GPS Switchology

These little gizmos seem to double the price of GPS installations. Heres why. (P.S. You gotta have em.)

When we finally bite the bullet and take the ol air machine in for a shiny new IFR-approved GPS, we usually focus on the features and price of that wondrous box. What sort of display, how many waypoints and so on. Things that make us buy Brand B, over Brand G or Brand T.

Then we get a look at the bottom line for the installation quote. Thats about the time NOAA records a weather phenomenon over the airport where the barometric pressure drops two inches and the temperature changes 60 degrees in 30 seconds.

The bottom line price for this GPS gizmo may be double the $4500 unit cost. One option that accounts for this cost run-up is the GPS/Nav switching accessory. What is it and why do they cost so durned much?

Rationale for Switching
In order to use GPS for IFR, you need a CDI installed where you can see and follow it. The electronic CDI on the GPS unit doesnt count. Thats not my rule. The FAA even says so.

If you have extra panel space, you could elect to have a dedicated CDI indicator. There are several on the market that have all of the requisite indicator lights and even an OBS card for Bendix/King and Garmin units. However, most of us don’t have a 2- or 3-inch hole to spare. So, we opt to share the GPS with the VOR/ILS indicator or an HSI.

Thats just great. Now, en route and terminal and GPS approaches will be a snap. All of the guidance information will be presented conventionally. Roll out on the desired heading, keep the CDI centered and youre home free. When youre cleared for the ILS, switch back to the number 1 Nav and smoothly return to the ground.

If it sounds simple, its only because the hard part is done electrically. A single switch on the panel swaps VOR/ILS and GPS. What you don’t see is between six and 24 individual wires being redirected behind the instrument panel. You don’t see the CDI left/right, TO/FROM and flag lines getting shifted. You don’t realize that the six wires on the HSI OBS need to be relay switched from the VOR to the GPS. That single little button on the panel doing all this work is one reason GPS installations are so expensive.

When loran first came out, a single panel switch threw a relay from the VOR to the loran. If the VOR was in ILS mode, it automatically reverted back to the nav, for the localizer. Then along came GPS with its waypoint annunciators, acknowledge switches, approach enablers and a host of functions and switches that depend on the manufacturer and certification basis. And the number of relay contacts continues to grow.

The logical step was to combine these required annunciators, switches and relays into a single unit. This saves labor and simplifies certification and operation. You still need the same number of wires interrupted and rerouted, but they all can be connected to a common point closer to the radios. The good idea caught on and today several companies build switching accessories.

Gotta Have It
What about those pretty switches? A discrete relay and properly labeled switch kit can easily cost more than $900. In addition to the relay wiring, you must add the labor to install the individual switches and annunciators, add their wires, add some sort of dimming and a test circuit and cut all the holes in the panel.

There are four manufacturers of these GPS switching units. Three manufacturers, Ameri-King, Avionics Innovations and Mid-Continent are TSO approved, while MSI Avionics is not. They rely on PMA documentation and an FAA field approval.

Four players makes this a fairly crowded market, because the population of approach installations for GPS is not exactly exploding. IFR GPS, while popular, is making its way through the fleet more slowly than loran. Hmmm, I wonder if the price, installation and certification has anything to do with it. . .

Mid-Continent Instruments
The real pioneer in the integrated solution is Mid-Continent Instruments. They backed into this business by manufacturing the MD-40. This is a very sharp little CDI indicator that contained all of the annunciations for GPS. It was a natural progression, since they worked with all GPS manufacturers and knew the requirements, to produce a switching unit. Their MD-41, at a list price of $995, quickly became the industry standard in GPS accessories.

Mid-Continent makes versions for all GPS manufactures who need switching. All are the same price. The MD-41 is available in both vertical and horizontal mounting configurations. The unit is about 4 1/2 inches deep and a standard 1/3 ATI panel cut out; the squashed rectangle with the chopped off corners that requires either lots of filing or a special punch to get right. Still, many, many aircraft have that size cut out, used for those things we used to have, like DME indicators.

