December 8, 2009
One of the primary benefits of an electronic ignition system for airplanes is the ability to optimize the firing of the spark plugs based on the ambient conditions, type of fuel being burned, and other variables. In today’s ignition systems, the magnetos are set to fire the spark plugs at only a fixed time in the compression stroke. It’s simple and it works, but it is not exactly elegant.
Full authority digital engine control (FADEC) made its debut in jet engines a couple of decades ago. There, the pilot simply sets the thrust levers to the takeoff position once on the runway. After takeoff, he slides the levers back one click to max continuous thrust. In cruise, he may click them back one more detent. Computers consider the ambient conditions and other variables and take care of the rest.
Similar systems have long been envisioned for piston airplanes, with the promise of lower workload, better fuel economy, longer engine life because the computers can better manage the powerplant than can ham-fisted pilots, and the ability to run the engine on unleaded fuel. Numerous electronic engine controls have been developed for piston engines and a few have been certified. Teledyne Continental Motors has long been a leader in such developments. With its partner, Aerosance, TCM certified and began delivering a piston-engine FADEC system a number of years ago, but it hasn’t exactly caught on. TCM recently brought to market a simpler system that considers fewer ambient conditions, but still does an admirable job of managing the engine.
During a recent visit to TCM’s headquarters in Mobile, Alabama, I had the chance to fly a Cirrus SR22 with an IOF-550-N engine, the “F” the indicating that it is FADEC equipped. With TCM Pilot Phillip Grice in the right seat, I turned the key in the panel, allowing battery power to flow to the FADEC system. I then pushed the “ignition enable” button and then the “start” and the engine started—no fuss, no muss. In cruise, I set cruise power at 75 percent and, with the engine page showing on the Avidyne multifunction display, watched the FADEC system fine-tune the fuel flow for best power. It was during this process that it first occurred to me that the Mixture lever was missing from the airplane. We truly were managing the engine with just one lever.
Punching the “Best Econ” button puts the FADEC system into best economy mode, which, over the course of a couple of minutes, caused the engine to smoothly go lean of peak, shaving a couple of gallons off of the fuel flow. The FADEC system is redundant with a computer box controlling each pair of cylinders. Each box has two separate systems inside and each is connected separately to dual electrical supplies. The likelihood that any one failure would take down the system is highly unlikely. But the system continually self tests and alerts the pilot to any anomalies. A caution light indicates a bad sensor or other minor problem that should be fixed as soon as practical. A warning light requires you to land right away.
Back at Mobile Downtown Airport, we shut the airplane down with another push of the ignition enable button and waited a few minutes for the engine to get good and heat soaked to try out one of FADEC’s most promising assists to the pilot: managing a hot start. Starting a hot fuel- injected Lycoming or TCM engine can require a flurry of hand movements among the usual three levers and the boost pump, and often a few choice words of encouragement. With the FADEC, it was simply a matter of hitting the button and waiting a second for the system to run the fuel pump and then the engine started; religion still intact.
A FADEC’s ability to vary the spark timing means that it can—to a limit—allow the engine to run safely on a lower-octane fuel, such as will likely result when we move away from leaded avgas. The lead additive in avgas increases octane and raises the detonation margins, especially in high-compression engines, such as the IO-550. Without the lead, reduced detonation margins increase the possibility of engine damage under certain circumstances. FADEC and other types of electronic ignition systems can manage engine power and other variables to reduce the likelihood of damage.
While FADEC and related systems show promise, one of their greatest promises—the ability to easily allow the engine to run lean of peak—has been eclipsed by other systems as FADEC certification and acceptance drags on. Once elusive, LOP operations are now commonplace thanks to sophisticated engine analyzers now standard on new glass-cockpit airplanes and easily installed aftermarket systems. Nonetheless, with leaded avgas most certainly on its way out over the next 15 years or so, electronic control of engines will become more mainstream. TCM is busy preparing its entire product line to run on unleaded fuels. More on that to come.
E-mail the author at firstname.lastname@example.org.
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