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September 1, 2005
By Peter A. Bedell
Twenty-first-century airplanes have arrived with huge flight displays that are revolutionizing the way pilots fly light airplanes. Then why is engine management still in the Dark Ages? Cirrus Design has made inroads to simplify engine management by eliminating the propeller control in its SR-series airplanes, but there's still the misunderstood and misused mixture knob that tends to perplex pilots, especially those new to general aviation. Teledyne Continental Motors, partnered with Aerosance, plans to change all that with the latest version of its PowerLink engine control system.
For years, pilots have dreamed of automotive simplicity being applied to airplane powerplants. We go out to our car and turn a key; it starts every time and runs smoothly while a computer adjusts everything for best efficiency depending on myriad conditions. But in our airplanes, we have a giant, magneto-fired engine that gives us fits if it's too cold or too hot and requires a good bit of operational knowledge to keep it happy and living a long time. Almost two years ago, TCM and Aerosance rolled out its full authority digital engine control (FADEC) system and had it initially approved for aftermarket installation in Beechcraft Bonanzas. We flew a Bonanza with one of the first FADEC installations and were impressed with the lowered pilot workload and ease of hot starts. Unfortunately, as with any new system, there were some bugs — most notably in idle characteristics and jittery transitions from best-power to best-economy modes.
Since those first systems were introduced, the TCM/Aerosance team has been busy fixing the glitches, and we recently visited the TCM factory in Mobile, Alabama, to experience the results of the work. TCM has been test-flying a Cirrus SR22 equipped with the latest-generation IOF-550-N (the F stands for FADEC).
PowerLink completely removes the mixture control from the airplane, greatly reducing pilot workload and simplifying procedures. As previously mentioned, Cirrus airplanes don't have a propeller control, so the result in the FADEC-equipped Cirrus is a true single-lever configuration. PowerLink replaces the magnetos with engine control units (ECUs) mounted on the firewall. Each ECU contains two computers, each responsible for running one cylinder. The ECUs are tied to sensors that detect crankshaft position, manifold pressure and temperature, cylinder head temperature (CHT), exhaust gas temperature (EGT), and throttle position. The computers adjust timing and mixture to maximize either power or fuel efficiency. Logic also is built into the system to protect the engine from exceeding any limitation. For example, if a cylinder's CHT climbs above 420 degrees Fahrenheit, the ECU in charge will add fuel to that cylinder to cool it off.
TCM's Cirrus demonstrator has redundant electrical systems so there is no need for dedicated batteries to run the FADEC in the event of electrical failure. Aftermarket retrofits, such as those in the Bonanza, will require two backup emergency batteries to keep the plugs firing in the event of complete electrical failure. With the redundant power sources, PowerLink will make its own sparks regardless of what cooks the electrical system, just like magnetos currently do.
The system uses a sequential-port fuel-injection system. Typical fuel-injected Continental engines are fed a continuous flow of fuel through the injectors, which is a tradeoff between simplicity and efficiency. PowerLink monitors all of the parameters and tells solenoids at the injectors to squirt only the precise amount of fuel for the conditions. (Automotive enthusiasts may be shaking their heads in disbelief, but this is exciting new technology for general aviation.) The solenoids that meter the fuel at each fuel injector are the only moving parts in the PowerLink system. In theory, since only the required amount of fuel is being injected at the required time, there should be some fuel savings to be had in FADEC engines.
An all-new engine control panel (ECP) resides to the left of the Cirrus' primary flight display. The new box is more attractive and functional than those installed in the first few Bonanzas in the aftermarket, which had to marry old tech with new tech with mixed aesthetic results. With the clean-sheet design applied to the Cirrus, PowerLink has found a nice home.
The new ECP displays percent power and whether the engine is running at best economy or best power. It also has the ability to display faults in the system in the form of cautions or warnings with annunciations that clue in the pilot as to what's wrong. The red Warning light indicates a serious problem that requires landing as soon as practical. A yellow Caution indicates a fault that won't require a diversion but should be attended to soon. After a warning or caution event, information from PowerLink can be downloaded to a laptop computer or personal digital assistant (PDA) to determine its source, potentially saving technicians hours of troubleshooting. Data can be transmitted directly to techs at TCM through the Internet. There also will be support available by telephone.
