In our last installment, we discussed single-engine, muffler-based heat exchanger systems. This time, we’ll review the combustion-based heater systems found in most twin-engine aircraft.
The design of a combustion heater system is similar to a household forced hot-air system in that you have a heat source with the basic ingredients of any combustion process—fuel, air, and ignition. Fuel (aviation fuel) is fed from the aircraft fuel system to the heater, where it is combined with heater intake air (not the cabin heat air) and ignited with a spark plug. The system is either on or off based on a thermostat control. There are no controls for the amount of heat produced within the unit. In addition, there is an overheat safety shut-off valve in the system for protection.
The No. 1 concern with any aircraft cabin heating system is carbon monoxide. But, surprisingly, combustion heaters are much less likely to vent CO into the cabin air system. This is because the combustion chamber in the heater unit is at a fairly low pressure and constantly venting out the dedicated exhaust for the heater. If an air leak did occur between the combustion chamber and the heater air system, it is more likely that the cabin heat air would flow into the heater, rather than contaminate the cabin air with exhaust gases. This is the opposite situation than found in a muffler heat exchange system, where high-pressure exhaust gases can easily flow into the heater air through any leaks in the system.
This isn’t to say that inspecting the integrity of the combustion chamber isn’t important; it is. This, along with the proper operation of the fuel and ignition systems, must be maintained to ensure proper operation and safety. All aircraft heating systems should be inspected prior to seasonal use, even if that does not align with the annual inspection schedule. As I said in the last article: 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.
The inspection should begin by following the fuel system, checking the lines, safety valve, and fuel solenoid for proper operation and leaks. Any blue staining is immediate cause for concern and further inspection. Next, the ignition system should be inspected, including the heater spark plug.
Follow the airflow through for the combustion process itself, from the intake through the combustion heater’s dedicated exhaust. This is definitely not a place you want to find a bird’s nest or other contamination. Then, follow the cabin heat air system, beginning at the air intake. As with muffler systems, there is typically 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 less than $10 per foot.
Airworthiness directives, inspections, and overhaul requirements comprise another factor to consider with combustion heater systems. Most combustion heaters must be overhauled at 500 hours of use. If you have a dedicated hobbs meter on the heater itself, it can be a long time before you reach 500 hours of use. However, if you do not have a reliable way to measure actual heater use, you will have to go with the FAA-approved guideline of 50 percent of the total aircraft service time. In other words: 1,000 hours of tach/hobbs time of the airplane equals 500 hours counted for the heater.
Winter brings more than a few challenges to safe flying. With some proactive inspection and maintenance, your cabin heat system doesn’t have to be one of them. And, I’ve also heard that a properly working heating system does wonders to cutting down on cabin noise … from the passengers in the back!
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 available for iPhone, iPad and Android, and on the Web at www.SocialFlight.com.