Many implementations of single HUD are extensions of Cat 2; a similar concept with reduced minima enabled by technology, beware false credit, must consider the overall operation.
Some situations are like 'being wrong' in Cat 2; technology gets you into a situation, but with HUD failure the operation depends on human recovery. Single HUD requires a head down pilot for monitoring and GA. The head-up PF has to revert head-down, and/or change of control for a GA, need to focus on the new task and reinterpret conventional instruments.
The reduced minima for manual landing with HUD may be not be sufficient to continuing a manual landing without HUD. The change of view, new mental task, takes time. Although an aircraft is capable of a low GA, the time-consuming mental process could increase the height loss.
There is anecdotal evidence that pilots do not 'look through' the HUD, decisions are made on fewer cues than specified - we perceive what is expected, and landings are made with FD opposed to using the visual scene as normally assumed - because FD is an easier mental task and usually results in 'good' landings.
The manual Cat 3 trials used a HUD (Mono HUD); a single eye system using the then current technology from helmet mounted displays (1978), but with a simpler format than shown. There were no computed flight path or flare aids, no FD or GA mode.
There were advantages when monitoring the progress of an auto or manual landing, and for flying a GA after auto failure; but never really cost effective when considering true Cat 3.
Beware what some authorities have approved - check the assumptions: HUD can generate 'illusions', false security from technology, and false sense of human capability.
https://rochesteravionicarchives.co....t-demonstrator
Click Documents for tech description; download pdf, note different formats and other evaluations / uses.
As tested in BAC 1-11
https://cdn.rochesteravionicarchives...og/2_D0549.pdf
Video:-
https://rochesteravionicarchives.co....civil-aircraft
Expand to full screen. Fifth approach ~ 4:30, Cat 3b 150m 50ft DH, these were the trial test limits for manual takeover and landing.
HUD shown was not as used; other autoland clips including DC-9.