Thomas Dunn, RAM Aircraft, L.P.
Carbon Monoxide (CO) is a colorless, odorless gas that is a byproduct of hydrocarbon (AVGAS) combustion. Due to its nature, many pilots overlook the danger that CO poses to the safety of flight. According to the NTSB Case Analysis and Reporting Online (CAROL) Query system, ten Factual Reports from 2013 to 2020 for aviation accidents contain ‘Carbon Monoxide – Pilot’ in the Findings. Nine were fatal, one had serious injuries. Prior to the advent of the searchable CAROL system, we found the following from the FAA brochure “Carbon Monoxide: A Deadly Menace”: “What is not known is the full extent of carbon monoxide poisoning in aviation. Analysis of toxicology samples from fatal U.S. aircraft accidents between 1967 and 1993 showed that at least 360 victims had been exposed to sufficient carbon monoxide before or after the crash to impair their abilities. Non-fatal carbon monoxide poisoning in aviation is likely a more common occurrence than currently believed.” 1 In this article, we’ll go through how CO affects the human body, how you get exposed to it, and how to protect yourself and your passengers from CO poisoning.
Inhalation of CO results in poisoning with symptoms similar to hypoxia (lack of oxygen to the body tissues). According to the Mayo Clinic website2, the symptoms of CO poisoning may include:
- Dull headache
- Nausea or vomiting
- Shortness of breath
- Blurred vision
- Loss of consciousness
Carbon Monoxide binds with hemoglobin (which normally transports oxygen in your blood) forming carboxyhemoglobin (CO-Hb), which prevents the delivery of oxygen to the tissues of the body. Unlike typical hypoxia, CO poisoning does not clear up quickly when the patient is moved to fresh air. The typical half-life for carboxyhemoglobin for a resting adult under room air ventilation at sea level is 4 to 5 hours.3 CO poisoning is accumulative and much more dangerous than simple hypoxia. See Table 1 for likely clinical symptoms at given levels of CO-Hb saturation in your blood.
The rate of accumulation of carboxyhemoglobin varies from person to person based on their age, overall health and, the concentration of CO in the atmosphere. Just keep in mind that exposure to CO prior to the flight increases the likelihood of incapacitation if a CO leak occurs in the air. If you are a smoker, you may already be close to clinical symptoms of CO poisoning.
Carbon Monoxide is very dangerous, but where does it come from? How does it get to you? The three most common sources of CO are your engine(s)’ exhaust gasses, your combustion heater, and other airplanes’ exhaust when on the ground.
For All Aircraft:
Any failed external seal could allow exhaust gas to draft into the aircraft. While on the ground any openings (like failed seals, open doors, or windows), may allow exhaust gas from other aircraft (or your own engine(s)) to draft into the cabin while you sit on the ramp or taxi to the runway. Idling an engine on the ramp with the door open can pose a significant risk to the flight. Make sure all of your door seals, window seals, air ducting, steering boots, land gear compartments, gear actuator boots, and firewalls are in good condition. Aftermarket air conditioning systems’ air inlets may be vulnerable to exhaust gas contamination on the ground. As Pilot-in-Command (PIC), you should pay close attention to these areas and those below for single and twin engine aircraft during your pre-flight inspection.
For Twin Engine Aircraft:
Your engines are safely isolated on the wing, but if the airplane is pressurized, an exhaust leak can still migrate into the cabin through a leak in the turbocharger system. If your plane is equipped with a combustion heater, you have a source of CO in the nose of your plane. An exhaust leak in the heater can allow CO into the cabin through the heater ducts. There are a number of Airworthiness Directives (AD) related to combustions heaters from various manufacturers. Check your specific heater for any applicable ADs.
For Single Engine Aircraft:
With your engine mounted in the nose of the aircraft, all it takes to allow CO into the cabin from the engine is an exhaust leak and a leak in the firewall. Cabin heat for single engine aircraft typically comes from a shroud wrapped around a heat-exchanger section of the exhaust system. A crack in the heat exchanger will allow exhaust gasses to flow freely into the cabin when the heat is on. Additionally, pressurized singles have the same vulnerability from the turbocharger system as the pressurized twins.
FAA Recommended Maintenance:
According to Special Airworthiness Information Bulletin (SAIB) CE-10-19 R15:
“1. …use a CO detector while operating your aircraft.
- To detect leakage of gases into the cabin due to cracks in the exhaust system, conduct engine run up tests with cabin heat on and check for CO in the cabin with a hand-held CO detector during 100 hour and annual inspections.
- Continue to inspect the complete engine exhaust system during 100 hour/annual inspections and at inspection intervals recommended by the aircraft and engine manufacturers in accordance with their applicable maintenance manual instructions.”
