EHSI #1 would be driven from
- NAV1 (KX155A, VOR/LOC/GS)
- NAV2 (KX165A, VOR/LOC/GS)
- GPS (KLN94)
EHSI #2 would be driven from the same sources.
Currently I have the standard "NAV/GPS" switch which selects the source for the 525 HSI: NAV1/GPS.
There is no NAV2 option on the current HSI (NAV2 drives a CDI, and an RMI). AIUI, a 3500 can accept just about all the signals available and one selects the required one on the 3500 itself. So there is nothing gained in installing a 3500 and not feeding it absolutely everything available.
My panel is here
http://www.peter2000.co.uk/aviation/crete2/tb-panel-big.jpg
It is almost totally factory standard.
The other reason for operating the AP from HSI #2 would be to enable the RHS pilot to do all the flying from his seat.
I would be interested in knowing the legalities of this. I assume that one would need a clear annunciation of the current AP source (HSI1/HSI2). That could be done with an illuminated switch - same as my current NAV/GPS selector switch.
If HSI #2 doesn't
drive anything, its heading bug will be virtually useless - except for manual flight.
In terms of wiring, I can see one would need a multipole relay to switch all the signals over. One can get very high reliability sealed relays (not cheap but nothing in this game is).
Incidentally, a friend has a nearly new KI-825 EHSI, on the shelf. It came out of a Jetprop. This has a silly price tag but he might sell it cheaply. Is it any good? I have flown with it; it is very sunlight-readable.
I need to understand the issues because UK avionics shops are well known for delivering a "90% solution", knowing the customer will still pay up because the alternative is to rip it all out.