Deadly CO

A renewed manhunt for the invisible killer

The NTSB is frustrated. Year after year, they’ve had to investigate the same airplane crash. It’s a morbid version of Bill Murray’s Groundhog Day. Each time the cause is the same, only the tail number is different.

The groundhog in this case is carbon monoxide (CO). That odorless, colorless, highly toxic gas you learned about in private pilot ground school. That powerplant that moves you through the air also has all that’s required to move you to the next life in an unpleasant manner, because lurking in the exhaust of a reciprocating engine is deadly CO.

Smell exhaust in the cockpit? Assume carbon monoxide is there, too. That’s easy. But sometimes the CO finds its way onto the flight deck without exhaust, where it enters your lungs with stealth that would make a ninja proud. Once there, your own biology works against you. The oxygen-transporting hemoglobin molecules in your blood have a real taste for CO. They’re practically addicted to the stuff. They would much prefer to carry CO than carry oxygen, which, of course, is their job. The result? When they hook up with CO, we end up deficient in oxygen and suffer hypoxia, just as if we were up in the flight levels in an unpressurized cabin with no supplemental oxygen.

But this flavor of hypoxia, which is the most common form of hypemic hypoxia, isn’t like altitude hypoxia in which you can just descend to resolve it. Carbon monoxide sticks to your hemoglobin as if attached with super glue. It takes days to clear, all the while leaving your body oxygen deprived. This can lead to permanent brain and heart damage, among other nasty side effects, which is why you need to seek immediate medical attention if you are—or suspect you have been—exposed to CO.

Of course, like any poisoning, there are warning signs, and the primary symptoms of CO exposure that pilots are taught include headache, dizziness, drowsiness, and nausea. And we’re further taught that once we recognize that we’ve been exposed to CO, we should promptly execute the following steps: Turn off your heater, if on, as most cabin heaters use a shroud that wraps around the exhaust as a heat source, and cracks in the system can pump CO into the cockpit. Next, increase cabin ventilation and go on oxygen, if available. Then descend to a lower altitude, as CO binds to hemoglobin both more readily and more strongly at higher altitudes. Of course, notify ATC about what’s going on, and finally, get on the ground ASAP.

The problem is that the symptoms start off exceedingly mild, and they’re easy to attribute to other causes. And by the time they become severe enough to wake you up to the problem, you’re already highly impaired and can no longer realistically recognize that you are affected. It’s like drinking four glasses of punch at a party before being told it’s 50-percent vodka. Now it’s too late. In the case of CO, your brain will be too muddled for you to fly well. And it can happen fast. If the exposure continues, you’ll literally pass out. And that, as noted, leads to the NTSB showing up—once again—at the scene of a CO crime.

An NTSB review of accidents from 1982 to 2020 attributed 31 accidents and 42 fatalities to carbon monoxide poisoning, and the NTSB estimates more cases may have gone undetected. The NTSB recently issued an aviation investigation report, urging the FAA—for a second time—to require all enclosed-cabin GA airplanes with reciprocating engines be equipped with CO detectors. They also released a Safety Alert YouTube video, written a blog post on the subject, and called on AOPA and the Experimental Aircraft Association to remind their members of the dangers.

Most aircraft owners aren’t too keen on more mandates for installed equipment; and most pilots aren’t too keen on more airworthiness items to track. But with our traditional first line of defense—recognizing the symptoms of CO poisoning—being unreliable, what else can we do? Of course, you should always remain alert to even the slightest symptoms that match up with those of CO poisoning, and keep the possibility of CO in mind, but there are three other things you can do to keep safe from CO poisoning: conduct a thorough preflight; invest in some sort of CO detector (and monitor it in flight); and up the ante during routine maintenance to discover vulnerabilities that could lead to CO getting into the cabin.

In a perfect world, before each flight we’d do a thorough check of the exhaust system, the source of CO. But to really check it requires removing the cowl and the heater shroud—and in some aircraft, even the exhaust system itself, neither practical nor legal for non-A&P pilots. But get your fingers dirty by physically checking the exhaust stacks for movement and get your eyeballs on as much of the exhaust and heater systems as you can. Look for cracking or soot. With the cowl open, check your firewall from inside the cabin to ensure you can’t see light. Run a finger over the door seals and feel for cracks or missing chunks.

CO detectors
There are nearly two dozen aviation CO detectors on the market, ranging from passive “spot” detectors that use a chemical reaction to alert you to the presence of CO by changing color, to portable electronic devices with aural alarms, to panel mounted devices with lights, bells, and whistles. This is an investment that on the lowend costs less than one gallon of 100LL gasoline; and on the high end is still less than the annual premium on your life insurance policy (see sidebar). Of interest, only one of the airplanes in the long line of bent metal in the NTSB reports had a detector, and that one was a five-buck spot type. It was a 90-day-life model—that was more than a year old.

Proactive inspections
The NTSB’s investigations revealed that some of the aircraft involved in CO crashes had developed exhaust system problems before their mandatory inspection periods, leading the agency to urge owners to work with their maintainers to be proactive and inspect exhaust systems more frequently than the recommended minimum. Other areas for attention during maintenance include steering boots and landing gear compartments. At a minimum, consider having a conversation with your A&P/IA about how much increased attention to these areas would add to the cost of the inspection.

So, throw some shade on the groundhog and free the NTSB from its repeating nightmare. A critical eye during preflight, any sort of detector, and a few extra breaths of fresh air during maintenance can go a long way toward stopping those headaches that can end so badly.

William E. Dubois

William E. Dubois is a widely published aviation writer and columnist. He is an FAA Safety Team rep and a rare "double" Master Ground Instructor accredited by both NAFI and MICEP. An AOPA member since 1983, he holds a commercial pilot certificate and has a degree in aviation technology. He was recognized as a Distinguished Flight Instructor in the 2021 AOPA Flight Training Experience Awards.

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