Carbon Monoxide Hazards and Mitigations in Aircraft Exhaust Systems

#FlySafe GA Safety Enhancement Topic

Published in

Cleared for Takeoff

7 min readSep 19, 2024

Imagine a beautiful day for flying: clear skies and a clear forecast. You completed an uneventful engine runup and takeoff and were just starting to appreciate the joys of flying, when a voice in your head starts saying something is wrong. Or in this case, it’s a voice in the cockpit. Your carbon monoxide (CO) detector sounds off that there is a potentially dangerous concentration of this colorless, odorless gas accumulating in the cabin. Because of the alert, you find a landing zone and touch down safely, cutting short an otherwise perfect flight.

Fly Safe illustration that says: Protect yourself from carbon monoxide poisoning.

Do you have a CO detector installed in your aircraft, or carry one on board with you when you fly? Do you thoroughly inspect your exhaust system every 100 hours? Let’s spend a little time discussing the source of this hazard, some common themes from both accidents and from successful landings, and what actions you can take to mitigate your own risk.

I’m sure you are aware of the dangers of carbon monoxide within an enclosed space, and it’s also likely that you have a CO detector in your home to protect you. But do you also consider the hazards it poses while operating your aircraft? Carbon monoxide is a by-product of combustion, and as long as aircraft burn fuel to generate power, the risk that CO finds its way into the cabin must be considered.

When inhaled, CO is absorbed into the bloodstream and binds to the hemoglobin in red blood cells with an affinity several hundred times higher than oxygen. This means it effectively blocks the ability of our blood to carry oxygen to our cells, and even after moving to fresh air, it takes time to clear the CO from our system. A National Institute of Health report notes that pulse oximeters overestimate oxygen saturation in cases of CO poisoning. If you are using supplemental oxygen because of suspected carbon monoxide exposure, do not rely on a pulse oximeter to indicate that your oxygen levels are in the safe zone. At high enough concentrations, CO exposure alone results in death; however, in an aircraft it becomes hazardous at concentrations below the lethal level. Similar to hypoxia, early CO exposure symptoms can include: headaches, drowsiness, nausea, or shortness of breath. Continued exposure leads to impaired judgement, which can lead to a loss of control of the aircraft.

Since 2010, there have been 12 fatal aircraft accidents where CO impairment was the primary root cause. A common element identified by the accident investigations in the majority of the cases was pre-existing damage to the exhaust system. The figure below illustrates a common architecture of the cabin heat system in many aircraft.

Using engine waste heat is a common method to heat cabin air. (Illustration courtesy of Boldmethod)

In a properly designed and functioning exhaust system, the exhaust gases are segregated from the airflow entering the cabin; however, over time, degradation can allow mixing of exhaust gases into the cabin air. Accident and Service Difficulty Reports (SDRs) show the most common causes of CO contamination are exhaust system cracks around the muffler and heat exchangers. Spot welds in these areas can fail, creating holes in the ducting. Corrosion and oxidation reduce material wall thickness causing cracking or pin holes. Dissimilar materials can also crack due to differing rates of expansion or corrosion.

Often the exhaust muffler area is covered by a shroud, and the complexity of the system makes a visual inspection of the interior area difficult. In the photos below the shroud has been removed, and a crack is visible in the muffler. In this case, exhaust would be able to enter the cabin through the heater system, but with the shroud on, this crack might remain hidden.

Exhaust muffler cracks visible after the shroud is removed.

In several accidents where CO impairment was causal, there was evidence that previous inspections and maintenance failed to identify and repair an existing exhaust system fault. Using remote visual inspection aids such as a mirror with a ball joint, magnifiers, or using a borescope can help to inspect these areas. Removal of the shroud also aids visual access.

Of these 12 fatal accidents, accident investigators were able to make a positive determination of a CO detector in only two instances, and both were passive, spot-type detectors. While passive, color changing spot detectors are low cost, to be effective as an alerting mechanism, they require the pilot to incorporate a regular scan to check if the detector is changing color. Additionally, the eyes are very sensitive to a lack of oxygen, and loss of color vision is another possible symptom while suffering from CO exposure. The likelihood of noticing that a spot detector is indicating CO exposure is reduced if the crew is already impaired.

In contrast to these fatal accidents, since 2010 there were 81 reports from pilot crews who communicated to air traffic control that they had an emergency relating to CO in the cabin but were able to land successfully. In 76 of these instances, the report indicated the crew was reacting to a CO detector alert. Usage of an alerting type of CO detector is a major difference between the fatal accidents and incidents where the pilot was exposed to carbon monoxide but was able to make a safe landing.

In the event of a CO leak into the cabin, pilots need to be awarene of the situation prior to their performance being compromised. The gold standard is a CO detector manufactured under Technical Standard Order (TSO) C48a, which ensures that the manufacturer of the device built it to a minimum performance standard and followed quality controls. The FAA has also approved several CO detectors under the Non-Required Safety Enhancing Equipment (NORSEE) policy. Detectors approved under this policy may be installed as a minor alteration. Detectors that can be set to alert at 35 parts per million (ppm) concentration can minimize nuisance alerts, but still provide timely detection. Detectors built to a TSO are more expensive though, and there are also electrochemical style detectors made for household use that can be brought on as carry on equipment. Limited testing suggests that installing the detector on the instrument panel provides a high probability of being able to detect at least 50 ppm anywhere in the cabin, and it also ensures that if the detector has visual alerting, that it is within the scanning range of the pilot. There are also headset options that have CO detectors built in that provide aural alerting.

So what should you do to protect yourself and your passengers? For many models of aircraft, there are existing Airworthiness Directives (ADs) that have been issued to address exhaust system cracking. Research the applicable ADs for any aircraft you operate and maintain compliance with them. Aircraft manufacturers have made improvements to the design of their exhaust/heater systems, but because this area is exposed to a high temperature and corrosive environment, regular thorough inspection remains important to detecting degradation before it leads to a hazard. It is important that both you and your aviation maintenance technician (AMT) understand the hazards that cracks and other damage in this area pose, and take efforts to detect and correct them. Finally, strongly consider utilizing an alerting type carbon monoxide detector when you fly. We urge pilots to adopt a safety mindset, be aware of the hazard of CO impairment, and take actions to mitigate that risk. Remember to #FlySafe.