Sandel Avionics SN3308 Electronic HSI: EFIS Becomes Affordable
The SN3308 is the world's first three-inch electronic HSI: small enough to fit in any GA panel, and at $7,895, priced about the same as an ordinary mechanical HSI. But, that's where the similarity ends, because the SN3308 is a magnificent multifunction color display that serves as an HSI, RMI, and moving map, and provides an integrated EFIS-style readout of nav data from your VOR, GPS, Loran, ADF, DME and marker beacon receivers. AVweb editor Mike Busch recently installed one in his Cessna 310 and still can't wipe the smile from his face. Here's Mike's in-depth review of what he calls "a glass cockpit for the rest of us."
A couple of months ago, I had Avionics West replace some of the avionics in my 1979 Cessna T310R. In particular, I changed out my decade-old Loran for a II Morrow GX50 approach-certified GPS. At the same time, I yanked out the airplane's original-equipment horizontal situation indicator — an old vacuum-driven Century NSD-360 — and installed the new Sandel Avionics SN3308 Electronic HSI in its place.
My decision to install the Sandel SN3308 was highly uncharacteristic, because anyone who knows me (or has read much of my writing) knows that I'm not exactly what you might call an "early adopter" when it comes to new technology for my airplane. I tend to avoid buying anything for the airplane — at least anything expensive or safety-critical — until it has proven itself in the field for at least a few years, and I often advise other aircraft owners to do the same. When Cermicrome cylinders were all the rage in the early 1990s, I urged caution. As early as 1991, I started discouraging owners of low-utilization aircraft from using Mobil's highly-touted AV 1 synthetic oil. In both cases, I wound up looking pretty smart in 20-20 hindsight. I held off buying a handheld GPS until the third-generation units came out. And although ANR headsets have been available since 1989, I didn't start using them until LightSPEED introduced the first reasonably-priced ANRs eight years later. (Not only am I cautious, but I'm also something of a cheapskate.)
So why in heaven's name would I install something like the Sandel SN3308, a brand-new, relatively unproven, moderately expensive gadget that uses bleeding-edge technology and comes from a new, start-up company with no track record in aviation? A primary flight instrument, no less?
That's easy to explain: I simply couldn't help myself. Once I took a close, analytical look at the Sandel SN3308 EHSI, I simply had to have one. And once I started flying with the SN3308, I simply can't imagine how I ever managed without it. I'm not exaggerating. Sandel's new instrument is that compelling.
I first read about the new Sandel instrument shortly after the prototype was first unveiled at NBAA and AOPA in October 1997. Not long afterwards, Tom Rogers of Avionics West saw the unit at the annual Aircraft Electronics Association trade show and told me how impressed he was. I first saw the SN3308 "in the flesh" at Oshkosh in August 1998, and drooled over it for days afterwards. But to be honest, I was too much of a skinflint to spring for the $8,000 purchase price plus installation. So beyond lusting in my heart, I did nothing.
A couple of months later, I had the opportunity to take a ride from Wichita to
Washington, D.C. and New York and back in one of Cessna's new Mach .92 Citation X muscle
machines. After dropping off all the pax at DCA and TEB, it was just me and the two pilots
on the return flight to ICT. So naturally, I spent the whole time standing in the cockpit
door looking over the shoulders of the cockpit crew. The Citation X was equipped with the
very latest Honeywell glass cockpit setup, with five giant 8-by-10-inch color displays
taking the place of virtually all conventional flight, navigation and engine instruments.
I asked a million questions about how the system worked, and reignited my visceral lust to
put an EFIS in my airplane.
Not long thereafter, my vacuum-driven NSD-360 HSI started acting up. On several occasions, the slaved heading gyro suddenly wound up 20 or 30 degrees in error, causing the autopilot to take me on an unplanned off-route excursion each time. Although I subsequently diagnosed the problem as being a clogged central vacuum filter, it reminded me that the NSD-360 was a pretty vulnerable instrument, and one that had required (and would continue to require) overhauls every few years at a cost of around $2,800 a pop. Somehow, that made the $8,000 price of the Sandel seem a lot more reasonable.
In October 1998, I saw the Sandel SN3308 again at AOPA Expo in Palm Springs. Sandel's booth was only two aisles down from AVweb's booth, and I found myself going back time and again to take another look and ask more technical questions of Sandel founder and president Gerry Block. The more I learned about the instrument, the more impressed I became. By the time Expo was over, I'd decided that I couldn't hold out any longer. I ordered a unit from Sandel Avionics and asked Tom Rogers to put me on the Avionics West schedule for what would turn out to be a month-long installation effort.
The Sandel Avionics SN3308 is the first really new general aviation flight instrument to come along in the last 20 years, and certainly the first one expressly designed for use in the GPS age. It's the first all-electronic flight instrument to fit in a three-inch panel opening, and consequently the first one that can be accommodated by a standard GA instrument panel layout.
Accomplishing this was no mean feat. The largest conventional CRT or LCD color displays that will fit in a standard three-inch opening have a viewing area of about 2 1/4 inches square. That's simply not big enough for a primary flight instrument. An EHSI based on such a display would have a compass rose substantially smaller than a conventional three-inch mechanical instrument, and would have no room left for all the other information that an EHSI should display.
The technological breakthrough that makes the SN3308 possible is a unique optical system that uses a small but extremely high-resolution color LCD display that's roughly an inch square and located near the rear of the instrument case. The image from that display is then projected by means of an extremely sophisticated optical system onto a rear-projection screen which constitutes virtually the entire face of the instrument. This approach makes it possible to use literally the entire 3-by-3-inch instrument face as useable display area. The resulting presentation is slightly larger and much easier to read than the face of a three-inch mechanical HSI. The display is razor-sharp and drop-dead gorgeous.
The Sandel SN3308 is also the first electronic HSI that's reasonably affordable for the owner of a piston aircraft. Of course, "affordable" is a relative term. The SN3308 sells for just under $8,000, which is quite comparable to the price of a conventional three-inch mechanical HSI system like a Bendix/King KCS-55A or Century NSD-1000. By the time you get it installed, you're looking at something between $10,000 and $14,000 depending on what equipment you already have in the airplane, what additional stuff you need to buy (such as a remote electric heading gyro and/or flux gate), and whether you buy those new or reconditioned. In any case, that's not exactly chump change.
