Garmin GFC 700: jet quality performance: the integrated G1000 autopilot offers a powerhouse of performance and features, but not without a few kinks. Serious training is a must.
Because the system is deeply integrated into the G1000 flight deck itself, it's hard to simply describe the autopilot. It's also tough to make global statements because the exact implementation varies with different makes and models. We'll focus primarily on how the GFC 700 works in a piston-powered airplane, but understand that its performance is largely similar for the kerosene crowd.
In our own flight tests and talks with others, everyone's experience is that the new autopilot flies with greased-rail smoothness. This comes partly from the physical systems and servos that use the latest technology, but also from the data that's available to the system. It gets inputs not only from the Attitude Heading Reference System (AHRS), which is the digital equivalent of an attitude gyro, but also the GPS and air data system. This yields a degree of attitude sensitivity impossible with an iron gyro or a turn-coordinator rate-based system and it delivers the ability to anticipate control scenarios rather than react to them.
Small deviations between where the wind is drifting the aircraft and the correct track on an ILS approach are corrected before they're detectable to the pilot, for example. Light turbulence did get the system making quick and aggressive corrections, but no more than was necessary. We also had some fun with a Cessna 182 we were flying by alternately extending and retracting 20 degrees of flaps while flying the approach. Pitch corrections were immediate and perfect.
The system is also capable of flying holds and procedure turns with perfect rollouts every time. Since it knows the precise winds drifting the aircraft it can apply exactly the right correction. This is more a function of the newer G1000 software than the GFC 700, but the autopilot hardware flies the commands of the navigation software as well as we could hope.
The system uses the flight director/autopilot logic that's familiar to pilots of high-performance aircraft, but can be confusing to pilots transitioning from Skyhawks and Archers. Here's a quick review: The flight director is a system that tells the pilot where to fly to stay on course. The pilot can follow the commands of the flight director by handling the yoke and pitching or rolling the airplane to put the triangular aircraft symbol inside the complementary triangular space under the flight director command bars. (GFC 700 systems in turboprop and jet installations have another flight director style option, the older style two-bar display which looks like the two crosshairs of an ILS.)
The pilot can alternately engage the autopilot to follow the flight director. Think of the flight director as the brains of the operation that can't actually fly the airplane and the autopilot as the dumb brute that actually moves the flight controls to do whatever the flight director commands.
Since the flight director gets all the benefits of perfect turn anticipation and course corrections, a pilot following the flight director precisely looks like an ace. You might need to do this just to keep your hand-flying skills up or if a servo were to fail and disable only the autopilot.
You activate the system by pushing either the flight director (FD) button to bring up the flight director, or pushing the autopilot (AP) button to bring up the flight director and have the autopilot follow it. We'll assume for simplicity that the autopilot is on and we'll just call the FD/AP combination "the autopilot."
The GFC 700's status bar is top center on the primary flight display (PFD) and makes it clear exactly which mode you're in and what to expect. The status bar is actually key to operating the GFC 700 and we commend Garmin for packing so much information into that space--even if the more advanced modes take some training to interpret.
Pushing AP puts the autopilot into roll mode for lateral navigation and pitch mode for vertical navigation. This means the aircraft will maintain whatever pitch and bank it had when you turned the autopilot on, with slight pitch and bank deviations assumed to be unintentional and reset to straight-and-level flight.
The system has the familiar heading (HDG) and altitude-hold (ALT) modes you d expect. There's also a familiar vertical speed mode where you can set a vertical speed in hundreds of feet per minute up or down. If an altitude is set on the altitude bug, the system will climb or descend in a vertical mode until capturing that altitude.
New to many light-aircraft pilots is the flight-level change (FLC) mode. The mode pitches the aircraft as needed to hold an airspeed rather than a rate of climb. Maintaining a 1000 FPM climb might result in excessive pitch up in a normally-aspirated aircraft as power declines in the climb. FLC lets you hold Vy or Vcruise as needed all the way to altitude, adjusting the pitch as needed. You can use FLC for descents, too by setting a speed and reducing the available power. This can be a boon if you must maintain a speed per ATC restrictions or in icing conditions.
The integration of the system really shines in more advanced modes. Navigation (NAV) mode will follow either GPS or VOR/Localizer guidance, although GPS guidance is required for all that fancy flying of turns and holds.
The approach (APR) mode actually looks at the kind of approach you loaded to decide what to do. If you loaded a GPS approach, it will attempt to arm the best vertical glidepath available to help guide you down as well as guide you left and right. If you loaded an ILS, I the ILS frequency is automatically tuned and identified. When you intercept, the GDI source switches automatically to the localizer with the glideslope armed.
WHAT'S IT DOING?
The status bar or the PFD keeps you in the loop through all of this showing what the autopilot is currently following and what it's armed to capture. Don't want the glides-lope or GPS glidepath to capture? Just fly the approach is NAV mode. You'll get the same top-notch navigation performance but with no vertical guidance. VOR approaches are flown in APR mode with enhanced sensitivity, but they get a dedicated VAPP annunciation so you know the system is doing the right thing.
