Synthetic vision, certification judgement
 

A recent incident involving a synthetic vision display triggered thoughts about the certification of increasingly compelling instruments against the caveat of "not to be used for primary navigation nor rely on it ..."

At what point should certification judge operational acceptability.
If the reliability of individual equipment and format is judged to be suitable for 'use', then how should the wider system involving single point failure of an input sensor be judged.

Is this form of technology creating more operational 'catch 22s', or should any form of synthetic vision display be considered as a primary display for certification?
"Don't use this system because we told you not to"; but the system is so compelling that without removal or warning it is most likely to be used.
Who judges, how, and where does the responsibility for this reside in certification - the suitability for use of complex systems?

The aircraft appears to be have been certificated for single crew operation, thus may not be a Part 25 aircraft.
For a Part 25 certification, or equivalent for IFR rated aircraft, I would expect that SV to meet the requirements of not displaying hazardous misleading information, and if applicable fitted with a comparator system with a greater availability than the display.
There is some information in CS 25, which appears to indicate that any synthetic vision display must meet the requirements of the host display - EFIS, HUD.

Further to the issues in #1, the certification reference is at:-
AC 20-167A https://www.faa.gov/documentLibrary/...AC_20-167A.pdf

This supports the view that the accident aircraft did not meet this guidance; perhaps the reason for the warning, but that may not avoid instinctive use of a compelling display.
Also, that although the regulatory guidance for synthetic vision appears similar to conventional instrument hazard assessments, it may assume too much about pilot involvement. Without a comparator system or stand by 'instrument' for synthetic vision, then the crew has to detect and resole a failure condition; choosing between a 'real world' display and the EFIS attitude indicator.