But to put things in perspective, consider that until Sandel introduced the SN3308, the lowest-cost electronic HSI available was a Bendix/King EFIS 40 system that cost upwards of $50,000 by the time it's installed. Furthermore, the EFIS 40 utilizes a four-inch cockpit display that is simply too big to fit in most existing GA aircraft without tearing the instrument panel apart and starting over. If you fly anything smaller than a King Air, an EFIS 40 is probably pretty much out of the question.
The Sandel SN3308 provides an integrated display that combines all your navigation information in one place. In addition to the usual functions of a slaved HSI, the unit offers the additional functions of an RMI, GPS-driven moving map, weather-avoidance display, ADF indicator, DME indicator, remote marker beacon annunciator, and all the remote annunciation and switching functions required for an approach-approved GPS installation. Truly one-stop shopping for navigation information.
Compass functions: The EHSI heading display is driven by an external gyro-stabilized heading source. If the airplane already has a heading indicator with a syncrho ("bootstrap") output, the SN3308 can obtain its heading information from that. Alternatively, the instrument can interface with almost any conventional remote-mounting directional gyro (Bendix/King, Collins, etc.) and flux gate.
HSI functions: The SN3308 provides electronic analogs of a conventional mechanical horizontal situation indicator: compass card, heading bug, course pointer, course deviation bar, to/from indicator, vertical deviation (glideslope) indicator, and nav flags. The HSI display is larger and much easier to read than a mechanical instrument. Heading bug and course pointer settings include digital displays that make it easy to set precise headings and courses to the exact degree. The "NAV" button allows the HSI to be switched among up to four different nav sources (two VOR/ILS receivers and two GPS receivers), and the HSI pointers change color to indicate clearly which nav sensor is selected (green for Nav1, yellow for Nav2, cyan for GPS).
Sync functions: TheSN3308 features two extraordinarily useful "sync" functions: Heading sync slews the heading bug to the aircraft's present heading, while course sync slews the course pointer to the bearing of the active navaid or waypoint, automatically centering the deviation bar. Thus, "fly present heading" and "proceed direct" each require just a button press rather than a knob-twisting exercise.
RMI functions: In case you've never flown with one, an RMI ("radio magnetic indicator") is an instrument with a slaved compass card plus two indicator arrows. Typically, one of those arrows is hooked to the aircraft's ADF, while the other is driven by a VOR receiver. The arrows point toward the navigation station (NDB or VOR), and the slaved compass card allows you to read magnetic bearing to the station. The RMI is an extremely useful instrument for maintaining situational awareness, and is especially helpful for certain maneuvers such as DME arcs, course intercepts and holding pattern entries. The SN3308 incorporates extended RMI functionality by displaying two "bearing pointers," each of which may be enabled or disabled by the pilot and switched to any of the nav sensors connected to the instrument, including GPS, VOR and ADF receivers. On approach, for example, the HSI could be displaying the ILS, while the #1 bearing pointer points toward the outer compass locator (ADF receiver) and the #2 bearing pointer points toward the airport (GPS).
Map functions: The SN3308 incorporates a high-resolution moving map display which can be enabled or disabled by pressing the "map" key. At present, the map displays only waypoints provided by the GPS receiver, but the next software release incorporates an internal Jeppesen NavData database of airports, navaids and special-use airspace to support the map display. (More about this later.) Map range is adjustable by pressing the up- and down-arrow keys. The map function can be superimposed over the conventional HSI display, in which case it provides a 360-degree view with the aircraft in the center. Pressing the "vue" key toggles the display to a 120-degree arc view with the aircraft at the bottom of the display and a forward-only map display. The map is really beautiful, and makes the tiny monochrome map displays offered by most panel-mount GPS receivers look like a bad joke. (Garmin GNS 430 excepted, of course.)
Other displays: The corners of the SN3308 display contain alphanumeric displays. The upper-left corner contains the fix name and distance of the currently-active navaid or waypoint that corresponds to the HSI display, plus the digital course pointer setting. (The fix name is available only if the HSI is switched to GPS, while the distance is derived from either GPS or DME as applicable.) The lower-left corner displays the nav source, distance and bearing associated with the #1 bearing pointer, and the lower-right corner displays the same information with respect to the #2 bearing pointer. Finally, the upper-right corner shows the current map mode, map range, and the digital heading bug setting. The present heading is displayed digitally at the top-center of the instrument, while CDI and VDI displays are shown at the bottom and righthand edges of the display. Various annunciators also appear on the face of the instrument, driven by the marker beacon receiver (OM, MM, IM), the GPS (MSG, HOLD, APPR, ACTV, ARM, WPT, etc.), and the DME (HOLD). Finally, if the SN3308 is connected to a WX-500 Stormscope, weather returns may be displayed as small green Xs on the display.
A-B memory: A particularly clever feature of the SN3308 is its capability to keep track of two separate views, referred to as A and B. You can configure a particular desired display (map selection, map range, bearing pointers, and 360/120-degree view) in the A-memory, then press the "A-B" key to switch to the B-memory and set up a different display configuration. Now, you can quickly toggle back and forth between the two by pressing the "A-B" key. It's an extremely useful feature once you get the hang of it.
During my recent visit to Sandel Avionics just north of San Diego, Calif., I had the opportunity to talk at length with company founder and SN3308 inventor Gerry Block about the history of the company and his vision for the future. I also had the chance to see the SN3308 opened up and to get familiar with the unusual construction and remarkable technology incorporated into the instrument.
As you might guess, the inside of the SN3308 is mostly air. The projector lamp, LCD display panel and some very sophisticated optics occupy the rear half of the instrument case, and the unusual construction is dictated by the requirement that nothing get in the way to cast shadows on the rear-projection screen that forms the face of the instrument. All the electronics are contained on three full-length circuit boards that hug the bottom and both sides of the instrument case. There is no circuit board on top, facilitating ventilation and maintenance access to the lamp and optics.