Starting requires the push of a guarded switch on the ECP labeled Ignition Enable. This starts the boost pump running, activates the FADEC system, and primes the engine for start. Position the single power lever against a detent and push the Engine Start button and hold it until the engine fires. Technically, you could keep holding the Start button as long as you want because the system will disable it at any rpm above 300. This provides protection from inadvertently engaging the starter while the engine is running. The boost pump continues to run at any rpm below 1,500 to aid hot starts and purge vapor from the system. Although it's really only required for hot starts and climbs on hot days, TCM and Aerosance elected to have the boost pump run anytime below 1,500 rpm for simplicity purposes. In other words, it's one less switch that the pilot can misuse.
From startup, PowerLink is programmed to run the engine at the optimum mixture for any given condition. On the ground it'll run very lean to eliminate plug fouling and keep the engine free of excess lead caused by a too-rich mixture. The cold idle is very smooth right down to 600 rpm, something the early systems weren't able to do.
Engine runup is similar in procedure to what we do now, but what's happening under the cowl is entirely different. On the ECP the pilot selects channel A and then B, which fails one side of the six-cylinder engine's three ECUs, assuring that the engine will run on the remaining channel, similar to a magneto check. It simultaneously checks that the backup power source will run the system. In the case of the Cirrus, that would be the second electrical system. In aftermarket applications, this check would confirm that emergency batteries will pick up the load. Although, technically, there should be no more fouled plugs because of the optimized mixture, you could clear one by simply increasing rpm. During the runup, the ECP will display the rpm loss on either channel.
During takeoff and initial climb, PowerLink leans to a best-power mixture without individual cylinder optimization. There is no fuel-enrichment feature that pours extra fuel to the engine for cooling as we have now, so power should be slightly higher. Below 75-percent power, the system allows the user to choose a best-economy mixture setting once parameters are met. PowerLink does not allow the engine to be operated at best economy if power is deemed too high. If rpm is increased above 2,650, it assumes that you need it all, such as during a missed approach, and reverts to a best-power mixture.
We leveled at 4,000 feet and set power at 22.5 inches manifold pressure. The Cirrus' automatic propeller control had set an rpm of 2,500 or slightly higher. The ECP said we were making 71-percent power at the best-power mixture setting. True airspeed settled on 172 knots as the cylinders were running between 60 and 100 degrees F rich of peak. Based on TCM's engine model specifications and all the parameters monitored by PowerLink, the FADEC set a fuel flow of 15.7 gallons per hour. This translates to brake specific fuel consumption (BSFC) of 0.43. (BSFC is calculated by taking the horsepower divided by pounds of fuel burned per hour.) Cirrus' pilot's operating handbook (POH) calculations for a magneto-equipped SR22 flying in the same conditions show a fuel flow of 16.5 gph for a BSFC of 0.47.
In a steady cruise, with rpm between 2,000 and 2,650, pilots can select a best-economy mixture setting by simply pressing the Best Econ button. Once the button is depressed, the system begins a leaning cycle that can take anywhere from two to four minutes depending on altitude. During this time, the ECP displays an Autolean message while PowerLink is figuring out peak EGT for each cylinder and determining a best-economy fuel flow based on TCM's engineering data. During our test flight, after the system was done with its work, Autolean disappeared and was replaced by a Best Economy message on the ECP display. The system displayed 63-percent power, and the SR22 settled on 168 KTAS at 13.6 gph with the cylinders running smoothly 30 to 60 degrees F lean of peak EGT. That translates to a BSFC of 0.387.
Compared to the numbers in the SR22's POH, PowerLink saves some fuel. But for a real-world comparison, we had an experienced Cirrus operator duplicate the conditions of our flight in a magneto-fired SR22. He was able to obtain similar fuel economy numbers. In fact, TCM's Maintenance and Operations Manual for a standard, magneto-equipped IO-550-N shows that a BSFC of 0.375 can be had without FADEC. The catch is that those numbers can be achieved only by a knowledgeable pilot using a multiprobe engine monitor (for example, in conjunction with software such as Avidyne's EMax lean assist function on the FlightMax Entegra system in SR22s). For now, it's safe to say that PowerLink's gains in fuel efficiency are modest until we have more experience with the product.
