Well Stated aterpster, ref the B727 SP50/150
I concur... but note...
All we had back then was essentially the Caravelle Lear MC1, the Trident complete kluge of a mess, and the B707 PALS and PB-20, the HUD on Col. Klopstein's N262 trying to get it on the Mercure, with the promise of finally doing it right with the B747 SPZ-1, either dual channel or triple channel, albeit still with no rollout capability. From that foundation, Lockheed (and Douglas) finally both did a design that was much better (Fail-Op as well as reversionary Fail-Passive), that led to the great system on the 1011, ...and great "attempt" on the DC10, but nonetheless fouled up execution on the PB-100 on the DC-10 (until it was fixed much later after partial resolution of the UAL/Douglas "who pays" fight).
So the B727 at the time looked pretty good in comparison!!!
This was all while the DC9's with their pre-906 AFDS, and the B737's Jurrasic APs never did get much traction, and never got much farther than somewhat useful Cat II.
Even the C141 (AWLS) and C5 were all fouled up at the time with AWO issues, which partially led to the Specked Trout AL and AWO effort, and the ILM on AF 624133 (both "my" jets in that era), and the massive Pi-Fax T39 effort at IPIS/IFC, and the Mythical Landing System (MLS) in parallel.
And even as late as the 80s, the A300's were still all fouled up for AL (many reasons,.. from delta-psi preset course error sensitivity, to stby AI reference drift, to pitch thrust coupling, and nearly complete inability to tolerate irregular terrain). So by Dec 1, 1977 (signing of AC120-28B), and opening up of the 30+ AN/GRN-27 ILSs, as Type II ILSs, to support Cat III 50'DA(H) RVR700 ops,... it looked like a much better and safer thing to do to use the B727's AL capability than continue with the countless low-vis (manually flown) accidents we were having, starting with Williamsport in '59, that led Ed Burke (AAA) to try to take action at the "All-WX-Flying Committee", ...long before even the DeCelles/Oliver era.
So while the B727 did have "non-optimum" AP/AL aspects, it was ultimately successful and safe, and finally help break through the many operational barriers to doing Cat III, ...since it was "good enough", and it used so many more destinations than the L1011 where Cat III minima could be applied. In the end, it was as least as important to ultimately making Cat III practical globally, than the L1011, and much more so than any European jet or set of airports.
B Regards!!!