A320 FCOM LIMITATIONS. AUTOFLIGHT. LIM-22-10
USE OF NAV MODE FOR APPROACH
VOR, VOR/DME, NDB or NDB/DME approach procedures may be performed, in NAV, or NAV and FINAL APP mode, provided AP or FD is used, and:
‐ GPS PRIMARY is available. In this case, the reference NAVAID may be unserviceable, or the airborne radio equipment may be inoperative, or not installed, provided operational approval is obtained.
‐ Without GPS PRIMARY:
• The reference NAVAID and the corresponding airborne equipment is serviceable, tuned, and monitored during the approach, or
• The radio NAVAID coverage supports the RNP value, specified for the approach procedure, and an operational approval is obtained