Aircraft Maintenance: Carbon monoxide season is here

The majority of single-engine piston aircraft are heated through a simple, yet effective heat transfer system that routes outside air over the hot exhaust system and into the cabin. The typical system uses a shield around the muffler with inlet and outlet air ports to heat the incoming air. In some cases, the muffler has dozens of studs welded to the exterior, increasing the surface area and efficiency of the heat transfer process. The heated air then travels to a mixing valve where it is combined with cold air controlled by the pilot through the heat control cable.

The beauty of the system is its simplicity and efficiency. However, this simplicity comes with some degree of risk. The main goal of the exhaust system is to contain and conduct hot and noxious post-combustion gases safely outside the aircraft. Any leaks in the exhaust system could allow deadly exhaust gases such as carbon monoxide (CO) into the cockpit. By intentionally directing cabin air over the exhaust system, we increase the risk that CO will find its way into the cockpit. Therefore, it is imperative that the system be carefully and routinely inspected to ensure that no exhaust leaks are present.

Regardless of the schedule of your annual inspection, I recommend that every aircraft have its heating system inspected prior to first use as the weather turns cold. This includes the entire exhaust system.

Beginning with the exhaust system, check for signs of leakage at the flanges where the exhaust meets the cylinders, then move on to the slip joints, mufflers, and tailpipes. Leaks will appear as dusty marks or “plumes” emanating from joints cracks in the system. Although heating air does not usually come directly from the lower cowl area, exhaust leaks from anywhere in the engine compartment can find their way inside the cabin.

Next, inspect the heating system from the air inlets through to the outlets in the cabin. Ensure that all scat tubing is secure and free of holes, cracks, or gaps. Remove the muffler surround and carefully inspect the muffler for leaks. Be especially diligent around welded seams and studs in the muffler. Finally, inspect and lubricate the mixing valves to ensure proper operation and that the seals in the valves are working properly. The valve seals are usually made of silicone baffle material and break down over time. They can often be replaced or rebuilt very reasonably, and it’s well worth the effort to keep this critical component working.

The last check should be for proper operation of the air cutoff valve (if the aircraft has one). This is your last line of defense against CO in the cockpit if detected in flight. Therefore, it’s critical for it to work properly in case you need it.

CO is often referred to as the “silent killer.” It’s colorless, odorless, tasteless, and nonirritating. Victims of CO poisoning frequently lose consciousness before they are able to recognize and react to the danger. In the United States, more than 400 people die from accidental CO poisoning every year, and 50,000 visit the emergency room because of it, according to the Centers for Disease Control and Prevention.

Given the risks involved, be sure to use an active CO detection system in your aircraft. There are a variety of inexpensive, portable detectors available that are specifically designed for use in aircraft, in addition to certified systems that can be permanently installed. You owe it to yourself, your passengers, and your family to ensure that “the silent killer” has no home in your airplane.

Armed with the confidence that your heating system is in good condition and that you will be alerted to any CO in the cockpit, you can focus on enjoying the smooth air and high performance that comes with cold weather flying. Until next time, happy flying!