General Aviation’s Silent Killer in the Sky

By Michelle Watters, MD, PhD, MPH

As the weather gets colder and using your aircraft’s cabin heater becomes more of a necessity than a luxury, there’s no better time to start thinking about a plan for handling carbon monoxide. Commonly called the “silent killer,” carbon monoxide is best known as the cause of household poisonings from oil or gas furnaces, stoves, water heaters, or portable generators or fireplaces. For general aviation pilots, carbon monoxide exposure poses a particularly concerning threat because impairing levels can build quickly in an enclosed cabin, and even nonfatal levels can lead to tragic consequences in flight.

For example, in 2017, a private pilot was flying his newly purchased Varga 2150A airplane on a visual flight rules cross-country flight. After flying for about 80 minutes, the airplane suddenly entered a spiraling descent from cruise flight. Witnesses observed the airplane flying erratically at low altitude before it impacted an open field near Bowling Green, Ohio; they stated that the engine was running until impact. Toxicological testing of the pilot’s blood found 55% carbon monoxide saturation (toxic level is 20 percent).

Carbon Monoxide and the Danger of Exposure

So, what is carbon monoxide and why is it dangerous? Carbon monoxide is a simple chemical formed from the incomplete combustion of carbon-containing compounds, such as aviation fuel. It’s odorless, tasteless, and colorless, so your senses don’t provide much of a warning if you’re exposed! (Although, if you do smell exhaust fumes, always assume they contain carbon monoxide.) Carbon monoxide is harmful to people because it competes with oxygen to bind to hemoglobin, an iron-containing protein in your red blood cells. Not only does it outcompete oxygen—which means there’s less oxygen circulating in your blood—but it prevents the blood from unloading oxygen to the tissues and vital organs that need it—including your brain.

What happens when you’re exposed? At low concentrations, symptoms of exposure are mild and vague, and include headache, nausea, and fatigue. You might think you’re just feeling a bit off that day. As the concentration of carbon monoxide in your blood increases, so does impairment, and you’ll start experiencing dizziness, confusion, and disorientation. For longer exposures or high enough concentration levels in your blood, symptoms can be incapacitating and include unconsciousness, coma, and even death.

In the case of the Varga pilot, exposure to carbon monoxide explains his loss of control—he likely suffered confusion, disorientation, and loss of consciousness. But how was he exposed to carbon monoxide in the first place? Examination of the Varga’s heat exchanger showed that the outside casing had either previously been repaired or had been originally constructed of metals with different properties. About half the casing was discolored and exhibited varying signs of corrosion. Small holes from corrosion were found in the casing material, which provided a means for carbon monoxide to enter the cockpit from the exhaust system.

Internal Combustion Engines and Carbon Monoxide Exposure

Wherever there’s an operating internal combustion engine, carbon monoxide is likely being produced. Many airplanes with internal combustion engines are heated by air warmed from circulating around the exhaust system using a heater shroud. As in the case of the Varga pilot, a defect or leak in the exhaust pipes or muffler can introduce carbon monoxide into the cockpit. Although piston engines produce the highest concentrations of carbon monoxide, exhaust from turbine engines can also cause carbon monoxide poisoning.

Our accident investigations show that there are one or two fatal or serious aircraft accidents each year in which carbon monoxide is a finding, contributing factor, or probable cause. Although these accidents are more prevalent in colder months, carbon-monoxide-related accidents happen throughout the year (for instance, the Varga accident occurred in June).

Maintenance and Inspection Issues

Maintenance logbooks indicated that the Varga’s most recent annual inspection was completed less than a month before the accident, and the logbooks didn’t contain any record of heat exchanger repair or replacement. The heat exchanger’s condition at the time of the accident indicates an insufficient annual inspection.

A Federal Aviation Administration (FAA) report found that inadequate maintenance and inspection has contributed to many carbon-monoxide–related accidents. Deficiencies included poor welds, unapproved modifications, and missed holes or cracks on visual inspection. The FAA also found that, for carbon-monoxide–related accidents involving mufflers, there was a strong relationship between the muffler’s lifespan and its failure—the mufflers in the majority of these accidents had more than 1,000 hours of use.

