Using GPS as Nav Source for ILS and Other Approaches

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I am completing my instrument training in a Cessna 172SP with G1000
and I have a question. When flying a non-GPS approach (ILS, VOR, or
NDB), how much of the approach can I fly using GPS as my navigation
source with or without the autopilot? In other words, when do I need
to switch my CDI to the legal source (ILS, VOR, or NDB) indicated on
the chart and away from GPS?
My guess is at the FAF but I have been unable to find this in the
FARs. Any help if appreciated.
Todd

-- Todd Ramming, February 5, 2010

Answers

The Garmin 430/530s will start to complain with a flashing MSG light if you haven't switched to VLOC. I think the flashing starts at the FAF. I haven't seen a FAR governing this, though.

-- Philip Greenspun, February 5, 2010