The silent killer in the sky — carbon monoxide

As the weather gets colder and using your aircraft’s cabin heater becomes more of a necessity than a luxury, there is no better time to start thinking about a plan for handling carbon monoxide.

Commonly known as the ‘silent killer,’ carbon monoxide is best known as the cause of household poisonings from oil or gas furnaces, stoves, water heaters, portable generators and 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.

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 is odourless, tasteless and colourless, so your senses do not provide much of a warning if you are exposed! Carbon monoxide is harmful to people because it competes with oxygen to bind to haemoglobin, an iron-containing protein in your red blood cells. Not only does it outcompete oxygen, which means there is 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 are exposed? At low concentrations, symptoms of exposure are mild and vague that include headache, nausea and fatigue. You might think you are just feeling a bit off that day. As the concentration of carbon monoxide in your blood increases, so does impairment and you will 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.

Internal combustion engines and carbon monoxide exposure

Wherever there is 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. 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. Accident investigations show that there are one or two fatal or serious aircraft accidents each year in which carbon monoxide has been a finding, contributing factor, or probable cause. Although these accidents are more prevalent in colder months, carbon-monoxide-related accidents happen throughout the year.

Maintenance and inspection issues

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, as well as 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 exposure to carbon monoxide

The first key step is preventing exposure is to make sure to routinely inspect your aircraft’s exhaust system and replace when required.During each 100-hour or annual aircraft inspection, ensure your mechanic thoroughly inspects the exhaust systems, air ducting firewalls, as well as door and window seals. During pre-flight 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. Do not rely solely on knowing the symptoms of carbon monoxide as your warning system, since they are not specific enough to be recognised as exposure before impairment sets in. You might have heard that your skin, lips, or fingernails turn red when you are exposed to carbon monoxide, but discoloration only happens sometimes and only at extremely high levels of exposure. If you do turn red, you are probably already too impaired to realize it and it is probably too late to recover.

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 colour when carbon monoxide reaches a certain level are undesirable. The colour change may be subtle in some lighting and these detectors require that you regularly scan the device. Also, colour-change devices need to be replaced regularly, whilst their useful lives may be shortened by exposure to direct sunlight, there is often no way to tell when they have 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 will 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 haemoglobin 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 is safe to use. Once on the ground, seek medical attention and do not continue your flight until the aircraft is inspected and repaired.