FLIGHT CONTROL SYSTEMS

Story by – Dave Higdon

Source: AVIONICS NEWS, October 2019

“So why do you feel you need an autopilot?” a pilot’s spouse pointedly asked the pilot, known in the local aviation community as something of a gadget freak. “You fly the airplane just fine without one.”

The pilot of the household paused, momentarily stumped because he considered autopilots as one of those signs that an aviator has “arrived,” if you will. Then the nonflying spouse let the pilot spouse off the hook. While the flying spouse stumbled to form an answer, the nonflying spouse tossed a well-known aviation magazine on the lap of the flying spouse.

“Why didn't you tell me about the safety benefits of an autopilot, and why didn't you say you need help sometimes?” she asked. The two previously flew together for nearly 2,000 hours, and all those hours flew without an automatic flight control system installed and operating in their complex piston single. The nonflying spouse regularly held the yoke as the flying spouse took pen in hand after the controller chimed in with the dreaded phrase, “November something-something, stand by to copy an amended clearance.”

The nonflying spouse then commented on the rough air to ask how the flying spouse handled such situations when flying alone, which was most of the time. “Very carefully,” the flying spouse responded, “very carefully.” You can probably guess how this discussion concluded: With the pilot spouse diving into research to identify the best flight control-system solution for their bird and their budget. “We can get it installed at annual,” the nonflying spouse offered, “and we'll have it to help when we fly home for the holidays.” What pilot would possibly decline that offer? That was years ago when the dominant single-axis systems worked well tracking a heading or following a VOR signal.

Two-axis models added altitude hold, while other so-called wing levelers did only what the name implies: Hold a wingslevel attitude independent of any navigation input. Such systems remain on the market, joined in recent years by flight control systems far more capable and sophisticated than anything short of a business-turbine panel – back then. Today, the choices available would have seemed farfetched for light piston aircraft – flight control systems capable of all the above functions, plus following a GPS course, following the flight plan in a GPS navigator, even navigating complex instrument approaches, including descents to decision height on short final.

These functions once stood as the exclusive purview of highly sophisticated flight-management systems used in turbinepowered aircraft, from light general aviation up through and including business jets and airliners. Best of all, many of these systems are available for far less than our turbine-flying friends' systems, with many within reach of owners of even the most modest aircraft. And the selections are significant.

Some basics

For decades, the dominant autopilot sensing technology for light GA aircraft used a rate-based system driven by the electric turn coordinator to sense attitude changes. Prior to those systems, pneumatic-driven systems used air pressure to move the appropriate control surface in the correct direction. The other sensing technology employed in autopilots is the attitude-based system, which senses roll and pitch changes from the attitude indicator. The rate-based approach enjoys some advantages and disadvantages, like most any aircraft system.

On the safety side, electric turn coordinator rate gyro works independently of the attitude gyro, which predominantly gets power from the aircraft vacuum system used to drive the attitude gyro. For the attitude-based system, the failure of either the attitude gyro itself or the vacuum pump powering the attitude indicator spells the loss of the flight control function. With the same failure – an attitude indicator failure of whatever cause – the turn coordinator and the rate-based autopilot suffer no ill effects. The rate-based autopilot continues to drive the flight control system unaffected by the failure of the attitude indicator or air pump.

Additionally, rate gyros won't tumble due to flight in unusual attitudes. That's why flight instructors teach their students to recover from unusual attitudes or loss of control using the turn gyro – either the turn and bank or turn coordinator instrument – to first bring the wings to level when recovering from an unusual attitude or upset. Speaking of attitude indicator and vacuum pump failures, rate gyros enjoy higher reliability than attitude indicators. Attitude gyros may suffer with progressive performance degradation over a period of time, typically due to worn bearings supporting the spinning-mass gyro wheel.

Bearing wear in an attitude gyro causes precession seen in the response performance of the autopilot. Rate gyros continue to function with worn bearings to a high level of performance until the spin motor fails completely. Then there are the performance issues. Since rate gyros can't “tumble,” they operate consistently at any attitude, nor are they damaged or worn excessively by unusual attitudes. In addition, since a consistent turn rate requires a lower bank angle at lower airspeeds, rate autopilots often provide better aircraft turn control at low airspeed. Still, many operators prefer an attitude-based flight control system, citing smoother performance, more-accurate course and localizer intercepts and altitude changes. Time was when the choice was primarily budget driven as much as pilot preference. The rate-based systems typically offered a budgeting advantage over the attitude-based systems. Many of these considerations became moot over the past decade as avionics makers moved toward using solid-state sensors to detect attitude changes and digital air-data sensors for altitude control. Sure, some of the spinning-mass gyrodriven models remain available and function well. But don't look for any new flight control systems using those old-fashioned gyros for sensing. And that's where we pick up our exam of today's autopilot options. First, we examine the changes in sensing technology and how those changes apply to various functions available in today's modern flight control systems. This article focuses most on the newest-technology autopilots, with a brief review of long-existent models.

