Clyde and Steve,
If the legal requirements are such as mentioned by Steve then there’s not much to do.
As to the #1 GPS dying: which GPS is driving the ARNAV screen? Is it only #1 or both. If it’s both couldn’t you just track your course on the big screen? This might be a totally dumb question, since I’ve got only the VFR ticket, so pardon my ignorance.
Only the #1 GPS drives the Arnav; if it goes the Arnav can’t pick up position from GPS #2.
Even if it could, I’m not sure I’d want to track the course just by looking at how close the “airplane” figure is to the pink line. It would depend on what range was set, and really doesn’t seem like a great way to navigate. Particularly if one was already stressed due to GPS #1 going out. Even without an external CDI, I’d probably rather follow the Garmin CDI display rather than try to follow the pink line on the screen. Of course, it depends what the goal is. If you’re VFR enroute, following the pink line seems more realistic than if you’re trying to shoot an approach in the clouds.
Also, after posting that previous message, I found an http://www.aopa.org/whatsnew/newsitems/1999/991213gps.htmlAOPA document, which, in the course of discussing using GPS in lieu of DME & ADF, mentions this:
Neither IFR enroute or approach receivers require a separate CDI by the TSO 129A specifications. However, both probably do require a separate conventional CDI display, or connection to an existing VOR-CDI, to meet Advisory Circular 20-138 installation requirements for display within the pilotÂ´s field of vision/scan.
AOPA’s web site also has the full text of AC 20-138 in the member’s section, but it’s pretty dense reading. Of course, do you (noticed your .ch email address) and Clyde have to follow FAA regulations?