Definition
A colorless, odorless, tasteless gas produced by the incomplete combustion of carbon-based fuels such as gasoline, oil, and wood. In aircraft, it is most often generated by the engine's exhaust system and can leak into the cabin through a faulty exhaust-based heater or cracks in the firewall. When inhaled, CO binds to the hemoglobin in the blood roughly 200 times more readily than oxygen, blocking oxygen delivery to the brain and tissues and causing symptoms ranging from headache and dizziness to unconsciousness and death.
Plain English
A poisonous gas you cannot see or smell that is produced when fuel burns. In an airplane it usually comes from the engine exhaust, and if it leaks into the cabin (often through the cabin heater) it starves your body of oxygen.
Context Anchor
Pilots encounter CO in discussions of exhaust leaks, cabin heat, engine operation, and cockpit health hazards.
Derivation
From Latin carbo (coal, charcoal) and Greek monos (one) plus oxide. The name reflects the molecule itself: one carbon atom bonded to one oxygen atom. The 'mon-' part is what distinguishes it from carbon dioxide (CO2), which has two oxygens and is harmless to breathe in normal amounts. CO has only one — a sign of incomplete burning, and the reason it is dangerous.
Why Pilots Care
Even low concentrations can impair judgment and lead to loss of consciousness before the pilot notices any problem.
Grounding Statement
A small exhaust leak into the cabin during flight can let CO build up while everything still looks and smells normal.
Intuition Check
Do not assume CO is harmless because you cannot see or smell it. Also do not confuse carbon monoxide with carbon dioxide; carbon monoxide is the exhaust-related poison that can quickly reduce a pilot’s ability to think and act.
Example Sentence 1
During preflight, the pilot checked the CO detector on the panel and confirmed it had a current expiration date.
Example Sentence 2
A carbon monoxide detector mounted on the glare shield gave an early alert during a night flight with the heater running.