There are two aspects to the legality of obsolete data: navigation and information.
The legality of using obsolete data for navigation can be found in your Flight Manual Supplement. The “Limitations” section of the Garmin 530W Flight Manual Supplement simply states IFR en route and terminal navigation is prohibited unless the pilot verifies the currency of the database or verifies each selected waypoint for accuracy by reference to current approved data.
While it’s clear that the source of that validation must be current, there’s no guidance on how the accuracy of waypoints must be established. Are you supposed to check that the VOR didn’t move by comparing the lat-long from a current chart to the recorded position in your database? Right. About the best you can practically do here is to look at where the GPS is taking you and see if it passes the “that-looks-about-right” test that you’re going the right direction.
The Flight Manual Supplement flat-out prohibits GPS approaches unless the 500W-series unit’s approach data is verified by the pilot or crew to be current. The flight manual supplement for the Bendix/King KLN 94 contains virtually identical language. Top this with the AIM prohibition—not technically regulatory, I know—against GPS approaches without a current database, and you’re bucking a violation of FAR 91.9 (Complying with the flight manual) and 91.13 (Careless and reckless) if you get caught flying GPS approaches without a current database unless your flight manual specifically describes an approved way of flying an approach without a current database.
In between these is the use of an out-of-date database in a GPS in lieu of DME or an ADF. There’s usually no statement on this in the flight manual. The AIM “Requires current database or verification that the procedure has not been amended since the expiration of the database.” OK, what do you do if it has been amended since the database update? Unless the DME reference or NDB moved, it’s hard to think of any reason you’d be careless and reckless to use your out-of-date GPS. Our opinion is that’s legal.
Your average MFD also has multiple internal databases (terrain, NavData, approach charts, etc.). MFDs and similar other devices are advisory in nature. The only legal requirement is the ubiquitous “all available information” from FAR 91.103 (Preflight action). If there’s data that’s obsolete on your obstacle database or digital charts, you’re only required to know what it’s supposed to be and act accordingly.
This is all under Part 91 only. If you’re flying Part 135 or 121, your company’s Op Specs may necessitate current data in other places, including portable devices. But when it comes to legal under Part 91, there’s usually nothing required unless you’re flying an actual GPS approach.