The question of GPS substitution for DME on instrument approaches has a regulatory answer that is both clearly documented and routinely misapplied in practice. AC 90-108 and AIM 1-2-3(c) together authorize the use of a suitable RNAV system — defined by TSO-C129, TSO-C145, TSO-C146, or TSO-C196 certification with a current database — in lieu of DME for a defined set of functions: determining position relative to a fix, navigating to or from a DME fix, holding over a DME fix, and flying DME arcs. The authorization is real, it is FAA-published, and it applies to IFR-approved WAAS GPS units such as the Garmin G1000. Instructors who categorically state that an approach with "DME" in its title cannot be flown without a physical DME receiver are applying an overly broad interpretation that is not supported by the cited guidance. The confusion persists partly because the authorization has limits, and those limits hinge on a specific technical distinction that is rarely explained with precision.
The critical variable is whether every fix required to execute the full procedure — including the Missed Approach Point — is an independently published, named waypoint that exists in the GPS database. When step-down fixes and the MAP are named fixes that the database can resolve and the avionics can sequence to, GPS substitution under AC 90-108 is straightforwardly authorized. The problem arises when the MAP is defined solely by a raw DME distance from a navaid rather than a published named fix. A distance such as "1.2 DME from the LOC transmitter" may not correspond to any waypoint in the navigation database. A G1000 may generate an internal waypoint at that position using approach procedure data, but if that point is not a formally charted, database-resident fix, the pilot is relying on avionics-computed geometry rather than an officially published RNAV waypoint — a distinction that matters for both regulatory compliance and crew coordination. This is the actual mechanism behind the guidance some pilots receive about certain LOC/DME approaches, and it explains why the answer cannot be a blanket yes or no across all procedures.
The LOC/DME approach at KLOT illustrates how the NOTAM system resolves this ambiguity through a formal, published mechanism. When a procedure carries a NOTAM stating it is "NA except for aircraft equipped with suitable RNAV," the FAA has affirmatively answered the substitution question for that specific approach: the DME requirement is effectively superseded for operators flying with compliant RNAV equipment. This is not a workaround or an informal interpretation — it is the FAA designating that the procedure is navigable via RNAV and that GPS-equipped aircraft meeting the TSO standards of AC 90-108 are explicitly authorized to fly it. For the LOC/DME at KLOT, an aircraft with an IFR-certified WAAS GPS, a current database, and the ability to identify all required fixes is legally equipped to execute the approach under that NOTAM. Pilots and instructors should check approach-specific NOTAMs as a standard pre-approach step, because these annotations frequently resolve the exact ambiguity under discussion.
The broader operational takeaway for IFR pilots and Part 91, 91K, and 135 operators is that GPS-in-lieu-of-DME authorization is procedure-specific and database-dependent, not aircraft-name dependent. The presence of "DME" or "VOR/DME" in an approach title signals that DME was the original design basis for fix definition, but it does not automatically prohibit GPS substitution. The correct workflow is to verify that every fix on the procedure — including the MAP — is resolvable as a named database waypoint or is specifically authorized via NOTAM for RNAV substitution, and that the GPS unit meets the applicable TSO. For operators flying glass-cockpit aircraft without legacy DME receivers, maintaining awareness of which approaches in their regular operating environment carry RNAV-authorization NOTAMs, and which still require DME for an unambiguous MAP, is a practical risk-management discipline that directly affects approach planning and alternate selection.