Well according to this Garmin doc , the interface is indeed needed: https://www8.garmin.com/aviation/brochures/GFC_500.pdf
dejwu wrote:
Btw: The G5 undoubtedly killed the AI/HSI market and I wonder if anybody still buys or let repair those devices – it just does not really make economical sense any longer.
I’m not sure. I talked to my avionics shop about replacing a mechanical HSI with a G5. He indicated that it would be a major task considering that you have to install a magnetometer, rewire various interfaces etc. Add to this problems of interfacing legacy autopilots. The total cost could easily be much higher than repairing or even overhauling the mechanical HSI.
@dejwu:
As stated in the beginning : it all depends on your personal strategy…
Shipping will be about 200 add the tax of 4200 will bring it to roughly 26000 – not 30000.
But that is for AP + G5 + full featured Nav/Com/Navigator.
Concerning double install – of course it can always be more (G500txi …), but I think that was not the question.
I was looking at the minimum viable configuration for real instrument flying and that has to be compared to the alternatives.
If you look at the other FAA STCed autopilots you have to consider that they are not able to fly ILS, no VNAV (I mean integrated with your navigator), no IAS hold, rate based ?, no indicator, only bank angle protection. I think these are also interesting products, but in a different category.
I don‘t say it is cheap, but it is a lot cheaper than what has been on the market before.
I agree, the new kids on the block are for a different purpose and they do not pretend to be a fully old fashioned autopilot.
Yes, they will fly ILS only GPS simulated and not coupled to radio navigation.
They do fly vertical guidance with WAAS/EGNOS navigators using GPSV, again GPS simulated only.
Yes, they do fly procedures GPS simulated only, which means you should not fly VOR approaches where the VOR has been shut down ;-).
Yes, they only can simulate IAS hold by GPS speed hold, which is kind of so-so.
They do have indicator support, but most don’t use that.
They do feature bank angle protection and overspeed/underspeed only based on GS, again which is so-so.
The point is – if used with care and knowledge, is it good enough? For many, Yes, and it saves a lot of money.
Somebody going medium to hard IMC and radio navigation with all bells and whistles will disagree, but for the typical VFR day pilot, it is ok. No more, no less.
Garmin is eager to get their systems out, a Dual G5 is usually quoted at quite a bit less than 2x 1 G5. Package a GFC with it and there is additional rebate.
Get quotes from Garmin.
GS hold and in particular bank protections based on GS sounds downright dangerous to me. With that not only the pilot but the autopilot as well will be able to do the famous downwind turn followed by a stall.
Airborne_Again wrote:
I’m not sure. I talked to my avionics shop about replacing a mechanical HSI with a G5. He indicated that it would be a major task considering that you have to install a magnetometer, rewire various interfaces etc. Add to this problems of interfacing legacy autopilots. The total cost could easily be much higher than repairing or even overhauling the mechanical HSI.
I got mine to replace a King HSI which I suspect had a dead gyro. All told it was less than €4k to get the G5 installed and that got me GPSS roll steering and backup AI. The King would have been half that amount at least to get working again.
dejwu wrote:
Yes, they only can simulate IAS hold by GPS speed hold, which is kind of so-so.
GFC500 speed hold is enabled by the G5’s pitot static derived IAS signal, not GPS calculated unless I’ve missed something?
the new kids on the block are for a different purpose and they do not pretend to be a fully old fashioned autopilot.
I would suggest that an autopilot which cannot fly an ILS is an almost total waste of money.
Unless of course
Somebody going medium to hard IMC and radio navigation with all bells and whistles will disagree, but for the typical VFR day pilot, it is ok.
it is for VFR-only 
and for that an autopilot is great too, but – to an uninformed prospective buyer, of which there are lots – I don’t think one can compare the prices directly.
I also think it is a real wasted opportunity, to install all that hardware, the servos, the metalwork, etc, and not have proper IFR capability for Europe. I know some people spend 200k+ on “VFR only” planes (most of the Rotax market really) but I don’t really think they keep clear of cloud the whole time 
Also their sales – we had a thread on this a while ago – are really very small in Europe, despite the hype.
An ILS is the ultimate lifesaver and will be for many years to come, if you fly internationally within Europe.
LPV is becoming more useful than ILS, but I imagine you mostly meant horizontal and vertical guidance to a runway
