I have a good basic understanding of the capabilities of an FMS in larger airplanes from my line of work - not as a pilot, but as an employee of a large avionics company (neither Garmin nor Avidyne).
Aha! You must be a Collins guy.
So with that disclaimer out of the way, here are some things the Avidyne does for which I have not yet found equivalent support on the GTNs:
- Gaps in flight plans. An FMS usually calls that a discontinuity, or short "discont". Not all flight plans are contiguous from take off to landing. Quite often a STAR ends with "expect vectors to an approach" or something along those lines. The IFDs accurately reflect these situations by having an explicit gap between the defined portions of the flight plan. A gap may be there by design, or ir may tell the pilot that the flight plan construction isn't quite complete yet. It's actually quite a useful thing once you've seen it in action a few times.
- More complete support of ARINC leg types. GPS and thus most RNAV procedures are meant to fly from one point over the ground to another. However, many legacy procedures have leg types such as "fly a heading to an altitude" (VA) or "fly a heading to intercept the next leg" (VI) or "fly a course to an altitude" (CA). Some of these are maybe getting rare in the US; BROAK1 at KPHX is an example of a departure procedure which depending on the transition uses several of these examples. A more common example is a missed approach leg like "fly runway heading to 2,500, then..." which the IFD handles automatically when connected to an EFIS or Air Data Computer.
- Loading multiple approaches at the same time. This is one of those tools which you won't need very often, but boy can it come in handy at times. Example: setting up for an instrument approach on the runway favored by the wind direction, but with high minimums (say LNAV only). You can then pre-load an approach at the same airport from the other side with better minimums and be ready to start that second approach as you go missed on the first one. The other example would be lining up multiple approaches at nearby airports in a quick sequence to do practice approaches for currency - you can set that all up on the IFD before you start the flight.
Aha. OK, now we're getting good information here.
TL;DR: I see the difference, but it's unlikely to actually make any difference for 99% of piston-driven airplanes.
I'll address these one at a time - 2nd, then 3rd, then 1st.
For the leg-type example you give - the GTN *does* do that. I even loaded the BROAK1 into the GTN simulator on my iPad and "flew" it from runway 26 (climb heading 258 to 1635 MSL and then proceed direct JUTAK. I don't think it supports RF (Radius to Fix) legs yet for RNP approaches. Does the IFD?
For the multiple approaches - Yes, that's true. However, outside of training, I can't think how that'd be a great idea. If I don't have time to load another approach - I'm not ready for the next approach. Get vectors somewhere, take a breath, brief the second approach, THEN fly it. And in training, IMO it would lead to bad habits to load all of the approaches prior to starting the first one. But it is an interesting ability, and the Garmin stuff doesn't have it. That said, I don't know of anyone else who does either - I'd sure like ForeFlight to support something like that before I did it on my panel so that the panel doesn't screw up my EFB's flight plan. Does Avidyne do flight plan transfer with ForeFlight or only with their own app?
That brings up a workaround, too... If you really need that second approach ready to go, load it in ForeFlight, and beam it to the panel when you miss the first one.
Finally - The gaps. That's a cool function, but even flying a turbine-powered airplane, in the real world it's incredibly rare to actually fly those procedures as charted IME. For example, we do LEEDN.GOPAC2 into KMKE frequently, which ends with the BONOT fix, fly track 076, expect vectors to the approach. I have literally NEVER made it to BONOT. I think I have gotten a heading after a fix once (ie "after Badger fly heading 090...") but that was a simple case of waiting for the fix and pushing HDG.
So, I get that there are some "missing" things on the GTN, but they're not likely to be things we'd be doing in our little piston pounders. I think the only place I've ever gotten a STAR in a piston was Houston, and I have yet to get a SID in a piston. Plus, as you mentioned, the ones that have the heading legs aren't too common even then.