One of the three circuit boards houses a high-performance 32-bit microcomputer consisting of a Motorola PowerPC processor plus a graphics generator, four megabytes of RAM and four megabytes of non-volatile flash memory. The flash stores the operating software, user-specific setup and configuration parameters, and (starting with the next software release) a Jeppesen NavData database of airports, navaids and special-use airspace to support the moving map feature of the SN3308. Sandel makes both software and database updates available for download from the Sandel Avionics Web site, whereupon they can be uploaded into the instrument via a data cable that plugs into a panel-mounted jack without removing the instrument from the airplane (using a laptop computer running Windows 95 or 98).
The remainder of the circuit board space is largely occupied by the logic that supports the SN3308's unusually large number of external interfaces, including dozens of analog inputs and outputs plus five serial ports. Also on the circuit boards is a regulated switching power supply and circuitry to regulate projection lamp voltage and cooling fan speed under software control.
The heart of the SN3308 display system is a tiny active-matrix LCD display panel that's only about one inch square, yet offers an astonishing 432,000 pixels, organized into 480x300 color triads. White light from the projection lamp is split into tri-color beams, then passed through the LCD panel. The resulting image then passes through a series of lenses which focuses it precisely on the rear of the instrument face, a translucent rear-projection screen with its own integrated Fresnel lens to provide uniform edge-to-edge brightness and focus. The result is a razor-sharp color image with no hint of distortion or "jaggies."
The front-panel controls deserve special mention. The nine keys — three each along the bottom and both sides of the instrument face — are actually transparent plastic arms that are artfully arched to avoid casting shadows on the screen. Each arm ends up at a pushbutton switch mounted on one the circuit boards. Just ahead of each switch is a tiny green LED that provides illumination to the keys for night use. The plastic arms serve as "light pipes" and each one has a tiny silvered indentation that serves as a 45-degree mirror to direct the light from the LED forward to the tip that the pilot sees as a pushbutton. A truly elegant solution to a thorny mechanical design problem.
The front-panel course and heading knobs are also interesting. Each is attached to a shaft that runs back along the extreme corners of the instrument case until out of harm's way, then via a tiny universal joint to another shaft that angles inward toward the rear of the instrument behind the optics package. At the end of that shaft is an instrument-grade optical encoder that reports the precise position of the knob to the microcomputer.
In order to integrate all this navigation information into a single instrument, the Sandel SN3308 needs to interface with virtually all the navigation sensors in the airplane: GPS or Loran, VOR/LOC, glideslope, DME, ADF, marker beacon, and possibly a Stormscope. It also has to interface with the aircraft's autopilot to provide it with desired heading and desired course (from its heading bug and course pointer, respectively).
In a perfect world — say if we were flying a Boeing 777 or a Citation X — this would simply involve connecting all those boxes to the aircraft's local-area network. But despite the fact that we're on the doorstep of the 21st century, the world of piston-class general aviation avionics today is about as far from perfect as you can get.
Even though most of the avionics we fly with today are solid-state designs from the 1970s and 1980s, largely digital and controlled by embedded microprocessors, most of their interfaces use analog standards that date back to the 1940s and 1950s. I never realized just how far into the dark ages GA still is in this regard until my Sandel SN3308 installation put me face-to-face with the myriad of horrible old legacy analog interfaces in my airplane. Frankly, I was revolted. Worse, it seems as if every avionics manufacturer came up with its own interface standards, so boxes from ARC, Bendix/King, Collins and Narco are almost guaranteed to be incompatible with one another.
The traditional way of dealing with this problem is to use external adapters. With enough adapters, you can interface anything with anything. For example, you can hook a Bendix/King KCS-55A HSI system to Collins Microline radios and a Cessna/ARC 400B autopilot, but doing so requires a bunch of extra boxes, connectors and wiring that complicates the installation and increases its cost, size and weight significantly. Not to mention that it's just plain ugly.
In this regard, the engineers at Sandel have managed to pull off what I consider something of a miracle: They've come up with an innovative design that enables the SN3308 to interface directly with almost any navigation sensor and autopilot without the use of external adapters. How? Basically, the SN3308 has its discrete analog inputs and outputs (dozens of them in all) implemented via general-purpose A-to-D and D-to-A converters whose functionality is almost completely defined by software. By programming these inputs and outputs correctly, they can meet almost any existing analog standard: DC, AC, sine/cosine, quadrature stepper motor, whatever. The SN3308 also has five digital serial ports — three ARINC 419/429 and two RS-232/RS-422 — to interface with newer-technology sensors with digital outputs (Lorans, GPSs, DMEs, and the WX-500 Stormscope). The result is an instrument that can interface with virtually anything.
To date, Sandel has not encountered a single nav receiver or autopilot to which the SN3308 can't interface. About the only boxes it has not been able to accommodate are some older Narco DMEs. Sandel's installation manual has detailed schematics of the proper hookups with the most commonly-encountered ones; others sometimes require the installer to call Sandel for consultation. In rare cases, an external resistor or capacitor or transformer is required to get the interface to work, but usually not. Adapter boxes are virtually never required.
The installing technician simply powers up the SN3308 in maintenance mode and programs the desired interfaces on a series of 20 maintenance pages. These pages not only allow the installer to tell the SN3308 what sorts of boxes it is interfacing with, but also to fine-tune those interfaces right from the instrument itself. The technician can compensate the flux gate, adjust autopilot sensitivity and centering, correct VOR OBS errors, and make many other such adjustments while sitting in the pilot's seat. In fact, some of these (such as autopilot adjustments) are best made in-flight. Incredible!
Although the SN3308 is designed to be able to interface with almost anything, the large number of interfaces makes careful pre-installation planning absolutely essential. The installer needs to make an inventory of all the navigation equipment in the aircraft, then carefully review with the owner precisely which are to be interfaced with the SN3308. Lots of important decisions must be made at this point that will affect the ultimate functionality of the installation. The best way I can explain this is to take you through the decisions I had to make in my own installation.
First on the agenda was to decide where the SN3308 will obtain its heading information. In the case of my T310R, one option was to use the existing NSD-360 mechanical HSI as the heading source, moving the instrument over to the copilot's side and obtaining heading data from its "bootstrap" output. This would have been the least expensive approach, but I rejected it because one of my objectives in installing the SN3308 was to get rid of that old, unreliable, expensive-to-overhaul vacuum-driven gyro.
