IN SUPPLEMENTARY PART OPERATING MANUAL BOEING 737 .........
note.DO NOT USE VOR/LOC AFDS mode when conducting VOR approaches if the VOR/DME statin elevation is more than 5000 ft MSL , if there is no co-located DME trasmitter available at the VOR station to be used or when the DME is invalid for any other reason.........
WHY ???
SOMEONE CAN EXPLAIN TO ME THE REASON ????
PLEASETHANKS