A question was posed in a pilots group as to why “SFO R-101” is charted as a thin line within the localizer course on the outbound end of the arrow.

Let’s back up and imagine your aircraft, N1234A is equipped with VOR receiver only and with no DME or GPS equipment. There are no notes on the chart that requires DME or GPS so it should be possible for N1234A to fly this approach. The IAFs for this approach are DUMBA INT, EDDYY and ARCHI. However EDDYY and ARCHI are GPS waypoints which N1234A cannot identify. So DUMBA INT is the only viable IAF. Looking at the Low IFR chart, DUMBA is not located on any airway and there are no feeder routes charted from the enroute environment. So how does one navigate to the IAF?

ATC must step in not only to provide a transition route for the pilot between the enroute environment and the IAF but also to provide a safe altitude on this route. Appropriately, the note published on this chart confirms that either “RNAV 1-GPS or RADAR required for procedure entry.” FAA Order 8260.19K section 8-6-10(g) reads as follows:

Where use of Radar is the only acceptable method for procedure entry from the en route environment, enter EQUIPMENT REQUIREMENTS NOTE: RADAR REQUIRED FOR PROCEDURE ENTRY.” See paragraph 8-6-9, for additional equipment that may be used in addition to, or in-lieu-of Radar.

When ATC radar vectors N1234A to DUMBA, how do you know when you have arrived at the IAF? Looking at the chart, it appears the outbound localizer course 104°, Woodside VOR (OSI) R-036, Oakland VOR (OAK) R-138 and San Francisco VOR (SFO) R-101 all converge at the IAF. What is the necessity for so many radials to be charted? Section 7-5-2(a)(5) reads as follows:

Transitions must not be established from outside of the normal clearance and buffer areas unless they have been flight inspected and the minimum localizer clearance requirements are met. Where such a flight inspection is unsuccessful, an intersection must be established on the localizer course, or a lead-radial established within localizer coverage. When established on the localizer course, the transition route from a VOR or non-directional radio beacon (NDB) must be predicated on a NAVAID or fix which does not utilize the localizer (i.e., the fix must stand alone on a localizer course for definition) [see paragraph 8-5-2.g(3) and figure 7-5-4].

Comparing this diagram with the approach plate, it appears that DUMBA INT must be identified using any 2 cross radials from OSI, OAK and SFO. Presumably, one would use either OSI or OAK depending upon their arrival route and find the cross radial with SFO to get established on the inbound approach course.

Theoretically, localizer signal coverage area spans up to 10° on either side of the localizer course. The signal range is routinely flight checked. Adjustments to the approach are made over time to compensate for any deficiencies in the signal range. DUMBA intersection is 16.1nm from the localizer. At that distance, the signal could span up to 2.8nm on either side of the course. Coming from OSI to establish on the localizer inbound requires a 110° left turn and coming from OAK to establisher inbound requires an even greater 146° right turn. Waiting to turn inbound until arriving at the IAF may mean that N1234A will overshoot the localizer signal coverage area – traffic flying into KSFO are typically very fast moving. Similarly, turning early in anticipation may require commencing the turn prior to reaching the localizer signal area.

FAA Order 8260.3G (TERPS) section 2-4-3(a)(1) states:

When the angle between an initial approach course and intermediate course exceeds 90 degrees, a radial or bearing which provides at least 2 NM of lead must be identified to assist in leading the turn onto the intermediate course…

Even though SFO R-101 not a “lead radial” since it is not marked as LR-101, the spirit of this section may apply here. VOR signal is omnidirectional and charting it at DUMBA INT helps guide aircraft towards the final approach course. VORs (and not Localizers) are used for enroute navigation. Once we are guided within the localizer coverage area i.e., when the localizer “needle comes alive”, N1234A can flip frequencies to the localizer and get established inbound for the approach.

These days aircraft flying into KSFO are very likely RNAV (GPS) equipped. So it would be interesting to know how often this approach is actually being flown using only RADAR guidance. Perhaps it is charted only as a contingency option if/when GPS signals are disrupted in the region.