Preventing Carbon Monoxide Exposure

So, how do you prevent carbon monoxide exposure? The first key step is preventing exposure—make sure to routinely inspect your aircraft’s exhaust system and replace when warranted. During each 100‑hour or annual aircraft inspection, ensure your mechanic thoroughly inspects the exhaust systems, air ducting firewalls, and door and window seals. During preflight inspections, look for cracking at the ends of your muffler and evidence of soot, which might indicate cracking in the muffler. Follow the manufacturer’s recommendations for the lifetime limit on your muffler and schedule for replacement parts.

Even with best efforts, leaks may happen. Secondary prevention involves being alerted to the danger before it becomes a problem. Don’t rely solely on knowing the symptoms of carbon monoxide as your warning system—they’re not specific enough to be recognized as exposure before impairment sets in. You might have heard that your skin, lips, or fingernails turn red when you’re exposed to carbon monoxide, but discoloration only happens sometimes, and only at very high levels of exposure. If you do turn red, you’re probably already too impaired to realize it, and it’s probably too late to recover.

So how can you be alerted to dangerous levels of carbon monoxide? Just like for your home, multiple types of carbon monoxide detectors are available for your aircraft and can be placed on your instrument panel. Detectors that only change color when carbon monoxide reaches a certain level are undesirable. The color change may be subtle in some lighting, and these detectors require that you regularly scan the device. Also, color-change devices need to be replaced regularly, and their useful lives may be shortened by exposure to direct sunlight—there’s often no way to tell when they’ve stopped working. Detectors mounted on the instrument panel with audible alerts or flash notifications provide the best warning. The FAA report mentioned earlier in this article found that electrochemical sensors were most suitable for use in general aviation due to their relatively high accuracy, quick response time, and low power consumption.

The next thing to consider is what you’ll do if your carbon monoxide detector goes off, you feel symptoms, or you suspect carbon monoxide in your aircraft. Unlike other medical emergencies where your crew may be able to assist, carbon monoxide exposure affects everyone on your aircraft. Communicate with air traffic control immediately and tell them you suspect carbon monoxide leak and exposure. When flying to the nearest airfield, descend to the lowest safe altitude, as carbon monoxide binds hemoglobin more readily and strongly at higher altitudes. Turn off the heater. Maximally increase cabin fresh air ventilation, open windows if permissible. Consider supplemental oxygen if it’s safe to use. Once on the ground, seek medical attention and do not continue your flight until the aircraft is inspected and repaired.

The NTSB determined that the probable cause of the Varga aircraft accident was the pilot’s loss of control due to impairment from carbon monoxide poisoning. Contributing to the accident was the corrosion of the heat exchanger and the failure of maintenance personnel to adequately inspect and repair or replace the exchanger during the most recent annual inspection. These factors were all avoidable with a little extra care. Inspect your aircraft. Know the symptoms of carbon monoxide poisoning, but don’t rely on them for warning. Install a carbon monoxide detector. Take immediate action.

Interested in more information?

You can learn more by viewing these NTSB and FAA resources:

• NTSB Safety Alert—Pilots: Prevent Carbon Monoxide Poisoning
• NTSB Safety Alert companion video—Pilots: Prevent Carbon Monoxide Poisoning features a pilot who shares his personal story on carbon monoxide poisoning and prevent it from happening to you.
• Carbon Monoxide (CO) Contamination in Aircraft – Detection and Prevention (Advisory Circular [AC] 20-32B) contains information on the potential dangers of CO exposure from faulty engine exhaust systems or cabin heaters, means of detecting CO, and procedures to follow when exposure is suspected.
• Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair (AC 43-13-1B) contains inspection methods and repair techniques in chapter 8 to prevent exhaust system failures that could lead to CO exposure.
• Engine Exhaust and Carbon Monoxide Detectors (Special Airworthiness Information Bulletin CE-10-19R1) informs owners and operators of the importance of installing CO detectors.
• FAA brochure Carbon Monoxide: A Deadly Menace contains medical information concerning the symptoms of CO exposure and methods of avoiding exposure.