-32B Carbon Monoxide (CO) Contamination in Aircraft — Detection and Prevention (1972, still active); FAA-H-8083-25B Ch. 18.
Physiological mechanism
CO binds to haemoglobin in the blood with an affinity 240–270× greater than oxygen (source: AAMA Aviation Medicine Handbook / NIOSH). Consequently:
- Blood oxygen transport drastically reduced (even with normal O₂ partial pressure in the lungs).
- Carboxyhaemoglobin (COHb) accumulates over hours.
COHb saturations and symptoms (source: NIOSH, AsMA)
| COHb % in blood | Symptoms |
|---|---|
| 5–10 % | Onset headache, slight reaction-time loss |
| 10–20 % | headache, forehead pressure, fatigue |
| 20–30 % | throbbing headache, dizziness, nausea, impaired judgment |
| 30–40 % | severe headache, vomiting, clouded consciousness |
| 40–50 % | confusion, unconsciousness |
| > 50 % | life-threatening poisoning, death |
In the cockpit especially dangerous: symptoms resemble hypoxia and fatigue → often dismissed by pilot as "tiredness".
Symptoms — sequence
- Forehead pressure, mild headache.
- Dizziness, nausea.
- Slowed thinking, impaired judgment.
- Confusion, possible blackout.
- Unconsciousness without warning.
Source of CO in light aircraft
Heat-muff heating: cabin heat draws fresh air through a tube wrapping the exhaust manifold. With a crack / defect in the manifold, exhaust gases can enter the heating tube and thus the cockpit.
Other sources (rare in aviation):
- Defective engine seal.
- Fuel engine with disturbed combustion.
Detection
Chemical CO indicator patch
- Yellow turns to black on CO exposure.
- Adhered to instrument panel.
- Typical life 1 year.
- Inexpensive (€5–15), strongly recommended for every GA cockpit.
Electronic CO detector
- Measures CO concentration in ppm.
- Alerts at e.g. 35 ppm or above.
- Modern cockpits or as EFB add-on.
Immediate actions on suspicion
- Cabin heat OFF — immediately.
- All cabin vents to max fresh air.
- Open windows (carefully, aerodynamics).
- If oxygen system: breathe 100 % O₂ — displaces CO from haemoglobin.
- Descend (denser air).
- Forced landing for moderate-to-severe symptoms.
- Radio: Mayday/PAN-PAN.
Recovery
- CO half-life in blood: about 4–6 hours with normal breathing; about 1 hour with 100 % O₂; about 20 min hyperbaric (pressure chamber).
- → Pilot after CO exposure: do not fly for 24 h, see a doctor.
Prevention
- 100-hour / annual inspection of the heat muff (visual, possibly pressure test).
- At seasonal first heater use: brief ground run, then watch detector.
- CO detector on board — mandatory in some states for IFR, strongly recommended otherwise.