In the transition from best power to best economy, we did not see any engine roughness or indication of what was going on unless we were watching the fuel flow gauge. The system is transparent, allowing the pilot to concentrate on flying the airplane. If you change altitude or power setting, PowerLink will adjust the mixture accordingly. Attempts to fool the system by slamming the power lever forward and aft were met without hesitation or stumble. This airplane, which is being used for FAA certification, has seen nearly all a pilot can throw at it.
Back on the ground, the only operational change comes when you shut down the engine. Lift the guard and depress the Ignition Enable button and the prop stops. We let the engine cook under the cowl for several minutes and then tried a couple of hot starts. For simplicity, TCM has designed the system so that the start procedure is always the same. During a hot start a few more blades will pass before the engine fires. After a few shudders it rolls into a nice low idle. Anyone who has fought his share of hot-start battles with fuel-injected engines will definitely appreciate the hot starts of the FADEC.
Like your car, when you bring your PowerLink-equipped airplane to the shop for maintenance, a technician can download engine data through a panel-mounted data port into a computer for analysis and troubleshooting. For example, the owner of one of the Bonanzas equipped with FADEC had a Caution light illuminate on the annunciator panel. After landing, he downloaded the information, which pinpointed a problem with the bottom plug on the number-two cylinder. Sure enough, a lead had backed off that plug. Granted, an engine analyzer and a knowledgeable user could have made the same diagnosis, landed, and pinpointed the problem just as fast. However, not all pilots are engineers who fly.
While you throw out a pair of magnetos worth about 10 pounds or so, the FADEC system and its associated components bring a net weight gain of about 25 pounds to a Cirrus equipped with redundant electrical systems. Aftermarket retrofits of PowerLink require the emergency batteries, adding 12 more pounds per installed engine. As electronics tend to get smaller with time, PowerLink may similarly shed some extra pounds in the coming years.
Because the system is so new, it's too early to report how engine durability will be with FADEC installed. In theory, a leaner engine is a cleaner engine, and the more precise metering of fuel should reduce lead contamination in the oil from unburned fuel. Field reports seem to support that theory with owners and mechanics reporting cleaner oil in engines equipped with PowerLink. Field service and maintainability also have to be proven in the coming years.
PowerLink is being installed on the new Liberty XL2, and other manufacturers are evaluating it for their new airplanes. Currently, STCs have been obtained to install the FADEC system in the aftermarket on the Beechcraft Bonanza and Baron with more to follow.
TCM and Aerosance have made huge improvements to PowerLink, and the future of piston-engine control looks like it's finally catching up to that of turbines. Purists may denounce the system for taking too much control from the pilot. With a mixture control, pilots have an infinite amount of adjusting to do. While that's true, simply flying behind a PowerLink-controlled engine may convert some of those purists when they see how little attention is required for engine management. Of course, pilots who are not engine wizards will benefit from the fact that all they have to do is concentrate on flying the airplane, not the engine. With PowerLink, voodoo starting techniques are no longer necessary. Additionally, leaning techniques passed on through old wives' tales will be replaced with computer-controlled mixture settings based on factory-derived data.
The potential of PowerLink goes beyond power management through engine timing and mixture management. Someday we might see cowl flap operation controlled by the system or wastegate control of turbocharged applications. If the plug gets pulled on leaded avgas, PowerLink will be able to accommodate unleaded fuels with a software change.
On the maintenance side, the arcane techniques we currently use to determine engine health — as good as they are — will be replaced with computer downloads of information pinpointing the exact problem, saving hours of troubleshooting time and hopefully eliminating the he-said/she-said warranty claims with hard data. Since computers will take over control of our engines, longevity also is expected to increase. In fact, time between overhauls could someday be "on condition" as is done with turbine engines rather than the arbitrary calendar time/hour figures used today. Although it's just in its infancy, PowerLink and other systems in the works will bring the subject of piston-engine management out of the Dark Ages and match the advances we've seen in new-airplane instrument panels.
Pete Bedell is a first officer for a major airline. He holds type ratings in the Boeing 737, Canadair Regional Jet, and BAe Jetstream 41.
Links to additional information about PowerLink FADEC may be found on AOPA Online ( www.aopa.org/pilot/links.shtml).
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