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GTN650 and GTN750 into a TB20

Wow, yes – checked the GTN installation manual and you do need to connect up a lot of analogue lines and switch them – looks like 14 lines to switch once you have everything in including the autopilot.

I also see this note:



I have the later SN3500 but AFAIK they never supported a glideslope via ARINC429. I don’t think that even exists.

Shoreham EGKA, United Kingdom

What’s wrong with the current set up?

Has Shoreham LPV approach got approval yet?

If it has why not just swsp out ur king 89b or is it 94 and put in a IFD440

I have everything required for IFR in Europe – except LPV. Shoreham doesn’t have LPV yet but probably eventually will.

So, yeah, I could replace KLN94 and 1xKX radio with a GNS430W or a GTN650.

Problem: the KMD550 MFD doesn’t work with a Garmin GPS. The issue is so subtle that most pilots never discover it. The OBS mode is depicted incorrectly. I have no more detail since only 2 pilots I know of have had a GNSxxx with a KMD550; one of them was “illiterate” and the other has died. The latter one told me (before he died) that the OBS is correctly shown but remains frozen if you later change it.

So… change the MFD. To what? They are all

  • made by Garmin, and obsolete, or
  • made by Avidyne, and obsolete

and last time I went around this loop, none of the manufacturers would confirm 100% correct operation with a Garmin GPS. Not even Garmin

To be fair, however

  • these old MFDs are very cheap on US Ebay – of the order of 2k
  • there is basemap support from the vendor
  • the basemap doesn’t really need updating because airports don’t move, and the displayed route comes wholly from the GPS (my KMD550 has not been updated since 2010)
  • there is a possible future issue only if you want to display Jepp terminal charts on the MFD but those are so expensive anyway (€2k/year)…

The MFD I have is great for VFR (it has the VRPs) and most others aren’t. Not a huge deal because one has that on a tablet.

This leads to the “modern” 2×GPS solution which “everybody” is installing. And since the GNS430/W or GTN650 are too small to display a usable map (for strategic IFR flight management, where you want to see the next 200nm and see all the waypoint names) this leads to either a 650+750 (2×750 is too big) or 2×IFD540 (and take a risk on Avidyne). There is more or less no other choice.

Avidyne did do an MFD which was same size as the KMD550 (EX500?) and these still have Jepp map support but as I say I could never find out the above point. One could establish it with a bench test. On a homebuilt you could develop a little protocol converter to fix the data stream to the KMD550…

These threads tend to go off the rails, because nobody is spending their own money (and this job would cost some 30k) but this one is not too bad. Post #11 hits the nail on the head.

One could argue that a diversion to Biggin Hill and the ILS is only about 300 quid and you could get 100 of those for the cost of this refit

It is very tempting to just put in a GTN650 as I say above, because it would wire in easily, picking up the KLN94 and KX155A wires, and deal with the OBS issue somehow. Otherwise, this job has the potential to go badly wrong and ground the aircraft for months. My appetite for downtime is somewhere south of minus zero.

Shoreham EGKA, United Kingdom

Do it properly Peter. Get rid of the analogue wiring and external switching and it becomes much simpler and removes many potential failure modes. You only require 3 twisted pairs between each GTN and the Sandel. These are for NAV and GPS to the Sandel and OBS back to the GTN. Annunciation and all deviations are on the digital busses.

Avionics geek.
Fairoaks. EGTF

Do you fancy a freelance job, wigglyamp?

Shoreham EGKA, United Kingdom

I don’t share the enthusiasm for remote audio panels. You need the MFD screen on the GTN, and you can be sure that you will need it most when you are switching between audio sources.

Give me a load of separate buttons any day.

I feel much the same about the remote XPNDR, though not quite so strongly, as you change it much less often.

And you lose the data fields at the top, so it only really works with a 650 as well.

EGKB Biggin Hill

I kind of agree with @Timothy

Audio Panel is one of those you use quite a bit and it gets old when it’s hidden in some sub-menu. When I was going to put a GTN 750 in mine, I was exploring the remote audio (not an option on the 650). But the 750 was just too big, so in the end an audio panel + IFD540 ended up being the same size in footprint anyway.

@Peter what’s the obsession with two giant IFD540’s or 750’s these days everyone has? I don’t get it. When on earth does anyone need huge 2 gps when you got ForeFlight, SkyDemon, JeppView on your tablet anyway? I look at my tablet much more than anything else I use (and I think most do). If you must have a 2nd GPS, get a 650 or a IFD440 and free up some panel space, no?

Last Edited by AdamFrisch at 03 Oct 00:04

Not that I’ve ever flown with a setup like that but what is it you like about the MFD?

I flown with plenty of the old Garmin stuff 150,155xl, 250xl and with a tablet I find it excellent.

But then all I ever do is hack around OCAS and in Class D.

For VFR you don’t need it because you can run a good moving map on a tablet, and anyway there are no panel mounted products that run a decent VFR chart (the map copyright issues, and Jepp’s dropping of their VFR charts in 2013 have taken care of that).

In a busy cockpit like mine I like to have all the stuff in the panel. You pick waypoints to ask for shortcuts to and when you get one you select it and the autopilot takes you to it. A decent size MFD is great for situational awareness.

I am not too bothered about having two GPSs. There are extra costs e.g. two pricey WAAS antennae. But there are no MFDs in current production AFAIK so if this is a criterion…?

The best system is a textual box used like an FMS (which the GTN650 does well – it makes a good keypad) and a big screen above it. This is what bizjets and above have and for a good reason.

Shoreham EGKA, United Kingdom
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