The AF 737 accident was indeed because of one on-board ADF receiver flying a two NDB approach. The two NDBs are "in-line" with one apx. 5 miles from the runway end and the other at the LMM position just short of the runway. That was a legal approach under early (50's) PANS-OPS standards, basically, the approach designer used two trapazoids-one expanding from the NDB on final, merging with a trapazoid narrowing toward the beacon just short of the runway. Absolutely requires two ADF receivers to legally fly. The point of the design was to allow lower mins due to the terrain "eliminated" from the design with two trapazoids. Also provides a positive MAP not using timing. I do not believe this is still a PANS-OPS approach design. Any comments??
GF