Gig,

the initial approach fix (IAF) is defined by the NDB. how would you
find this point without an ADF? You'd get around that with radar
vectors, but you couldn't fly the full procedure without the NDB. And
the missed approach is a hold on the NDB. Same problem.

BUT: An approach certified GPS with the NDB in it's database can
replace for the ADF.

--
Thomas Borchert (EDDH)