Exhaust gas is a mix of gasses and contaminants, some of which you can smell, along with odorless CO. If you can smell exhaust fumes, you must assume CO is present and take steps to protect yourself and your passengers. Don’t be complacent though! You can have CO contamination from the exhaust even if you don’t smell the other contaminants in the exhaust gas. Rather than relying on your nose alone, consider having a functioning Carbon Monoxide detector in the plane. The NTSB says in their Safety Alert 069: “Detectors mounted on the instrument panel with aural alerts and a flash notification are more likely to draw your attention and alert you to a potential hazard.”6 Technology has improved significantly in this field since your airplane rolled off the factory floor – they are inexpensive and can be a lifesaver! We recommend you research available products and select the one that suits your application the best. The FAA has a certification process for this class of devices called Non Required Safety Enhancing Equipment (NORSEE). You can search for NORSEE-approved detectors if you want a permanent installation in your airplane vs a portable detector.
Bear in mind, that a pulse oximeter is not a substitute for a CO detector.4 CO-Hb looks like oxygenated hemoglobin to a pulse oximeter. Essentially, CO poisoning will show an elevated oxygen saturation level on your pulse oximeter.
If you suspect (or have confirmed) CO contamination in the cabin, you should take immediate steps. Close off sources of CO and get as much fresh air in the cabin as possible. If your airplane is equipped with supplemental oxygen, start oxygen immediately provided you can do so without a safety or fire hazard. Land as soon as practical. Declare an emergency if necessary. Seek immediate medical attention for everyone on board. Do not fly the airplane until the source of CO is found and corrected. The PIC should evaluate their aircraft for potential sources of CO and establish a procedure checklist for CO contamination. An emergency is a terrible time to learn a new process.
Here is a first-hand telling of an accident told by a very lucky pilot:
An ‘Unconscious’ Landing
Plane Lands Itself in Hayfield as Pilot Slumbers
(excerpted from FAA Brochure #OK05-0270)
“ PHYSICIAN ROBERT FRAYSER had lifted off in his Comanche 400 from the North Bend, Kansas, airport at 7 a.m., en route for a meeting in Topeka. He was flying alone, cruising at 5,500 feet on autopilot, with the sun coming up on a clear, beautiful day. Per established routine, he switched the fuel selector to the auxiliary tank and set up the navigation system for nearby Topeka.
About 90 minutes later, Dr. Frayser found himself in a hay field. The engine was silent. He was confused, disoriented, and groggy as he struggled to rouse himself from a deep sleep. His head was throbbing.
Thinking he was still in the air, he went through his landing checklist. As he became more oriented to his surroundings, a new reality dawned: The airplane’s right wing was nearly torn off from an impact with a tree, but the plane was otherwise intact. Aside from a fractured wrist, minor cuts, and bruises, he seemed to be relatively uninjured. But he had no idea where he was. He had no memory of landing.
Dr. Frayser stated that there were no early warnings or symptoms to alert him. “I just went to sleep.” The plane, trimmed for cruise flight and on autopilot, flew a perfectly straight course over Kansas and into Missouri until it ran out of fuel, and then the autopilot gently brought the Comanche in for landing.
Since the engine had stopped, no one heard the aircraft glide to a landing on the open field. “I was alone, disoriented, injured, and had a severe headache and ringing in my ears,” he said. The aftermath of a near-fatal accident caused by carbon monoxide poisoning.
Extracting himself from the aircraft, he struggled a quarter of a mile through snow-covered fields for help, finally stumbling onto a farmhouse. Dr. Frayser was taken by ambulance to a hospital, where the emergency room physician put him on 100 percent oxygen to overcome near-fatal blood levels of carboxyhemoglobin.
Carbon monoxide poisoning from a cracked muffler had allowed the deadly, odorless gas to seep into the cabin through the heater and caused him to fall asleep. The crack, which had apparently opened after the last annual inspection, was concealed by the heat shield and could not be detected during the pre-flight inspection. “The crack could have been there for a long time, just waiting for someone to turn on the heater,” he said. Frayser did not have a carbon monoxide detector on board to alert him of its presence.
Another 30 minutes in the air might have been fatal. Carbon monoxide poisoning would have claimed another victim.”1
In conclusion, Carbon Monoxide is a colorless, odorless gas that is highly toxic. Carbon Monoxide poisoning can result in impairment or death. Your airplane produces this gas in the exhaust and you must protect yourself and your passengers from it. Maintenance of your airplanes, engine(s), firewall(s), heater, seals, and exhaust system will keep the exhaust going where it should. Be aware of exhaust gas from other airplanes and/or your own depending on wind conditions – it may draft into the cabin while on the ground. A CO detector is a highly recommended piece of safety equipment. If you have CO contamination in the cabin, take necessary steps – close off potential sources of Carbon Monoxide, put on supplemental oxygen, and land as soon as practical. Remember! Carbon Monoxide poisoning is accumulative. Exposure to CO prior to the flight can push you close to, or into, the thresholds of saturation where you experience clinical symptoms.
1 FAA Brochure #OK05-0270 Carbon Monoxide: A Deadly Menace by G.J. Salazar, M.D.
3 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6992844/ (National Library of Medicine)
4 https://pubmed.ncbi.nlm.nih.gov/9792574/ (National Library of Medicine)
5Special Airworthiness Information Bulletins (SAIB) (faa.gov) (Search for SAIB CE10-19 R1)
6 https://www.ntsb.gov/Advocacy/safety-alerts/Documents/SA-069.pdf (NTSB Safety Advisory 069)