Instead, I decided to rip the old NSD-360 system out by the roots and install a remote electric heading gyro to provide the heading data. The SN3308 can be made to interface with most remote gyros, but the two most economical and readily available candidates were the Bendix/King KG-102A and the Collins 332E-4. The Collins was a bit cheaper, but I decided on a Bendix/King KG-102A because it's well-supported by AlliedSignal and it interfaces particularly nicely with the Sandel instrument.
Buying a new KG-102A gyro and its companion KMT-112 flux gate would have cost me about $4,000, but being the skinflint that I am, I decided to look around for a better deal. I called around to a bunch of instrument supply houses to ask whether any of them would be willing to trade me a reconditioned KG-102A gyro and take back my serviceable NSD-360 HSI in exchange. Most of the firms I spoke with said they could not sell me a reconditioned KG-102A without getting a serviceable KG-102A core in return. But when I called Southeast Aerospace in Melbourne, Fla., Joe Braddock said he'd be happy to accommodate me. He quoted me $2,875 for the reconditioned KG-102A/KMT-112 with mounts and connectors, and offered me $2,000 core credit for my NSD-360 system in return, for a net out-of-pocket cost of $875. Much better!
Upon discussing this proposed trade with Tom Rogers at Avionics West, Tom suggested that I really didn't need the KMT-112 flux gate because the flux gate already installed in my airplane would work just fine with the SN3308, and leaving it there would also simplify the installation. I phoned Joe Braddock at Southeast Aerospace again, and he said eliminating the KMT-112 would bring the cost down to $2,300, and not returning my existing flux gate would reduce my core credit to $1,900. With shipping charges, my net out-of-pocket for the exchange was now down to $400. Joe, you got a deal!
My next decision was on which GPS to install in place of my ten-year-old Loran. I'd originally planned to go with an inexpensive VFR unit, but ultimately decided to bite the bullet and install an approach-certified unit. My choice came down to the Garmin GPS 155XL and the II Morrow GX50. I ultimately decided on the GX50 based on my preference for the II Morrow user interface (mostly, the non-resolver interface) and my past good experience with II Morrow customer support.
The downside of my GX50 choice was that II Morrow uses an RS-232 output rather than an ARINC 429 output. ARINC-429-capable GPS receivers like the Garmin and Bendix/King units output a richer serial data stream that includes not only present position and route waypoints but also nearby airports and fixes. They therefore produce a richer moving map display on the Sandel SN3308. However, I knew that this difference would soon become relatively moot when Sandel releases its forthcoming software update that includes an internal airport/fix/SUA database. So when the dust settled, I settled on the II Morrow box.
Another wrinkle I hadn't counted on concerned the fact that the GX50 requires a source of serial altitude data in order to operate in approach mode with the required RAIM capability. The encoding altimeter in my airplane provided parallel output only, so I'd either need to install a parallel-to-serial adapter or a blind encoder with both serial and parallel outputs. Since my encoding altimeter had been giving me some trouble when it got hot, I opted to install a new Trans-Cal blind encoder with dual-mode outputs. This tacked on another several hundred dollars to the tab.
DME presented another problem. My aircraft is equipped with a Bendix/King KNS-80 which integrates VOR, LOC, GS, DME and RNAV functions into a single panel-mounted box. It has been a great and reliable radio and I didn't want to part with it. Unfortunately, however, the KNS-80 was not designed to drive an external DME indicator and provides no external DME output of any kind. Consequently, the Sandel SN3308 would not be able to display DME distance unless I installed a different DME such as the Bendix/King KN-64 panel mount (about $3,300 new or $1,700 refurbished). Since DME seems to be on the way out in today's GPS world, I decided to leave things alone and forgo the remote DME display on the SN3308, at least for now.
Then, there was the matter of nav switching. The SN3308 provides a single set of analog inputs for the HSI display: course deviation, to-from, nav flag, glideslope deviation, and glideslope flag. To permit the HSI to interface with two nav sensors — say, VOR and GPS — requires an external relay box. As it happened, I already had one of these in the airplane: a NAT RS08 switchbox used to switch the old HSI between VOR and Loran. To interface the SN3308 with three sensors — say VOR#1, VOR#2 and GPS — requires two relay boxes and additional wiring. I decided to keep things simple by not trying to interface the HSI with VOR#2. However, VOR#2 would be available to drive the SN3308 RMI bearing pointers, a simple hookup that requires no additional relay switching.
(Incidentally, integrated GPS/VOR/ILS receivers like the new Garmin GNS 430 handle the switching internally, so no relays are required and the hookup is considerably simplified.)
Finally, it was obvious to me that I was going to have to rearrange the panel a bit. The path of least resistance would have been simply to install the new GPS where the old Loran had been located. Unfortunately, the Loran was mounted in a hard-to-see, hard-to reach location below and to the left of the control yoke, and that position would never do for what was going to become my primary navigation receiver, particularly in view of its own moving map display. So I asked that my KR-87 ADF receiver be moved over to the location vacated by the Loran, providing enough room in the central radio stack to mount the GX50 in a prime location just under the KMA-24 audio panel. Making this switcheroo would require a bunch of rewiring (and some additional labor hours), but it was clearly a must.
Once these decisions were made, it was time to "down" the aircraft and start the actual installation process. The job fell to Tom Knoll, the most experienced installer at Avionics West, who has been doing this kind of work for more than 20 years. Tom had done previous avionics installations in my airplane, and I had a great deal of confidence in his ability to handle this admittedly complex installation. But because I was curious (and because I knew I was going to be writing this article), I asked Tom to let me look over his shoulder, ask questions and take photos as the installation progressed.
Tom's first step was to round up installation manuals and schematics for all the various avionics installed in the airplane to make sure that he had the connector pin-outs for each unit. He then created a big Excel spreadsheet that had columns corresponding to each item of avionics, and rows for every signal that needed to be hooked up. The SN3308 took up three columns because it has three 37-pin D-connectors on the back (111 pins in all) to accommodate the extraordinary number of inputs and outputs. The cells of the spreadsheet contained the relevant connector pin numbers for that signal. Constructing this spreadsheet took hours and hours of painstaking work, and by the time it was done, it had a dozen columns (avionics connectors) and well over 100 rows (signals) with pin numbers at each appropriate intersection. This spreadsheet would become the wire list that defined the wiring harness and hookups, and would ultimately be placed in the aircraft maintenance records for the benefit of any future avionics technician that winds up having to work on the airplane.
