Can one fly an ILS, LOC, or VOR approach using the radios in a GTN 750 which has outdated databases? Assuming one has another source of current charts in the plane, like Foreflight or printed.
If you aren't using any of the GTN's database information during approach (i.e. tuned the radios and following the procedure based on info on a current paper or EFB chart or plate) I don't see why not.
Consider table 1-1-6 in this part of the AIM if you were doing a GPS approach with the GTN and expired database:
Note 3 to the table column "IFR Approach" which applies only to the row "IFR En Route, Terminal, and Approach" states: "Requires current database or verification that the procedure has not been amended since the expiration of the database." Note 2 on the other columns indicates that IFR enroute and terminal use "Requires verification of data for correctness if database is expired."
They don't say how verification should be done, but I assume you'd somehow display the intended procedure on your aircraft GPS and cross-check all its elements with a known up-to-date paper or EFB copy. If they match it looks like you're legal to use it. If not, I presume you can't use that GPS approach.
So for any of those non-GPS approaches I would extrapolate that so long as you've verified the procedure is unchanged compared with a current plate you could use the same info for a non-GPS approach.
If you are just using the radios to feed data to ILS glideslope and localizer needles (on a traditional or a digital display) then of course the database is not involved. If you get ATC instructions to a fix you perform your own database lookup on the paper/EFB chart on your lap. Back in the day that is how you would navigate. Of course if there is an autopilot involved then that's a horse of a different color.
(You know I don't actually have an IFR rating yet. So anything I say on the subject is dubious.)