The buttons on the MD-41 are large, we’ll spaced, with clearly legible lettering and a 3/16-inch long switch throw. You wont have any difficulty pushing these guys. The dimmer circuit affects the annnunciations, which have a broad intensity range. Bright enough for broad daylight, dim enough for the darkest night.

The NAV/GPS button (and OBS/LEG, if youre equipped with Bendix/King) is a latching switch. The difference between in (NAV) and out (GPS) is more than 1/8-inch, with a very positive feel. This unit has quality everywhere.The only gripes we have are the color of the legends and the heat build up. With the backlighting on, the unit gets very warm to the touch, gaining 20 degrees C in less than 20 minutes. The MD-41 example we saw has a green GPS and white NAV legend. We thought that the industry had settled on blue for GPS, green for VOR/ILS. It doesnt matter what your airplane is, as long as your Flight Manual Supplement reflects the colors. (And the FAA approves it.)

The only other flaw in the MD-41 was a trivial misalignment of the bezel that caused part of the logo to be shadowed when backlit. If the logo were never illuminated, youd never notice the flaw. What price vanity, huh?

If the panel space isn’t there, or if you prefer a tidy remote relay package, Mid-Continent offers the MD-41-13XX unit. Less than half the height of the MD-41, you can find a place in any panel for this guy. You spend more for less size, though. A typical KLN 90B unit lists for $1325.

This begs the question, why spend more for a remote relay when you can have a compact panel unit? There are several reasons. Panel space is the most obvious one. Some installation, such as an EFIS or air data system, may need more than the 19 poles provided by the panel unit. The 13XX series has five more relay poles than the panel mounted version.

The sort of airplane that needs these relay contacts probably has remote mounted radios. A King Air or Citation, perhaps. It would be silly to run 75 wires from the radio bay into the cockpit and back out again. This version of the MD-41 is a product born of experience. Most folks wont need it, but those who do will be glad it exists and will pay for it.

A small company with big ideas is Ameri-King Corp. Their unit is, like the company, also quite compact. The AK-950 is one third the volume and half the weight of the Mid-Continent unit. Naturally, it requires less panel space. With a street price of less than $600 for the KLN 90B version, it takes less wallet space, too.

Like the MD-41, the AK-950 has versions for all GPS types. Ameri-King thoughtfully included the GPS companys logo on the front. Just in case you didnt know that you had a Bendix/King or II Morrow GPS.

Unlike Mid-Continent, Ameri-King provides both sets of bezels for the vertical and horizontal installation. They also provide something very useful: A wired connector. Their units, even though less expensive than the competition, come with a wiring pigtail that can be easily spliced into the existing harness. This saves lots of labor, considering that its a subminiature, 78-pin ultra dense connector.

The harness supplied to us was good quality, too. The power wires were #26 gage, easily able to handle the AK- 950s minuscule current requirements, but a bit thin for aviation. The risk here is in breakage under handling or vibration.

Although the unit and its buttons are smaller, the annunciator legends are larger than the others weve seen. The dimmer circuit was not responsive, and as a result, we never saw the display at full or minimum brightness. Could it be that we were attempting to run a 28V unit with a 14V power supply? Possibly.

Some units (we received 5 AK-950s to try) were marked, others did not have their check box marked to indicate their flavor. In general we found the quality of these Ameri-King units to be inconsistent, the ever-present hazard when many versions are available. We saw several cases of loose (or missing) screws, besides the improper labeling.

The small buttons are an improvement from the earlier AK-950s weve seen. But they are still small, without much touch feedback. A very short throw on the latching switch makes the annunciation vital for seeing the mode engage. The travel on the GPS/NAV switch is about 1/16th inch.

Another gripe we have is with the Ameri-King version for the Garmin units. The ARM and SEQ legends werent backlit although the buttons themselves were. There’s a certification rule of thumb applying to function labels. If you can read it in the daytime, you better be able to read it in the dark.