Next step was to open up the airplane. The seats came out, the glare shield came off, and so did a few side panels to provide access to the wiring. All panel-mounted radios were removed, the instrument panel was partially disassembled, and all radio racks were removed from their mounting rails. By the time Tom was done with this phase, the airplane had that "I'll never fly again" look, and I made him promise not to get run over by a truck or eaten by a bear until the airplane was back together.
The old NSD-360 HSI and its associated slaving amplifier were removed from the airplane, boxed up, and shipped to Southeast Aerospace as part of the exchange arrangement I'd made. Now Tom had to find a suitable mounting location for the new KG-102A remote gyro and the Trans-Cal altitude encoder. The encoder mounted nicely where the old slaving amp had been with easy access to the aircraft static system. The KG-102A presented more of a challenge because it is rather large, sits high on its shock mounts, and has to be mounted level to function properly. Tom wound up having to fabricate a moderately complex mounting bracket and rivet it into the tailcone to accommodate the new gyro.
Next step was to cut lots of tie-wraps behind the instrument panel and then rearrange the radio stack in the fashion we'd decided upon during the planning session. As anticipated, there was not enough length in the wiring harness to the ADF to permit it to be moved over to the location of the old Loran, so a bunch of wires had to be spliced and extended.
The Sandel SN3308 requires a standard 3ATI instrument panel cutout, identical in form factor to my old NSD-360 HSI, so no metalwork was required. Tom did have to pick a spot on the panel for mounting the SN3308's external dimmer potentiometer and its data loader jack. He also had to mount a couple of new circuit breakers on the circuit breaker panel.
Installing the new GX50 was straightforward enough, but mounting the GPS antenna turned out to be a bit tricky. The old Loran had a bottom-mounted antenna, a site that clearly wouldn't work for GPS. Tom wanted to find a way to mount the GPS antenna on the cabin roof without having to drop the headliner, which is a messy and time-consuming job. There was room to mount the antenna immediately above the cabin loudspeaker, but the question was whether it was possible to snake the antenna feedline to that location with the headliner in place. After a lot of patient probing with a piece of stiff coat hangar wire, Tom managed to do exactly that.
The next step was making up the wiring harness. After making a bunch of length measurements and notes in the airplane, Tom moved back into the maintenance office and started work on the harness. This is painstaking, time-consuming work. Each wire is cut to size, repetitively marked along its length with the appropriate circuit ID, and crimped or soldered to the appropriate interface connectors. Many of the wires are shielded, requiring extra effort. The harness is made up in special wire looms, then meticulously laced up, tie-wrapped, and heat-shrinked into a complex-yet-tidy umbilical cord. The job isn't finished until every wire in the harness is "ohmed out" against the wire list to make sure there are no mistakes or shorts. A few extra hours at this stage could prevent some expensive fried avionics later.
Now it was back to the aircraft to install the harness and complete the final hookup and partial reassembly of the disassembled instrument. This went surprisingly quickly, and before long it was time for "the smoke test" . . . initial power up. I watched anxiously as Tom applied power to the airplane and turned on the avionics master. No smoke! Everything lit up the first time!
I told you Tom was good.
With power to the avionics, the time had come to program the SN3308 inputs and outputs for the particular interfaces required in this installation. To do this, the SN3308 is placed in "maintenance mode" by powering it up while holding down two of the front panel keys. This causes the instrument to display a series of 20 maintenance pages which can be navigated with the up- and down-arrow keys. Each page allows configuration of a particular interface: NAV, GPS, DME, ADF, marker beacon, Stormscope, autopilot, etc. In most cases, this is just a matter of selecting which kind of box you have from a menu of different choices. In some cases, however, some fine-tuning is required (such as to null out any errors in the flux gate or OBS interfaces). I found the fact that the SN3308 lets you do this with software, rather than by opening up boxes and tweaking trim-pots, to be incredibly cool.
In the course of going through this setup procedure, each individual interface is
tested for proper operation. Despite all the pains taken earlier, we did have a few
problems at first. We wound up having to reverse a couple of wires in the KNS-80 resolver
interface, and a couple more wires in the KR-87 ADF interface. (At one point, we convinced
ourselves that we'd miswired the flux gate interface, but after trying various
permutations we ultimately concluded that we'd gotten it right in the first place.)
Finally, we concluded that we'd done just about all the testing we could do on the ground, and it was time to test-fly the system. That meant closing up the rest of the airplane, reinstalling the glare shield and seats and generally making sure it was ready to fly.
By the time it was done, this turned out to be a fairly big installation job. Tom Knoll indicated that he had approximately 100 hours of work into it all told. Of that, he estimated that about 40 hours was attributable to the installation of the GX50 GPS and blind encoder, including mounting the GPS antenna and rearranging the panel to put the GPS in an optimum location. That left roughly 60 hours for the SN3308 installation itself.
Sandel says to expect a typical SN3308 installation to require 40 hours, but my installation took half again as much time. There were at least three good reasons for this. First, this was Tom Knoll's first SN3308 installation, so it required a good deal of reading and research and discussion with Sandel tech support that he won't have to do next time. Second, my installation involved the removal of the existing air-driven HSI and the installation of a new remote electric gyro, with all the bracketry and wiring that this entailed. Finally, I spent a lot of time looking over Tom's shoulder, taking pictures and asking him a zillion questions and generally doing my best to slow him down. (If you don't believe me, just ask him!)
In any case, this is the sort of installation that should not be rushed. Figure on at least 40 hours of labor, and remember that a few extra hours spent in meticulous pre-installation planning will pay great dividends in terms of winding up with an installation that works just the way you want it to, with no surprises.
Airplane maintenance is like going to the toilet: The job is never finished until the paperwork is complete. The Sandel SN3308 is FAA-approved under TSO C113 as a primary navigation instrument to replace a conventional DG and HSI. The TSO ensures that the SN3308 meets the FAA's minimum performance standards for such instruments. However, in itself, the TSO does not constitute an approval basis for installing the instrument in your aircraft.
