PRNAV based on DME/DME

[Skyjob]

AFM dictates what you can and cannot do with the airplane.
Chart says what is required for STAR/SID/...

The two do not necessarily say the same or require the same.

Thus a RNAV 1 chart may (or not) say DME is required.
If DME is required the MEL items prohibits you flying such procedure.
If no reference to DME required on RNAV 1 chart you are good to go as your other systems installed guarantee you RNAV 1 compliance.

An RNAV 1 procedure based on DME/DME essentially means that when the procedure was created, the following was undertaken:

1. If RNAV 1 is required for SID:
   • DME/DME and GNSS is assumed as the navigation sensor.
   • Greater of the following two types of areas is applied
     • GNSS criteria or
     • DME/DME criteria
   • Classified into Straight Departure and Turning Departure (Turn>15degrees)
   • It is assumed that aircraft climb straight until reaching 120m(394ft) after takeoff.
   • Procedure Design Gradient (PDG) for aircraft Nominal = 3.3% (200ft/NM) (Origin of the climb is assumed to be 5m above Departure End of Runway(DER))
   • Obstacle Identification Surface (OIS) Nominal = 2.5%
     • Normally no obstacle penetrates this surface.
     • In case of penetration:
       • Avoid the obstacle by turn, or
       • Raise PDG
2. If RNAV 1 is required for arrivals:
   1. If RNAV 1 is required for STAR:
      • DME/DME and GNSS is assumed as the navigation sensor.
      • Greater of the following two types of areas is applied; GNSS criteria or DME/DME criteria.
   2. If RNAV 1 is required for APPROACH (*Published as RNAV(GNSS) RWYxx APCH):
      • GNSS is assumed as the navigation sensor.
   3. GNSS Area width is derived not from navigation accuracy, but from RAIM Alert Limit (IMAL).
      • ATT (Along-Track Tolerance) = IMAL
      • XTT = IMAL + FTT(Flight Technical Tolerance)
      • Area semi width = 2 x XTT
        • 8.0NM (IAF outside 30NM, Missed Approach WP, DEP)
        • 3.0NM (IAF within 30 NM from ARP, IF , Missed Approach WP, DEP)
        • 1.2NM (FAF)
        • 1.0NM (MAPt)

The fact that an RNAV 1 approach is based on DME/DME thus just means that the accuracy tolerance is higher then for GPS.
The reason your MEL refers to BOTH required when procedure is based on DME/DME, is that it cannot guarantee GPS accuracy thus bases itself on most accurate signal (being DME/DME) being lost for navigational purposes.
Then, if RAIM would be out of tolerance, you would fly blind...