17.carbon-monoxide-poisoning. Carbon Monoxide Poisoning
Carbon monoxide (CO) is a colorless, odorless, tasteless gas produced by the incomplete combustion of carbon-based fuels. In aviation, the most common source is engine exhaust leaking into the cabin through a defective exhaust manifold heat exchanger — the same shroud that supplies cabin heat in most piston singles. Because CO cannot be detected by the senses, pilots are dependent on training, awareness, and detection equipment to recognize and respond to exposure.
Why CO is Dangerous
Hemoglobin in the blood normally binds with oxygen and transports it to body tissues. CO binds to hemoglobin with an affinity roughly 200–250 times greater than oxygen, forming carboxyhemoglobin (COHb). Even tiny concentrations of CO in inspired air will progressively saturate the blood, displacing oxygen and producing a chemical form of hypemic hypoxia. Once bound, CO is released slowly — the half-life of COHb in normal room air is approximately 4–5 hours, dropping to about 1 hour on 100% oxygen and around 20 minutes in a hyperbaric chamber.
The effects of CO are cumulative and worsen with altitude, because the partial pressure of oxygen is already reduced. A pilot who tolerates a small leak at sea level can become seriously impaired at 8,000–10,000 ft cabin altitude. Smokers begin every flight with an elevated baseline COHb (often 5–10%), reducing their physiological reserve.
Symptoms
Early symptoms of CO poisoning closely mimic the flu and other forms of hypoxia:
- Headache (often frontal)
- Drowsiness, sluggishness, fatigue
- Dizziness or lightheadedness
- Nausea
- Blurred vision or tunnel vision
- Loss of muscular coordination
Prolonged exposure leads to confusion, impaired judgment, unconsciousness, and ultimately death. A useful warning sign is the simultaneous smell of exhaust fumes in the cabin combined with cabin heat in use — although CO itself is odorless, exhaust gases carry other detectable byproducts, so any exhaust odor must be treated as a CO emergency.
Detection
Most light aircraft are equipped with an inexpensive chemical spot detector — a card with a tan or yellow indicator that darkens to gray or black in the presence of CO. These are effective but have limitations:
- Indicators degrade over time and must be replaced per the manufacturer's schedule (typically 30–90 days after opening, or by an expiration date).
- They must be mounted within the pilot's normal scan.
- They give no audible warning and respond slowly.
Electronic CO detectors with digital readouts (in parts per million) and audible alarms are far more reliable and are strongly recommended. Many panel-mount and portable ADS-B receivers now integrate CO sensing.
Pilot Response
If CO poisoning is suspected — whether by detector activation, exhaust odor, or symptoms — take immediate action:
- Turn off the cabin heat and any defrost/manifold-heat source.
- Open fresh-air vents, windows, or storm windows to maximum ventilation.
- Use supplemental oxygen at 100% if available.
- Avoid further physical exertion to reduce oxygen demand.
- Land at the nearest suitable airport and seek medical evaluation.
- After landing, do not return the aircraft to service until the exhaust system has been inspected and the leak corrected by a qualified mechanic.
Declare an emergency if symptoms are significant. Recovery from mild exposure in fresh air takes hours; cognitive deficits may persist for days, so a self-grounding period is appropriate.
Prevention
- Annual and 100-hour inspections must include the exhaust system. Cabin heat shrouds are a common failure point because hot exhaust pipes corrode and crack from the inside.
- A pressure check or leak test of the exhaust system is recommended at every annual, especially on aircraft over 1,000 hours since exhaust replacement.
- Replace chemical CO detectors on schedule and consider upgrading to an electronic detector.
- Be especially vigilant in cold weather when cabin heat is in continuous use, and during winter operations after the heater has not been used for months.
- Avoid running the engine on the ground for extended periods with the cabin closed and heat selected, particularly with a tailwind blowing exhaust forward.
Example
A pilot departs on a winter cross-country at 8,500 ft MSL with cabin heat selected. About 45 minutes into the flight, the pilot notices a mild headache and unusual fatigue, then sees the spot detector has darkened. The correct response is immediate: heat OFF, vents OPEN, supplemental oxygen ON if available, divert to the nearest airport, and have the exhaust system inspected before further flight. Continuing the flight while "feeling okay" is the classic error — by the time judgment is impaired, the pilot may not recognize the impairment.