Sandel also holds an STC (STC #SA00696LA) for the installation of the SN3308 in a Beech F33 Bonanza. Installation in any other type of aircraft requires a Form 337 one-time approval. Sandel's installation manual provides detailed guidance on how to fill out the 337 form, as well as a specimen Airplane Flight Manual Supplement and Instructions for Continuing Airworthiness.
To ensure that owners have no difficulty getting the Form 337 approved by the local FSDO, Sandel has obtained a letter from FAA Headquarters stating that the SN3308 is an approved instrument under the terms of FAA Flight Standards Information Bulletin FSAW 95-09 "Electronic Horizontal Situation Indicator (EHSI) Approvals" which gives FSDOs uniform guidance on approving such instruments. Attaching a copy of this letter to the Form 337 for your SN3308 installation should make its approval more-or-less a rubber-stamp affair. Avionics West had no difficulty getting the local FSDO (Van Nuys, Calif.) to approve the 337 for the installation in my airplane.
Finally, the day arrived that I'd been waiting for: Tom Knoll and I would test-fly the airplane and put the new installation through its paces. Before launching, however, there was one more little chore: "swinging" the flux gate to make sure it was providing accurate heading information. This is done by taxiing the airplane in circles, stopping at exactly each 30-degree heading increment (as determined by the airport compass rose or a sighting compass of known accuracy), noting the flux gate heading on the SN3308 "compass calibration" maintenance page, and writing down the error. Then the four calibration adjustments, one for each quadrant, are tweaked to reduce the errors to as close to zero as possible. In my case, I was able to achieve accuracy within one degree on all headings. You can't ask for more than that. While I was at it, I re-swung the wet compass, too, and adjusted it for minimum error.
Time to go fly! Tom and I spent 45 minutes putting the system through its paces. For the most part, everything seemed to work more or less as advertised. However, the autopilot badly overshot its turns in both heading and nav mode. The Sandel installation manual had prepared us for that. We brought up the autopilot interface maintenance page on the SN3308 and adjusted the heading and course gradient values until the overshoot was eliminated. The autopilot flew smoothly now, but we noticed that in nav mode, with full CDI deflection, its intercepts were much shallower than normal. We made note of the problem and continued the flight.
The biggest problem we had during the initial test flight was two unexplained compass card deviations in the course of the 45-minute flight. This concerned us greatly, because this was clearly an intermittent problem and one we had no confidence that we could reproduce on the ground.
After landing, we called Sandel technical support to consult with them on the problems we'd encountered. On the autopilot too-shallow-intercept problem, Sandel tech support said that they were confident that the problem could be resolved simply by reducing the course gradient adjustment a bit further. With respect to the intermittent heading problem, they advised checking to make absolutely sure that the three 37-pin connectors on the rear of the SN3308 were fully engaged into the mating connectors at the rear of the clamp tray. Sure enough, a flashlight-and-mirror inspection revealed that we did indeed have a connector engagement problem. A simple adjustment to the connector panel on the rear of the clamp tray resolved the problem.
Two days later, Tom and I went up for a second test flight. We adjusted the autopilot outputs once again and were able to get proper intercepts. We flew a bunch of intercepts and approaches using VOR, ILS and GPS and everything worked great. The compass headings remained solid as a rock, confirming that the connector engagement problem had indeed been the cause.
The following week, I went up for a third flight, this time with my good friend Chris,
an excellent instrument pilot who owns a Bonanza and a Cessna 195. We flew for an hour and
a half and put everything through its paces. By the way, it's absolutely essential to have
a safety pilot along for the first several hours after doing an installation like this
one. I found myself so totally engrossed playing with the SN3308 and GX50 that I could
easily have run into a mountain or another airplane had Chris not kept elbowing me in the
ribs when I screwed up. (Reminds me of a good friend who took up his R182 for a solo test
flight after installing a fancy new autopilot. He got so preoccupied with his new toy that
he wound up landing gear-up.)
Actually, I found the heads-down problem to be much worse with the GX50 than with the SN3308. Sandel created a user interface designed to be as conventional as possible, to ensure that users familiar with a mechanical HSI would have no difficulty using it. According to Gerry Block, this turned out to be an important issue with the FAA's human-factors engineers who were involved in the SN3308 certification process, and they provided a lot of valuable feedback to Sandel on how to make the instrument easy to use. As far as I'm concerned, they certainly succeeded.
However, the SN3308 has a great deal of functionality that a mechanical HSI does not have, and it takes a little practice to learn how best to take advantage of those capabilities. Even so, I found that "coming up to speed" on the SN3308 took me only a fraction of the time that I needed to get comfortable with the new GX50 GPS (which, by all accounts, has one of the easiest user interfaces of any approach-certified GPS). Using the SN3308 becomes second nature after a few hours.
For the most part, the symbology and color-coding used by the SN3308 is modeled after high-end EFIS systems, and will be familiar to anyone who has flown with EFIS. Most piston-aircraft pilots haven't, so they'll find much of it new (and cool). Once you get used to flying with the SN3308, of course, it is easy to step up to an EFIS-equipped airplane.
Until I flew with the SN3308 for awhile, I didn't fully appreciate how much color adds to an EFIS display compared with a mechanical instrument. For example, the HSI course pointer and deviation bar are color-coded to indicate which navigation source is in use: green for Nav1, yellow for Nav2, cyan for GPS. This means that there's no chance of confusion about which nav sensor is driving the HSI display. The RMI bearing pointers are similarly color-coded, so there's never any question about what each pointer is pointing at.
On the SN3308's moving map display, waypoints are also color-coded: The currently-active fix (i.e., the one you're flying to) is white, while rest of the flight-plan fixes are magenta. As you cross each fix, it turns from white to magenta, giving you positive confirmation of station passage. This is particularly important when navigating by GPS, because the to/from arrows aren't particularly helpful (since for the most part, GPS is strictly "to-to" navigation.)
