Glass Panel Redundancy
In a recent blog, where Paul Bertorelli described our flight without XM weather and playing my bile level "like a $2 harmonica," he also touched on the fact that my Glastar Sportsman is fitted with a Garmin G3X EFIS/engine monitor running, shall we say, commando. That is, I don't have backup mechanical gauges to rely on should the G3X go haywire. This fact, by itself, will make a lot of old-school IFR pilots slide forward on their Ikea office chairs, click "compose new message," and fire off an email describing my stupidity.
Paul mentioned that the G3X has internal redundancy, and that's partly true. In the current two-screen configuration, failure of either screen will result in all of the information being packed up and shown on the remaining good display. As a matter of fact, Garmin has done a really nice job with this; the combined screen is tightly packed, that's true, but all the major functions still work, are legible and generally will get you home without bending metal.
But truly redundant? No. For starters, the XM weather is resident in only the display fitted with the XM receiver; in my case, the MFD (multifunction display). Should the MFD, a GDU 375 in Garminspeak, opt for some impromptu vacation time, you'll lose the weather and XM radio function across the board. Even if it's just the display portion that's died. The G3X does not share XM data across the network, so if the box carrying it can't display it, you won't see it.
Also, there are interconnect issues. Each GDU has three RS-232 data ports (in and out), and one of them is configured to send GPS and control-logic data to the GX Pilot autopilot. And, yes, that's on the MFD, too. Lose the MFD, and now the autopilot is literally on its own. It can't follow a GPS-derived ground track, but will hold wings level and maintain altitude all by itself, so it doesn't go totally stupid—and that's one big advantage of having a separate, brains-in-the-box autopilot as opposed to an EFIS-based system. I also have the serial data from the Garmin SL30 nav/com ported through the MFD, so that data would be lost as well. (The GNS 430W sends all its navigation data through the ARINC 429 bus, which is part of the separate AHRS box.)
Paul might have mistaken the two screens for true AHRS redundancy, but that's not the case. The G3X system is intended to use one and only one AHRS box, which is the "brains" of the system, containing the solid-state sensors used to determine the airplane's attitude along with pitot/static pressure sensors and the sole connection to the external magnetometer. Should it fail, you have no attitude or engine information.
Other EFISes in the Experimental market allow you to have two independent AHRS modules (at extra cost, of course) to provide a backup in case the main set of smarts goes suddenly dumb. To Garmin's credit, it has given the G3X the ability to show as much as it knows, and provides a chart in the user's manual defining what data will be good in the event of specific subsystem failures. If it can't resolve attitude, you might still get airspeed and altitude.
Hardcore IFR pilots insist on redundancy, and the regs for certified aircraft demand it, because of a long and glorious history with instruments failing with predictable and depressing regularity. There was time when a renter pilot could tell if the Cherokee's attitude gyro was about to fail by the sound of the bearings, and predict when the vacuum pump might choke—pretty much any time the engine was running. We learned how to fly needle-ball-airspeed, and fitted backup vacuum systems and other devices for the inevitable inflight failures.
It's a new day, and the systems are vastly more reliable. I have yet to have a hard failure of any of the EFIS's run in the Sportsman, from Dynon, Grand Rapids and Garmin. And that's despite testing quite a bit of beta-test software. (I do insist on no IFR with beta software, though.) Still, an EFIS is a computer, and computers crash. I'm happy enough for now that the separate autopilot can at least keep the wings level should the EFIS go nuts, but the ideal would be to add an inexpensive backup attitude indicator, like the TruTrak 2.25-inch ADI.
I would also like Garmin to at least consider allowing the display acting as an MFD to show the GPS-derived flight panel page like you see on the GPSMAP 696, 496 and 396. (A GDU 37X is in many ways a flange-mount 696; if you power it up without an AHRS module hooked up, the screen looks just like the 696.) But Garmin's hard-over on the idea that once configured as a G3X system, it's always that, which is a shame—the programming is already in there! I've flown the "pseudo panel" on the 696 and, I'd be happy using it to back up the autopilot in the event of a partial EFIS failure. Add a switch to allow your IFR-approved GPS to drive the autopilot directly, and I think you're good to go.
So, Bertorelli can play on my angst all he wants, but I worry less about an all-out instrument failure than having to spend another 54 hours in Winnemucca.