Probably because (15 years after its introduction) everybody has just become comfortable with its menu logics an doesn't want to start all over again. :-)
I for example have to go for new unit because DHL managed to destroy mine beyond economical repair :(
I would guess from existing installations that need a replacement unit, or simple upgrades from x30 to x30W. Given that the additional mission capability is exactly zero, and the cost increase going to a GTN is HUGE (rewiring and paperwork for the GTN650; panel re-work for the GTN750), you need to be a serious gadgetophile to do that.
Only when you re-work the panel and spend lots of money anyway, or perhaps can avoid the cost of an MFD, the GTN650/750 become a more "obvious" choice.
Also, in the same way that people range from buying decades old used aircraft on a shoe-string to new aircraft buyers, I would guess that quite a few would rather save the $5k-$15k extra and spend it on fuel.
I wonder if the GTN boxes have something in their functionality, or database coding, which the GNS boxes don't?
I have never seriously flown with either, but the kind of stuff I have in mind might be
All variously esoteric stuff, though the hold entry one is impressive to show somebody
The only GTN capability which I believe is really operationally relevant - once actually flying a plane - is the keypad and the airway entry.
I'm putting in a 750 because although it was going to be fairly cheap to upgrade my 530 (for sale!) to WAAS, the paperwork was going to be expensive and wouldn't include LPV without still more paperwork. The 750 is LPV approved "out of the box".
I'm leaving in the 430 below it, switching off cross fill (there is limited cross fill between 750 and 430/530) and using the 430 for "what ifs" and backup.
Used avionics have always held up price wise long after they were obsolete. Before the GNS430 was introduced in 1999, the KNS80 was still commanding $3,500 and a 14 volt KX155 with a GS still commands $2500 today. A GNS430 can be purchased at around $4,000 (only an extra $1500 over a KX155) and it is another 3500 to upgrade to WAAS, so an extra 500 for a refurbed unit with a warranty is not out of line. The GNS530 gets an extra $2000. WAAS upgrade it for another 3500 and it can yield $9500, so $8000 is a good buy. In my opinion, the GTN650 has a poor user interface and is less desirable than a GNS530W. The GTN750 is the best of the lot, but the most expensive.
OK - an excellent point. I should have mentioned that.
The GTNx50 AML STC gives you LPV and PRNAV via the AFMS - for both N-reg and EASA-reg.
The GNS530W AML FAA STC gives you LPV and PRNAV.
The GNS430W AML FAA STC gives you LPV only. PRNAV needs the FAA LoA process which is very hard to do nowadays (1 year has been reported). And there is no good solution for an EASA-reg plane for either LPV or PRNAV (= EASA Major mod etc).
That means that for European IFR a GTN box seems the only option, if spending any real money, although an N-reg can get both LPV and PRNAV with a GNS530W.
the GTN650 has a poor user interface and is less desirable than a GNS530W. The GTN750 is the best of the lot, but the most expensive.
I was coming round to thinking the same...
Hmmm. I wonder if I can get a GNS530W and an EX600 in the space of a KX radio, KLN94, KMD550... I reckon it would fit. What won't fit is a GTN750 and an EX600, or 2 x GTN750.
there is limited cross fill between 750 and 430/530
In what way is it limited?
Is there any chance of Garmin offering a real physical keypad on the GTN, or anybody doing it for a GNS with a hacked crossfill?
I wonder if the GTN boxes have something in their functionality, or database coding, which the GNS boxes don't? I have never seriously flown with either, but the kind of stuff I have in mind might be
something subtle in missed approach handling
Minor improvements in the GTN series.
intercepting DME arcs
Even the non WAAS GNS430 has this.
intercepting airways (of course the GTNs support named airways)
The GTN series has very good airway support. All the Garmin GNS/GTN can intercept a specified course
automatic hold entry determination and hold flying
The GNS430W/530W support this as does the GTN series.
holds at any waypoint
Only the GNS480 provides this capability. I suspect it may get added to the GTN, it will never be added to the GNS430W/530W