Mixing RNAV and an ILS

If you fly an airplane with a suitable RNAV system (for most of us, that’s an IFR-approved GPS navigator in the panel), you’re accustomed to flying RNAV (GPS) approaches and other procedures, such as RNAV departures and arrivals. And since most RNAV navigators currently in use also support flying ILS and VOR procedures, you also probably still fly the occasional ILS or VOR approach, even if you prefer all-GPS procedures.

But as the shift to Performance-Based Navigation (PBN) continues, the FAA is publishing more approaches that include–and sometimes require–using both GPS-based RNAV systems and ground-based navaids.

For other examples, see An ILS that Requires GPS, and the ILS OR LOC RWY 21 at KSTE in WI.

Consider the ILS or LOC RWY 12 at Huron (KHON), a small town in South Dakota.

In many respects, this is a typical ILS. It offers a DA at 200 AGL and requires 1/2 sm visibility (the RNAV (GPS) approach to RWY 12 offers the same LPV minimums). It also has an old-school locator outer marker (LOM) at BEADY that serves as the final approach fix for the LOC-only version of the approach and as the anchor for the missed-approach holding pattern.

But read the note in the required equipment box.

In most light aircraft these days, you must also have an IFR-approved GPS to fly the feeder routes and to identify BEADY, because your panel probably doesn’t include DME or an ADF.

ATC could provide vectors to steer you from the enroute environment to the final approach course. But as I’ve noted elsewhere, if you filed IFR as an RNAV/PBN-capable aircraft, a controller can clear you direct to any initial approach or intermediate fix, even if you’re flying a conventional procedure like an ILS.

The enroute chart for the area around Huron shows why you might expect such a clearance. The closest VOR at Watertown (ATY) is nearly 40 nm away, and the airways that converge at KHON are all GPS-based T-routes. The VOR at HON has been decommissioned; only the DME component remains (for more information, see Stand-Alone DMEs on Charts).

A controller can avoid issuing a series of vectors and altitudes as you fly toward the airport and offer one simple instruction, “Cross HUMSO [or WEDEM] at or above 3000, cleared for the ILS RWY 12 approach.”

Your task is to brief the plan for changing from GPS guidance to fly the feeder route from either HUMSO or WEDEM to “green needles” to intercept and track the localizer as you turn inbound toward the airport. And decide, if necessary, how you’ll fly the missed approach.

So today, even if you’re flying to a GA airport far from the big city, you should be prepared to load and fly such hybrid procedures if, for training or practice, you want to fly an ILS, LOC, or VOR approach.

Using GPS on Conventional Procedures

I get many questions from pilots about how they can use an IFR-approved GPS while flying departures, airways, arrivals, and approaches that are based on or include navaids—VORs, DME, localizers—and sometimes even NDBs. The current guidance from avionics handbooks, instructors, and FAA publications, such as AC 90-108 and AIM 1-2-3, isn’t always easy to understand, and some details have evolved as new avionics have become available.

Note: FAA has released Draft AC 90-119 Performance-Based Navigation Operations, which updates guidance on this topic. When it is published, the new AC will also replace several existing AC and drive changes to the AIM and other sources of information.

The video presentation below takes a close look at the FAA guidance on the topic, including details in the draft AC 90-119 and uses specific examples to help you understand how you can use a suitable RNAV system–for most GA pilots, that means an IFR-approved GPS–to fly all or parts of departures, airways, arrivals, and approaches that are based on navaids.

For more information on this topic, see the following posts here at BruceAir:

To see more presentations for pilots, check this playlist at my YouTube channel, BruceAirFlying.