Extract from the above doc
INSTRUMENT PROCEDURES
4.1. Display of Endorsed Primary Flight Reference.
4.1.1. Anytime flight conditions (illumination, visibility, weather) or procedures (National Airspace System) require flight by reference to instruments, the pilot MUST select and continuously display an endorsed primary flight reference (PFR). The Standby Flight Display (SFD) and Helmet Mounted Display are not endorsed PFRs. Currently approved single medium PFR displays are heads-down, either:
4.1.1.1. Helmet Mounted Display Virtual Heads-Up Display (HMD v-HUD). Note that under some flight regimes, the horizon line and pitch ladder collides (coexists) with the airspeed, altitude and heading symbology, causing potential readability issues; or,
4.1.1.2. Full-color Electronic Flight Instrument (EFI). When using this display, pilots are prohibited from using the pop-up data entry keypad overlaid on the EFI.
4.1.2. The primary unusual attitude reference is the HMD v-HUD. Do not use the HMD or SFD to recover from an unusual attitude or when executing lost wingman procedures except when no other attitude reference is available.
Back in the 80s I wrote a paper that was later used at an AGARD confernce that was entitled:
Modern flight Instrument displays as a major military aviation flight safety weakness.
History is great stuff.