From simplest to sophisticated: Wing levelers, single and two-axis systems

When it comes to automatic flight control systems, nothing simpler exists than a stand-alone wing-leveler system. By whatever source it references, the wing leveler acts to hold level those wings. No heading or navigation input. Thus, to keep on course, the pilot must occasionally correct the aircraft heading; the wing leveler knows only to keep those wings level; drift varies with the winds aloft. Give the wing leveler the ability to follow a navigation input and that wing leveler evolves into a single-axis autopilot.

The ability to track the heading on a gyroscopic compass came first; then came the ability to track a radio signal such as a VOR transmitter. Today, the modern GPS navigator joins those VHF navigation signals and gyrocompass headings in helping steer the aircraft. Throw in the ability to hold an altitude and the two-axis autopilot brings true automated, dynamic flight control capabilities to the cockpit. The higher the capabilities of the autopilot, the more likely it is that it will sport a yaw damper to help counter a tendency of some aircraft to fishtail a bit, the nose hunting port to starboard and back. These are still considered twoaxis autopilots, even with the third surface of the rudder trim working to counter the yawing.

Capabilities beyond the axes

Today's modern, digitally driven GPS navigators help make sophisticated autopilots a powerful ally, effectively becoming a de facto flight management system, or FMS. In sophisticated business-turbine and commercial aircraft, the FMS is effectively the brains of the plane, with a database of navigation waypoints, VHF navigation frequencies, a VHF navigator and WAAS GPS navigator. All in a single box, typically found in the pedestal between the chairs of the captain and first officer. As digital autopilots grew in capabilities and GPS navigation became more prominent, pilots began tasking their autopilots to work hand-inglove with the GPS navigator in the FMS. Add the specifics of instrument approaches to the FMS database so that the autopilot can follow the approach plate and the so-equipped generalaviation aircraft enjoys the operational sophistication level of business jets, turboprops and commercial airliners. Here are a few examples.

AVIDYNE

This company's digital autopilot, the DFC90, adds the precision of an attitude-based flight control system and Avidyne's innovative, safety-enhancing flight Envelope Protection and Envelope Alerting capability, incorporates a Straight & Level button, employs Indicated Airspeed Hold, Enhanced Flight Director and more. The DFC90 can command a turn with a change in the heading bug of the gyro compass, allowing the pilot to make the autopilot command a right-hand 270-degree turn by turning the heading bug right by 270 degrees. The DFC90 features many other improvements, such as a dedicated Vertical Speed knob, pitch hold mode, wide autopilot engagement, additional annunciations in PFD 8.0, a dedicated GPSS button, automatic Back Course Approach selection, multiple color annunciations to indicate armed and active states, intercept mode annunciations, and Avidyne's hallmark ease of use. The suggested retail price is $9,999.

BENDIXKING

Since AirVenture Oshkosh in late July 2019, an innovative family company known as TruTrak has been part of the Honeywell avionics business after TruTrak's acquisition by BendixKing. (See story beginning on page 34.) And with that come the combined flight control product lines of both BendixKing and TruTrak. The products of BendixKing and TruTrak cover a broad spectrum of displays, size and capabilities. BendixKing's AeroCruze 230 provides owners of existing BendixKing flight control systems with a way to step up in functionality without biting the bullet for an expensive revision of many existing BendixKing installations. This unit is designed to fit within the existing form factor used by the legacy KFC 150 flight computer. BendixKing also designed the AeroCruze for remote mounting to provide space for KFC 200 and 250 installations.

The AeroCruze allows reuse of legacy KFC 150, KFC 200 and KFC 250 autopilot servos, offering owners the simplest, most costeffective upgrade path for their autopilot, according to the company. The KMC 231 Mode Controller gives the pilot control and annunciation functions with a glove-friendly touch interface and color LCD screen; the unit employs dedicated controls for frequent tasks. The unique mechanical design of the KMC 231 seamlessly integrates with the AeroCruze computer, allowing it to fit in the same mechanical location of the panel-mounted KFC 150 autopilot flight computer. Meanwhile, the TruTrak line remains one of the most diverse catalogs of flight control systems available for certificated and experimental aircraft. For experimental aircraft, TruTrak offers models starting as low as $2,100 for one of the Vizion models from a list with three different size configurations.

All three two-axis systems share the rich features list, which includes two standard servos, built-in ground track DG, track select mode, GPS nav mode, GPS steering mode, altitude hold, emergency AP level mode and far more. These interface with Dynon SkyView, Garmin G3X and G3X Touch, AFS 5000 Series, Grand Rapids GRT HX/HXr and others. For $5,000, operators of certificated aircraft can choose from among three size variations of the PMA Vizion, which share the same features list as the Vizion experimental models.

