FAA and members of the aviation community have almost completed a long process for updating the equipment required notes on IFR charts. In particular, FAA is consolidating and adding detail about performance based navigation (PBN) requirements.
For background on this topic, see New Equipment Required Notes and the topic 13-02-312: Equipment Requirement Notes on Instrument Approach Procedure at the Aeronautical Charting Meeting.
But FAA still hasn’t clearly addressed the confusion that many pilots have about when and how an IFR-approved GNSS (i.e., a “suitable RNAV system” as described in the AIM and various ACs–specifically, AC 90-100, paragraph 7) supplants the need for conventional avionics, such as DME and ADF.
For example, see the KBEH ILS or LOC RWY 28 approach chart.
The equipment notes say that RNAV-1 GPS (i.e., either a non-WAAS or WAAS GNSS) is required to fly the procedure. A second note says that DME is required if you fly the LOC-only procedure, a non-precision approach to an MDA with a missed approach point identified by a DME fix. Other waypoints along the final approach segement include WESUG and the visual descent point (VDP), each defined as a DME fix.
The notes seem to imply that if you fly the LOC-only version of this procedure, you must have DME. That is, if you have an IFR-approved GNSS, you must also have DME to fly the non-precision version of the procedure—you can’t use GNSS to substitute for the DME requirement unless you have a second GNSS that you can use to load the I-BEH localizer as a fix to provide distance information from the location of that DME transmitter. But that interpretation would make it impossible for most new aircraft (which don’t typically come with DME) or aircraft retrofitted with GNSS avionics and no DME to fly the LOC approach.
If you load that procedure into an IFR-approved navigator such as the Garmin GNS 750, the flight plan includes the following fixes:
You can use the distances shown relative to the FAF and the RW28 MAP to determine your along-track distance (ATD) to the VDP and the MAP, as described in the AIM and other references. You don’t need DME to identify the key fixes (and VDPs aren’t included in databases, so you must use ATD, either from a GPS or DME, to determine your position relative to a VDP).
Both AIM 1−2−3 Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes and AC 90-108 (it has the same title as the AIM paragraph) state:
Use of a suitable RNAV system as a Substitute Means of Navigation when a Very−High Frequency (VHF) Omni−directional Range (VOR), Distance Measuring Equipment (DME), Tactical Air Navigation (TACAN), VOR/TACAN (VORTAC), VOR/DME, Non−directional Beacon (NDB), or compass locator facility including locator outer marker and locator middle marker is out−of−service (that is, the navigation aid (NAVAID) information is not available); an aircraft is not equipped with an Automatic Direction Finder (ADF) or DME; or the installed ADF or DME on an aircraft is not operational…
1. The allowances described in this section apply even when a facility is identified as required on a procedure (for example, “Note ADF required”).
That guidance apparently is too subtle. AOPA has flagged this issue at least three times in written comments to FAA about the PBN requirements box. As an AOPA representive confirmed, “It is not necessary to add a ‘DME required’ note if RNAV-1 GPS is also required to fly the approach.”
FAA is aware of the confusion current notes cause, and my contacts at AOPA are checking with FAA to see if the agency is making progress on clarifying the language used in the notes and guidance such as the AIM and ACs.