Because all this automation can breed confusion in the heat of battle, we love the fact that each button push is really a toggle. If you're in NAV mode and you get baffled because the system did something unexpected, push NAV again to revert back to ROL mode. This is a step better than simply disengaging the autopilot to work something out.
Other smart bits include over-speed protection, clear cautions and warnings if systems are performing incorrectly and excellent integration with control wheel steering.
POWER OPTIONS, HEADACHES
The system has even more sophisticated options, but here the waters get muddy. A vertical navigation (VNV) mode in some installations let the autopilot fly an entire sequence of courses and altitudes from an IFR arrival through all the step-downs on an approach. It can even follow combinations of published altitudes retrieved from the database and ATC-assigned altitudes.
This feature rocks--when the multiple criteria needed are met, and you don't arm it too early or miss the cue to accept it, and you remember to set your altitude bug at or below the altitude in your flight plan. There are several annunciations and vertical path cues that aren't intuitive, in our view. The system also can get baffled by unplanned changes like a late decision to descend in a hold.
We give Garmin high marks for taking the bold move and making this level of automation available to pilots not flying jets. But you should expect to spend quality time with the manual to master these features and be ready to revert to a simpler navigation mode if the system doesn't do what you expect.
The other item to consider is what happens when components of the G1000 flight deck fail. Because the system is integrated, losing the AHRS in a single-AHRS installation will kill both your pretty PFD attitude display and the autopilot. Lose the MFD when that's the only place you have autopilot controls? The autopilot may still be on and try to capture the last modes you used. But you can't change anything and once you disengage it, you're on your own. This is the seedy underbelly of integration without redundant systems. It's far from a deal-breaker, but it's worth considering before you go pony up the extra cash for a new G1000 airplane vs. and late-model used one (see sidebar).
It's worth noting that the system disengages when put into go-around mode. This was done, we're told by Garmin, because they wanted to keep the pilot in the loop during the go around to assure that power is added. But this doesn't make much sense, since go around is initiated by pushing a button next to the throttle.
If we have nits to pick, however, they're more with the G1000 software driving the system rather than the autopilot itself. For example, the annunciation for the kind of GPS approach the system is flying is dead-center on the HSI, but it's subtle. We'd like to see something more attention-getting to clearly indicate what guidance the pilot has and what minimums are in play.
Also the enhanced vertical navigation mode of the autopilot makes the flight plan page on the MFD an excellent choice for navigating on arrival and approach. But the moving map on this page lacks any declutter function. Garmin has improved the G1000 flight deck software in some crucial ways from the first flying example and we expect many of the current annoyances will be likewise squashed with future updates.
The end analysis is that the GFC 700 is a standard-setting change in both autopilot performance and cockpit automation capacity. It's hard to imagine a scenario in general aviation flying that the GFC 700 wouldn't be up to with a competent pilot pushing the buttons. And it's likely that pilot will be wearing a big fat grin.
RELATED ARTICLE: KAP 140: Doomed?
Does the appearance of the GFC-700-equipped aircraft mean that older G1000 aircraft with Bendix/King. KAP 140 autopilots will become second- class citizens? Not in the slightest.
While the venerable KAP149 is far from our favorite autopilot, it offers two features that make used aircraft with these autopilots attractive alternatives to new aircraft with GFC 700s: built-in redundancy and roll steering.
If the AHRS takes a vacation, G1000 pilots flying a Bendix/King KAP 140 still have a wing-leveler and full altitude control. In fact, there are several failure scenarios where the G1000/KAP140 combination is more capable than the identical failure on G1000/GFC 700 systems. This is simply: because the KAP 140 is a stand-alone autopilot that talks to the GI000 rather than lives inside it. Diversity breeds robustness here.
The KAP 140 also can take commands directly from the G1000 to guide the aircraft rather than relying solely on keeping the CDI needle centered. This means a software update to the latest Gl000 software gives KAP 140 aircraft the ability to fly perfect holds and procedure turns just like its GFC 700 cousin.
Enhanced vertical modes are a different story, as they may require an upgrade to WAAS--a far from trivial cost in an older G1000 system. This may not be an option at all. Check with a knowledgeable shop about what upgrades are available and what the fly-away costs will be before you purchase a late-model G1000 airplane.
You do have to enter the altimeter setting in three places in the KAP 140 setup (the G1000, the autopilot and the backup altimeter) and the G1000 altitude bug is still nothing more than an alerter with the actual vertical control done on the KAP 140 itself. Older systems don't offer a flight director. But we wouldn't rule these aircraft out just because they don't have autopilot buttons on the Gl000 bezel.
RELATED ARTICLE: WHEN CAN I BUY ONE?
GFC 700 RETROFITS: DOABLE, BUT NOT SOON.
When Garmin engineers designed the digital GFC 700, they started with a clean sheet of paper with the intention of creating a 21st century flight control system for a wide variety of new airframes. The result: System architecture that's layers deep, a design loaded with redundancy to bridge failures and an overall logic not copied from any other autopilot.