DYNON

Since before the company added certificated avionics to its product mix, Dynon offered its experimentalaircraft customers an autopilot option for its PFDs. What was true for the experimental, amateur-built market remains true today for Dynon's certificated products. For example, a Dynon SkyView System becomes a full-featured dual-axis autopilot for the incremental cost of the servos: $750 each. That option represents a significant saving over the cost of a separate autopilot system. Every Dynon autopilot can fly magnetic heading, GPS ground track, and horizontal Nav from any connected compatible radio or GPS. Now with new IFR capabilities, the autopilot also handles fully coupled approaches, coupled VNAV, IAS Hold, Mode Sequencing, and Flight Director guidance.

Additional standard features include emergency 180-degree turn capability and a new Level button to immediately return the aircraft to straight-and-level flight. The SkyView Autopilot now also features an alternate new set of simplified controls that streamlines controlling the modes most-often used, such as HSI+ALT for following navigation sources from GPS flight plans, or TRK+ALT for flying in the directions chosen. And because Dynon's autopilot takes its input from the company's EFIS systems, the flight control function respects limits on bank angle, airspeed and G limitations.

GARMIN

Garmin's latest flight control systems, the GFC 500 and GFC 600 flight control packages, reflect the company's embrace of digital sensing and control. The revolutionary GFC 500 autopilot brings unprecedented levels of capability, reliability and affordability to light single-engine, fixed-wing general aviation aircraft. The GFC 500 leverages Garmin's advanced attitude-based flight control technology developed for the GFC 700 autopilot, which drives Garmin's most-advanced glass flight deck systems such as the G3000 integrated flight deck.

Garmin designed the GFC 600 for aftermarket installation on highperformance single- and twinengine piston aircraft, turboprops and light jets. Like its companion GFC 500, the GFC 600 offers an array of toplevel safety and performance features. Both offer envelope protection, automatic level recovery from extreme attitudes using solid-state attitude and air-data sensors in place of mechanical gyros. Smooth best describes the experience of flying behind both the GFC 500 and GFC 600.

GENESYS AEROSYSTEMS

Those simple wing levelers seem to stay in demand, though as the capabilities of low-cost autopilots grew, the demand for wing leveler systems declined. But a few remain on the market today, with Genesys Aerosystems' well-regarded System 20, a popular choice among certificated systems. Typically listed among singleaxis autopilots, the old S-TEC model incorporates the flight control hardware, sensing and switching electronics into a standard turnand- bank indicator, so it takes up no new panel space.

The System 20 is a rate-based system, which senses when the wings roll out of level flight and applies opposite roll input to level the wings through an electrically driven roll servo. Pilots can opt for a stand-alone altitude-hold capability with the System 30 ALT. With this option added to the System 20, it becomes the System 30. Trim the aircraft at the desired altitude, push the altitudehold button and the 30 ALT holds the altitude flown at system activation.

Or the pilot can opt for the System 30, which incorporates the altitude hold hardware to the same 3-inch case with the other control electronics. The pilot need only trim the aircraft at the selected altitude and engage the hold. An annunciator light signals if the pilot needs to adjust the trim setting. Genesys' newest, the S-TEC 3100 digital flight control employs a raft of new functions and capabilities in a package informed by digital MEMS sensors in its integral AHRS. The S-TEC 3100 is a two-axis Digital Flight Control System, with standard Automatic Pitch Trim. Whether the aircraft employs a conventional analog six-pack panel or an advanced glass EFIS panel, the S-TEC 3100 supports a wide variety of cockpit setups. The integral AHRS drives the precise movements of the servos in aircraft lacking a digital EFIS display. The 3100 also provides such advanced features as envelope protection and alerting, as well as a singlebutton recovery function to protect against entering unrecoverable unusual attitudes.

CENTURY FLIGHT SYSTEMS

Century's 4000 autopilot brings state-of-the-art flight control to today's fleet of general aviation aircraft, done in a way to allow each pilot the equipment needed to do the job. Century made the 4000 expandable, so as new requirements arise, the needed functions can be added as desired.

Among the Century 4000's standard lateral axis features are Heading Hold, Nav intercept and track, the ability to fly fully coupled ILS approaches, localizer back course, VOR approach outbound intercept and track, lateral navigation steering and sequencing including en route, terminal and approach segments with compatible GPS receivers providing ARINC 429 steering commands. In the pitch axis, Century gave the 4000 trim prompting, pitch attitude hold, altitude hold, vertical speed select and hold, glideslope capture and track from above or below the glideslope. And the 4000 is completely capable of LPV and LNAV/VNAV approaches with compatible GPS WAAS receivers. If needed later, automatic electric pitch trim, Flight Director or yaw damper expansions may be added to the 4000.

OTHER OPTIONS

Wing levelers remain available for experimental aircraft and for basic aircraft, and many experimental PFDs provide autopilotcontrol functions with the addition of the appropriate servos. Check those manufactures for their available options.