The SN3308 also uses color to make you aware of various error conditions. If you lose
VOR/LOC or GS, for example, a large red "X" is displayed over the top of the
corresponding deviation pointer … far more obvious and intuitive than the flags or
disappearing GS bugs on a mechanical HSI. Another example: If the heading gyro fails, the
compass rose changes from white to amber to let you know that the heading is no longer
gyro-stabilized and the SN3308 has gone into a reversionary mode where in which heading is
based on raw flux gate data. (If flux gate data is unavailable, the compass rose simply
I now have two panel-mounted moving maps, one on the SN3308 and another on the GX50. The GX50's map is a useful addition to the cockpit. But next to the Sandel's big, sharp, easy-to-read, color-coded moving map, the GX50's comes off looking pretty poor by comparison: tiny, cluttered, and monochrome.
On the other hand, my SN3308's map is pretty sparse, because all it presently displays are the route waypoints that it receives from the GPS. While the fixes along my route of flight are certainly the most important items on any moving map, they're really not enough. I'd also like off-route airports and navaids that are nearby, and particularly special-use airspace boundaries. As I mentioned earlier, this was a known limitation of the SN3308/GX50 combo that I installed.
GPS receivers with ARINC 429 outputs, such as those from Garmin and Bendix/King, provide not only route waypoints but also nearby off-route ones, and so the SN3308 presently provides a richer map when interfaced with those receivers. Even then, however, there's no depiction of special-use airspace.
All this will soon change, however. In the next major software release (presently in certification testing and scheduled for April 1999 release), the SN3308 will include its own Jeppesen database of airports, navaids and special-use airspace. This will enable the unit to include SUA on the map, and to depict off-route airports and navaids even if the attached GPS or Loran does not output that information. Database updates will be downloadable from Sandel's Web site, and uploadable into the instrument by means of the same data jack used to upload software updates (which are also available for download from the Web site).
I saw a preview of what the new moving map will look like when I visited Sandel recently, and it's absolutely magnificent. Standard aeronautical chart symbology is used to distinguish airports, VORs and intersections. Special-use airspace is clearly depicted. Even published holding patters will be shown if the GPS furnishes this information (as the new Garmin GNS 430 does). I'm really looking forward to the new release.
It takes a bit of flying with the SN3308 to really start to appreciate some of its capabilities. Let me tell you about a couple of features I find expecially useful.
I think my number one favorite feature of the SN3308 would have to be auto-slew. If you enable auto-slew and select the GPS receiver as your nav source, the SN3308 automatically slews its course pointer to the proper course as furnished by the GPS. Then, when the GPS sequences to subsequent legs of a flight plan or a GPS approach, the course pointer auto-slews to each new course in turn.
Auto-slew is pure dynamite when using the autopilot in nav-coupled mode, because each auto-slew causes the autopilot to turn automatically at each turnpoint on the route. Program your route into the GPS, climb to cruise altitude, engage altitude hold, and the airplane can virtually fly the entire trip automatically. When flying a complex GPS approach, auto-slew guarantees that you're pointed in the right direction and makes it almost impossible to miss a required course change or take a wrong turn. If you miss the approach, the course pointer auto-slews to the missed-approach course.
The SN3308's two RMI bearing pointers come in a close second to auto-slew on my list of favorite features. Once you start using this capability, you'll discover it has all sorts of valuable uses:
- VOR Intercept: When intercepting an airway or VOR radial, it's always unnerving
to see the CDI needle pegged and wonder how soon it will come alive. By setting one of the
RMI bearing pointers to the same nav source as the HSI, the pointer will provide a
continuous readout of the bearing to the station. When the bearing pointer gets within
10° of the course pointer, the CDI needle should start coming off the peg.
- Localizer Intercept: You can't couple an RMI bearing pointer to a localizer. But
you can accomplish the same thing by setting the pointer to your GPS and entering the
destination airport as the active waypoint. The bearing pointer will then point
continuously at the airport. When the pointer gets within two or three degrees of the
localizer course, the CDI needle should come off the peg. If the ILS also has an outer
compass locator, tuning it in on the ADF and setting the second RMI bearing pointer to the
ADF provides yet another valuable aid to positional awareness.
- Enroute Cross-Check: Suppose you're flying an airway and navigating by GPS. Tune
in the next airway VOR and set one of the RMI bearing pointers to it. The bearing pointer
should align with the course pointer so long as both VOR and GPS receivers agree. If the
two start to diverge, you're immediately alerted that something is wrong.
- Crossing Radial Identification: Set an RMI bearing pointer to an off-route navaid
(VOR, ADF or GPS) to identify airway intersections, crossing radials, step-down fixes,
airway changeover points, and the like.
- Airport Position: Set an RMI bearing pointer to point at the airport (using GPS or an on-airport VOR or NDB) to aid in positional awareness while flying circling approaches, visual approaches, or traffic patterns.
Two additional SN3308 features worthy of note are heading sync and course sync. Heading sync causes the heading bug to slew to the aircraft's present heading with a single keypress. Similarly, course sync slews the HSI course pointer toward the active waypoint, centering the CDI. Think of these as "fly present heading" and "proceed direct" keys that save lots of knob-twisting in these two very common situations.
Since I let it be known that I was installing a Sandel SN3308 in my airplane, I've received a good deal of email from other owners who are thinking seriously about doing the same thing. In the course of this correspondence, three questions seem to come up frequently enough that I thought it would be worth addressing them specifically.
Is the SN3308's rear-projection display readable in direct sunlight?
I worried about that situation myself, particularly because I'm flying a low-wing airplane. The answer is that bright sunlight at certain angles does degrade the contrast of the display, but certainly not enough to make the instrument difficult to read. The worst case seems to be flying with the sun at the eight o'clock position (i.e., over the pilot's left shoulder). That's the only situation I've encountered in which loss of contrast was noticeable, and even then it was definitely not a problem. Under all other conditions, the display offers all the contrast you could want. At night or in the soup, of course, it's just breathtaking.
What if the projector lamp burns out? Does the entire display go dark?
Yes it does.
As you might imagine, this was a significant issue during FAA certification of the SN3308. Projection lamp failure is considered similar to the failure of a compass card servo mechanism of a conventional HSI, or failure of the CRT or power supply in an EFIS system. Consequently, Sandel has taken steps to ensure that lamp failure is an extremely unlikely event.