Being a clean sheet design, the G1000/GFC 700 was certified first for new production airplanes, but the fact that it can fly jets and light single-engine pistons with equal precision and reliability means there's definitely a retrofit market--eventually.
While other autopilots excel at commanding light airframes, they may fall short in a faster, heavier airframe.
Integration is the byword in the GFC 700.
It should be thought of as a sub-system of the G1000 rather than a standalone or add-on autopilot. In fact, if not for the G1000, the GFC 700 couldn't function at all. This might be disappointing news for owners who hope to install a GFC 700 as an upgrade. But stay tuned--Garmin has the G600 in the works.
Examining the GFC 700/G1000 technical interface unmasks some of its tricks. Much of the autopilot's precision comes from the dual flight directors that live in each of the GIA63 IAUs or integrated avionics units. In modest airframes--a new single-engine Cessna or Beech Bonanza--these IAUs are fed by a single air data computer for airspeed, altitude, vertical speed and outside air temperature. A single AH RS feeds attitude, rate of turn and slip/skid data.
The autopilot continuously monitors data from each of the two IAUs for comparison and crosscheck, while the servos receive their data from both lAUs over a high-speed RS-485 data bus.
In all GFC 700 interfaces, there's dual flight director input which is fed independently from each IAU for crosscheck. While a GFC 700 failure in any aircraft would cause a pilot to sit up straight, this redundant flight monitoring through a high-speed ethernet data bus is truly revolutionary in the world of autopilots and explains the GFC 700 reliability.
The GFC 700 in any aircraft--jet, turboprop or piston--is comprised of nearly identical hardware. All of the drive servos are identical other than the roll servo, the only difference being a higher slip-clutch torque setting.
Through the dual RS-485 serial interface, the GIA63 IAUs command the GSA80 servo actuators--electromechanical units that control pitch, roll and trim. Some applications include a yaw damper which also needs a yaw servo. These servos are microprocessor controlled and consist of a solenoid and a brushless DC motor contained in a lightweight aluminum housing. (Each servo weighs about 3 pounds.)
The motor control circuit board monitors the incoming digital data and drives the brushless motor as necessary to obtain the desired flight control response. An integral monitoring circuit board watches servo speed, monitors output torque and engages and disengages the drive clutch.
There are several obvious advantages to digital servos. They're more precise than older analog designs but, more important, these servos won't hardover as traditional servos are prone to do.
One thing we've noticed about the GFC 700 is its lively pitch trim speed. Garmin realizes that it's prudent to run the trim as fast as the FAA will allow for certification and this is exactly what they have done, making for a crisp, wallow-free ride.
Maintenance folks will appreciate the hardware design of the servos, since the gearbox can be removed from the motor assembly without having to mess with bridal cables and tension settings. Detachable servo mounts with a gearhead that remains in place is a technician's dream.
And when it's time to troubleshoot a GFC 700, the process is streamlined with an in-depth, on-screen diagnostics mode that allows technicians to control each servo and view servo torque, motor speed, control board status and other parameters pertinent to system operation. A stored maintenance fault log can be downloaded and sent to Garmin for a look-see. This detailed maintenance capability could save lots of time and money.
OK, so it's a near perfect autopilot. Can you buy one for retrofit? According to Garmin, this won't happen any time soon, given the architecture that relies so heavily on the G1000. Then again, a few years ago, Garmin said no way when asked about G1000 retrofits, but now there's an STC in place for a G1000/GFC 700 combination retrofit in some C90 King Airs.
Given the advanced capabilities of the GFC 700, one would think the installation portion of the autopilot in a G1000/GFC 700 retrofit would be a huge undertaking. Not so, say Garmin engineers. Aside from the physical chores of opening the airframe, mounting the servos and associated brackets and hardware on the airframe, much of the GFC 700 electronic components already exist in a G1000 suite. In most of the GFC 700 applications for lower-end aircraft, the autopilot controls are housed in the bezel directly left of the MFD.
In our estimation, the system to watch in the coming year or two is the G600 retrofit PFD/MFD combination that uses AHRS inputs that could feed a GFC 700. This could be the ticket to GFC 700 retrofits for round-gauge aircraft, albeit an expensive one. Our prediction is that it may not happen soon, but it will happen.
Jeff Van West is Aviation Consumer's managing editor.
With lots of digital data available to it, the GFC 700 flies with greased-rail smoothness.
+ Rock-solid performance, with capability to fly holds, procedure turns and approaches flawlessly.
+ Lucid visual annunciation keeps the pilot in the loop.
+ Potential maintenance issues proactively addressed.
- High level features can be confusing; software needs tweaking.
- Expect a steep learning curve and the need to remain proficient.
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|Title Annotation:||AVIONICS FLIGHT TEST|
|Author:||Van West, Jeff|
|Publication:||The Aviation Consumer|
|Date:||Feb 1, 2008|
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