Most light airplane heaters use a shroud around the exhaust to warm ambient air, which is a simple and effective way to keep pilots and passengers warm. Unfortunately, a leak in the exhaust system means carbon monoxide (CO), a colorless, odorless, tasteless - and potentially deadly - gas, can enter the cabin.
This is dangerous because CO essentially tricks your blood into bonding with it instead of oxygen, starving your body of what it needs. With enough exposure, this can lead to symptoms of hypoxia (anemic hypoxia, specifically). At low levels, that might mean just a headache, but over a long enough period of time or with high enough CO concentrations, that might mean impaired decision-making or even incapacitation. Worst of all, your brain won't notice anything is wrong - if you don't have some way to monitor CO in the airplane, you might never know it's there.
Sadly, accidents can and do happen. In fact, I narrowly avoided one myself last year. A crack in the exhaust sytem of the helicopter I was flying meant unfiltered exhaust gasses were pouring into the cockpit every time I pulled the cabin heat knob. It was only because I had a good CO detector that I stayed out of the NTSB reports.
One way to avoid this scenario is to perform a good preflight inspection every time you fly, paying particular attention to the condition of the exhaust system. Look for any cracks, holes, or hot spots, which might indicate a leak. Since much of this is hidden with the engine cowl on, have your mechanic do a thorough inspection of all exhaust parts at every oil change and annual, too.
The easiest thing to do is use a carbon monoxide detector on every flight. If you buy a quality device and make it part of your regular checklist, you're much more likely to detect a leak before it becomes a major problem. All CO detectors measure concentration in parts per million (ppm), and alert pilots with some combination of lights, audio alarms, and even vibration.
So how much is too much? The US Occupational Safety and Health Administration (OSHA) uses 35 ppm as its lower limit, although it's important to note that this is based on a time weighted average (TWA), taken over 8 hours. OSHA uses 200 ppm as a 5-minute sample ceiling, and 1500 ppm as an instantaneous limit.
In our experience, pilots should err on the side of caution. Certainly a concentration of 200 ppm should get your attention right away: turn off the heat, open the fresh air vents, and consider landing. It's highly likely you have an exhaust leak. But pilots can also be impacted by much lower levels. Even 35 ppm, which could be caused by poor airflow or even the landing gear being down, can cause confusion, fatigue, and poor decision making if it persists for an hour. For that reason, we like units that alert at 35 or 50 ppm.
Flying with a CO detector is only helpful if you know how to use it. Make sure the batteries are fresh and the sensor is replaced every few years, as directed by the manufacturer. Also keep it in your direct line of sight - it doesn't have to be right in front of a vent, but it should be some place where you can see it without having to move your head much.
Carbon Monoxide Detectors
There are hundreds of CO detectors on the market. However, most of them are not well suited for aviation. They may not alert until CO concentrations reach over 100 ppm, or they may not have an alarm that's audible in the loud cockpit of a general aviation airplane. Here are three options we have flown with and recommend.
Tocsin 3. This is our overall top pick. It features three alert modes - a 90 dB audio alarm, flashing red lights, and vibration - so you will notice it in the cockpit (trust us!). The built-in screen gives you a real time indicaton of CO ppm, but it's still small enough to mount almost anywhere. You can use the sturdy clip to keep it attached to a seat belt or mount it to the panel so it's in view. The default low alarm is set at 35 ppm and the high alarm is set at 100 ppm. It also has a TWA setting for 8 hours, but this is less important unless you're troubleshooting a persistent problem. More Info
ForeFlight Sentry. This all-in-one ADS-B receiver does more than just receive weather and traffic. It also features a built-in CO detector that alerts pilots via a loud audio alarm, a flashing red light, and a pop-up alert in the ForeFlight app. This makes Sentry a solid safety tool, and it can be mounted out of the way if necessary. More Info
Aithre. This tiny detector takes a different approach. It is a standalone CO detector and alerter, but it also connects to a free iOS app to offer detailed monitoring. You can track ppm levels with a 15 minute history or get pop-up messages and Siri voice alerts. It uses a rechargeable battery and includes a handy mounting clip. More Info