Okay, enough picking on the little guy. Hey, they got the colors right, anyway. For the price, the AK-950 is an excellent value. If the fit and finish were a little inconsistent, so what? Ameri-King offers a two-year warranty. These are the guys who broke the $200 floor for encoders and ELTs, all because the company president, Keith Van, asked himself, Why does it have to cost so much?

Avionics Innovations
Avionics Innovation is a small company in Southern California thats known for manufacturing stereo CD players with an PMA approval. Tunes, dude! Their AI90B has a list of $895, making it price-comparable to the Mid-Continent units.

Its long, at 6.8 inches overall. It has tiny buttons that poke you in the finger nails, and very small legends. Its best feature is a short package, less than one inch tall. Avionics Innovations gets credit for being there when the market was young, but they havent quite kept up with the challenges.

MSI is a division of Microterm Systems Corporation. This company is probably the pioneer in these gadgets, but they havent had the exposure or market impact that a company as well-connected as Mid-Continent has. MSI Avionics is the lone non-TSO system. Frankly, we don’t care (but the FSDO might). As we said, a TSO doesnt exist for a switching/annunciator unit, so TSO C129 is, in our opinion, specious.

What the MSI Avionics units (called INSSDU, by the way) have is an FAA-PMA approval. If the unit is an approved part, because its traceable, thats good enough for me.

INSSDUs range from $695 for a generic unit, to $795 for the KLN 90B, so the price is right there with the MD-41 and AI90B. This unit has the same panel footprint as the MD-41, about 3 1/2 inches wide by 1 3/4 inches high. Its a rectangular cutout and with a mounting ring or riv-nut in the panel, it only requires a 3-inch wide hole.

Whats different about the INSSDU is that the unit has nice big, flat toggle switches. In turbulence, you grab the switch and hang on, instead of fumbling for a button. The legends on the switches are not as easy to see as a button, and the INSSDU mode annunciations are smaller than the other units.

In any GPS installation that shares a nav indicator, the installer must break the wires and run both ends to the relay contact, whether theyre in the panel mounted accessory or remote. In most airplanes, its relatively easy but by no means a picnic.

Anytime you get elbow deep in the radio rack you invite damage to other equipment, as we’ll as a tremendous potential for miss-wiring. In addition, many of these wires should be shielded to minimize needle oscillations and errors.

It can take as much as 35 man hours to add a GPS switching accessory for a complicated installation such as a Garmin or Bendix/King. These manufacturers, blessed as they are by being first out with approach capability, are burdened with the requirement to interface their units with the six wires of omni bearing selectors.

Not only does this burden the installer with an increased wire count, but many navigation indicators lack an OBS resolver. These are reserved for the HSI and high-end avionics. (Bendix/King KI 206 and Collins units.) Narco or ARC? No way.

If this field of GPS switching gizmos, can one make a choice? It comes down to a choice between economics and quality. The Ameri-King units are small and comparatively inexpensive. The Mid-Continent units cost more, but have an uncompromising quality behind them. This company has some Midwestern roots and values that would make you glad to know them.

Ameri-King, while full of determination and innovation, is lacking in that crucial support element and company integrity, as evidenced by a significant cash tip for the reviewer included with their samples. (Yup, thats right. We sent it back, of course.)

Still, for the price, their units can make the difference between affordability and an out-of-reach installation for the budget conscious. We wouldnt hesitate to use one of their AK-950s.

As for the Avionics Innovations and MSI units, their price structure places them too close to the leader but too far above Ameri-King. Without the nationwide support recognition of one of Americas largest avionics companies, such as Mid-Continent, or the price leadership and size advantage of the Ameri-King, these fine companies have their work cut out for them to get the consumers attention.

Also With This Article
Click here to view the GPS Switch Checklist.
Click here to view the GPS Switch Addresses & Contacts.

-by Gary Picou