Dave S wrote:
Roger wrote:
and don't forget the basic steam gages are still
required in addition to the glass panel and should be located where
they are easy to read in case the electronic system fails.

Please cite the appropriate rule that mandates that assertion.

For type-certificated aircraft, there's an Advisory Circular
which says basically either establish specified failure
probability, depending upon the importance of the instrument, or
please the exception herein for "redundancy." Thus, we may see
like a mechanical horizon gyro stuck in a glass panel somewhere.
For amateur-built and IFR, probably a good idea too.

Fred F.