Summer is over, the days are getting shorter, and the temperatures are dropping to the point that many pilots are reaching over to pull that heater valve knob to warm their toes and quiet the natives in the back seat.
Without the heat-distribution benefits of water-cooling systems on cars, aircraft have only two options for heating the cabin: Heated air from the engine (used on single-engine aircraft) or heated air from a dedicated combustion heater (used on twin-engine aircraft). Both of these systems leave something to be desired in terms of control, maintenance, and safety (should the system have a failure).
The No. 1 concern with any aircraft cabin heating system is safety. In the muffler-based heating systems found in single-engine aircraft, the top danger is from carbon monoxide. Muffler-based heater cabin heat systems pass cabin air through a sealed chamber which is heated by the exhaust gasses as they flow through the system. This exhaust contains carbon monoxide, which is a deadly, odorless gas that can quickly overcome an unsuspecting pilot. This is why maintaining the integrity of the seal between cabin and exhaust air is so critically important.
Muffler-based heating systems come in many variations, but they share the same basic design. Air is routed from outside the aircraft into the heat muff or chamber around the muffler. Following this, some systems will mix this air with outside air to moderate the temperature and allow adjustment. From there, the heated air is routed into the cabin.
The most common failure of this system is a leak in the muffler itself into the heater shroud. All mufflers eventually suffer from internal corrosion and heat distortion simply due to the extreme temps and corrosive elements in the exhaust. In addition, some muffler systems include studs to increase the surface area available to the passing air and, therefore, increase the efficiency of the heat exchange process. These studs are another area of concern for potential leakage.
All aircraft heating systems should be inspected prior to seasonal use; even if that does not align with the annual inspection schedule. Just because you had the airplane thoroughly inspected in March doesn’t mean you should blindly trust the integrity of the heating system in November.
Inspecting the heat system begins, surprisingly, at the exhaust risers and tubes. One of the things that you want to eliminate is the potential for exhaust gasses within the cowl itself. This is because carbon monoxide in the cowl could migrate into the heat or fresh air system through any available gaps in the heat system. Look for dusty residue where the exhaust manifold meets each cylinder, as well as at any slip joints in the system.
Next, follow the airflow, beginning at the air intake. Typically, there is a system of SCAT ducting and valves. Any holes, chafing areas, or other fitment issues in the SCAT ducting should be cause for replacement. Never try to repair SCAT when you can completely replace it with new for very little money.
The muffler shroud must come off so that the muffler can be thoroughly inspected. Again, look for signs of exhaust dust and heat distress. You can pressurize the system with a low-power vacuum on the “blow” side and a light spray of a water/soap mixture looking for bubbles. Be sure to wash all contaminants off prior to reassembly.
Finally, test the system in use and always use a good carbon monoxide detector in the cockpit with an alarm. The old “dark spot” detectors are of little value now that you can get a good battery-powered alarm system (even at your local home supply store). When it comes to aircraft heating systems, a small investment in prevention can save the lives of you and your loved ones. Fly safe!
Jeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Jeff is also the creator of SocialFlight, the free mobile app and website that maps over 10,000 aviation events. Free apps are available for the iPhone, iPad, and Android, and online at www.SocialFlight.com.