I have been flying my RV-8 now for 1000 hours and four years with the Grand Rapids EFIS and a Tru Trak Pictorial Pilot / Altrak for an autopilot. Before that, I (like thousands of other pilots) happily flew around IFR with a single vacuum pump feeding an artificial horizon and Directional Gyro with an electric Turn and Bank as a backup. Therefore, I have been very happy to have dual electronic, solid state systems to keep me upright in the clouds in the RV. But hey, if dual redundancy is good, then triple must be better – right? (I’ve got four GPS’s BTW…..) Well, not always, and there are caveats, but having an additional attitude reference for the primary display system is in the direction of “goodness” in my book, especially when there are redundant power sources and independent displays to show the data from the two platforms.
Now, a word on those caveats with regard to redundancy. If you have “one” sensor, you don’t know if it accurate or not. If you have “two” of a particular sensor, you increase the chances of having one “good” one, but if they give you different readings, you can’t tell for sure which is right. (There are ways of telling if they fail in certain, particularly sudden, ways, but if they simply drift apart, you don’t really know.) If you have “three” sensors, and you believe that the odds of three simultaneous identical failures are unlikely, you now have operationally meaningful redundancy. If one of those three is of a different type, the odds are even better. The GRT AHRS is designed so that the Display Units can do some checking on the AHRS hardware to help decide if one of the AHRS is headed out to lunch, but if the solutions simply start to diverge, you DO need a third, independent source of attitude control (and/or display) to keep you safe. For that, I like the Tru Trak autopilot, fully capable of flying the airplane without help from the EFIS, so that is why I tend to split my loyalties between two manufacturers (no offense guys!).
The Dual EFIS is, as GRT puts it, really two AHRS units in a common chassis. The footprint is exactly the same, which is nice if you are doing an upgrade – the mounting holes remain the same. The Dual AHRS is about a third again taller than the single – not twice as high – and fitting it in the same spot for me was not a problem. Here’s a comparison picture:
Swapping out he Single AHRS for the Dual is really not that involved of a process, except for the fact that each AHRS has its own magnetometer, so you have to run some cable through the airplane for that. My magnetometer is mounted back toward the tail, about halfway from the baggage compartment to the empennage fairing. Since I was doing an autopilot upgrade at the same time, and that required a new pitch servo cable behind the baggage area anyway, I striped the entire interior out (it was Condition Inspection time as well – a triple play!), and spent about four hours pulling the necessary conductors. My panel comes out fairly easily, so I removed it to our nice clean hangar apartment for the necessary mods, and that gave me access to my AHRS mount. I frequently get questions about where/how I mounted it in the RV-8, so here’s a picture of the bare platform:
And here it is with the AHRS (my 5 Ah backup battery Velcro’s to the top of the AHRS – still room for it with the Dual setup):
The wiring is fairly straightforward, and I spent more time rebundling all the cables behind the panel to make them neat than I did adding conductors. Basically, you need six wires going back from the AHRS to the Magnetometer, three power feeds and a ground to the AHRS, and an output line from each AHRS to each Display Unit. Each AHRS also requires a control line from a Display Unit, so pairing them gives maximum redundancy – DU1 controls the Primary AHRS, and DU 2 Controls the secondary. Every box in my EFIS suite has three power feeds – one from the main bus, one from the essential, and one from the Aux battery. You have to do a little re-plumbing in the pitot/static lines to add the second AHRS of course , but fortunately, the ports on the back of the AHRS are far enough apart that you can spin an elbow on one without bumping in to the other.
Once everything was buttoned back up, I simply had to go in to the “Setup” pages of the display units and make a few changes to tell them that there was another AHRS, and fire it all up. I have yet to do the mandatory Magnetometer calibration due to heavy rains yesterday, but that is a pretty quick process involving a compass rose and doing a few circles on the ground. I haven’t flown with the system yet, but it is comforting knowing that I know have a significantly greater chance of having a good AHRS – even though I have not had a single hiccup with the single system in over four years of flying – a testament to the robust nature of both the hardware and the software. I’ll report any issues I have with the flight test phase…but I don’t expect to see any.