The Monitored Approach concept was developed primarily by BA (BEA) in conjunction with BLEU at RAE Bedford for all weather operations on BAC1-11 and Trident aircraft.
The principle for approaches required one pilot to be head down and ‘fly’ the aircraft (generally via the autopilot), and the other to monitor the operation, but be head up for the decision and landing.
The head up pilot looks up at about 100ft above DH to seek visual cues for a landing decision. The head down pilot would announce ‘xx ft above’ (decision height) alerting the head up pilot, who would reply ‘land’ if the conditions were suitable. If there was no return call (land) or a go around instruction then the head down pilot flew a go-around.
If a system failed or there was an ILS deviation before a land decision, then the head down pilot would fly a go-around.
After a ‘land’ call, the head up pilot would either continue to monitor the landing visually or if there was a failure after decision altitude, fly a go around. Thus, there was a change of control at the point of deciding. Some operators also used a hand tap on the thrust levers to reinforce the ‘land’ call and the change of control; this was very effective in busy ATC environments where crew calls could be masked.
The procedure reduced human error of ‘press-on-itis’ or ‘lets have a look’ during low visibility operations. It can also be used during NPAs. Primarily it reduced the decision maker’s workload whilst maintaining good monitoring. For experienced crews it also enabled first officers to be the head up pilot and gain more experience in low visibilities; each sector could be flow turn and turn about.
The principle of the procedure was adopted by many European operators, but not in the USA due to the then ‘P1’ biased operating technique (some would argue otherwise, but their monitored approach was still P1 led). In recent years, the procedure has been adapted to suit more modern atutoland designs, HUD, etc, and lower Cat 3 minima.
IIRC, B Cal and Dan Air used the monitored approach, and possibly Brittania, but I don’t know what the current procedures are.
The principle of the procedure is sound and safe, but to some extent relies on good training, experience, and discipline. There is a very good paper continuing some the conceptual ideas and proposals for current operations -
Eliminating "cockpit-caused" accidents: Error-tolerant Crew Procedures for the Year 2000