The SN3308 has been life-tested in a lab under worst-case conditions — on a shake table under temperature extremes — and the halogen projector lamp was shown to last reliably for at least 400 hours. In an abundance of caution, Sandel specifies that the lamp be replaced at 200 hours or after one year, whichever comes first. For most owner-flown aircraft, the one year will be limiting and the bulb will never reach 200 hours; the lamp will simply be replaced at each annual inspection. Lamp replacement costs $30 and takes about 15 minutes.
According to Gerry Block at Sandel, there has been only one SN3308 projector lamp failure in the field so far. It occurred in a SN3308 that was installed improperly: wired to the aircraft main bus rather than avionics bus, so the instrument was subject to huge voltage excursions during engine start. Predictably, the lamp failed at engine start (on the ground) so there was no hazard. As a result of this failure, Sandel has redesigned the circuitry that regulates lamp supply voltage to make it virtually immune to voltage spikes. Needless to say, like any other piece of avionics equipment, the SN3308 should not be powered from the main bus, nor powered up during engine start. (Duh!)
As part of the lamp regulator redesign, Sandel has added the capability to actually monitor the condition of the bulb filament. By measuring the lamp's current draw at a precisely-regulated voltage, the software can estimate the amount of metal that has been lost from the lamp filament, and warn when lamp replacement is warranted. Depending on how this new capability works out in the field, it's conceivable that someday the SN3308 lamp will be replaced "on condition" rather than after a specific number of hours or months.
Part of the SN3308 installation is a lamp dimmer control. Operating the projector lamp at even slightly less than maximum brightness radically increases its life. I find that the instrument can be dimmed somewhat from maximum brightness in virtually all but the eight-o'clock-sun case, and generally do so. Nighttime operation is essentially "free" so far as lamp life is concerned.
Isn't is risky to purchase a primary flight instrument from a new, start-up company? How do you know Sandel will be around tomorrow?
This is a fair question, and one that bothered me as well. It's one of the reasons I decided to fly down to San Diego and spend a day checking out the company and speaking at length with Sandel founder Gerry Block. I came away from that experience feeling very good about the future of the company.
One obvious question is how Sandel's sales are going, and whether they're making money. As a privately-held company, Sandel does not publish its financial or production figures. When I asked Gerry Block, he just smiled and told me "let's just say we're right on plan, and doing very well, thanks." Given the Cheshire-Cat-from-Alice-In-Wonderland smile on his face when he said that, I was inclined to believe him.
Based on the serial number of the unit that went into my airplane, it appears that Sandel shipped roughly 500 of these instruments during the last half of 1998. The company appears to be selling all they can build, and based on what I'm hearing from my contacts in the avionics industry, everyone seems to think that Sandel has one of the two hottest general aviation avionics products of 1999. The other hot product I hear mentioned is the Garmin GNS 430, so Sandel's running in pretty good company. Actually, quite a few owners are installing Sandel SN3308s and Garmin GNS 430s together. One of those is AOPA, who will be installing this combo in their 1999 giveaway airplane, a "better than new" Cessna 206.
Another indication of stability in my view is the fact that Sandel signed an exclusive deal with DAC International to serve as the master distributor for the SN3308. DAC is one of the best-respected names in the avionics distribution business, a first-class operation by all accounts, and one that largely deals with higher-end avionics for the turbine market. The fact that DAC agreed to distribute the SN3308 says to me that some pretty sophisticated avionics people think rather highly of Sandel, and adds a great deal of credibility to this fledgling company.
Yet another positive sign is how seriously Sandel seems to be taking its customer obligations. Not only is their product superb, but so is their documentation and customer support. The SN3308 installation manual is a real work of art, spelling out every aspect of the installation in excruciating detail, providing schematics for interfacing the instrument to a wide range of other avionics, anticipating common mistakes and problems, and even providing detailed instructions for preparing the paperwork and securing FAA approval. I had the opportunity to see Sandel's customer support in action when we were troubleshooting my installation. They always had quick and responsive answers to our questions, and their troubleshooting advice was consistently spot-on. Some of the larger avionics firms could clearly take lessons from Sandel in this regard.
Sandel's small size is not necessarily a negative. The recent and sudden departure of Trimble from the general aviation market shows that dealing with a big, wealthy company is no guarantee of continued support. On the contrary, the fact that Sandel has been financed personally by its founder, Gerry Block, rather than some faceless venture capital firm seems like an excellent sign that the company is very serious about what it's doing.
My visit to Sandel gave me a deja vu feeling reminiscent of II Morrow, a company established on a shoestring budget by Chuck Morrow in the early days of general aviation Loran, and subsequently acquired by a deep-pocket firm (United Parcel Service). Gerry Block admitted to me that he's already had a few buyout offers, but has no plans to sell the company in the foreseeable future. Frankly, it seems to me that if Gerry ever does decide to sell the company, one of the avionics giants like AlliedSignal would be nuts not to buy it if only to acquire rights to Sandel's incredible rear-projection optical technology.
It's impossible to overstate my enthusiasm for the Sandel SN3308. It's one of the few aviation products I've reviewed that exceeded my expectations in virtually every respect. It represents the first real breakthrough in flight instrumentation for piston-powered aircraft in at least 20 years. The technology is innovative and superb. The design strikes me as being very difficult to improve upon, and I'm a pretty picky guy.
The $7,895 price is certainly not cheap, but it seems eminently fair for what you get. If you're considering adding an HSI to your airplane, it seems to me you'd have to have your head examined to buy a Bendix/King KCS-55A or Century 1000 when you can install the SN3308 for about the same cost and have vastly greater functionality.
If your aircraft is equipped with an old-technology air-driven HSI like the NSD-360 (as mine was) and you face a $2,800 instrument overhaul every four or five years, wouldn't it make more sense to apply that money toward the cost of an SN3308 installation? That certainly figured into my decision to upgrade.
But what if you already have a good electric HSI like a Bendix/King KCS-55A that's working reliably and not giving you any trouble? Is it worth pulling it out and installing an SN3308 instead? That's a much tougher choice.
If you're a serious IFR pilot like I am, sooner or later you'll wind up installing an
EHSI. Now that Sandel has shattered the EFIS price barrier, there's no turning back. The
only real question is whether to do it now or later. If you're stoic and iron-willed, you
may succeed in holding out for two or three years. But be forewarned: After one flight
with electronic instrumentation like the Sandel SN3308, you'll never again be happy with