Curves ahead

In the air, your GPS flashes before a course change. But a recent flight in a Cirrus SR22T G7, equipped with all the latest avionics magic, reminded me that those prompts alone may not provide all the information you need. You must look more closely to anticipate what may happen next.

The owner and I approached Paine Field (PAE) north of Seattle on an IFR flight from the east, planning to fly the RNAV (GPS) Runway 34L approach for practice on a cloudless night.

We weren’t on a published route, and when we checked in, the Seattle Approach controller cleared us to RARYO, which serves both as an initial approach fix when it anchors a hold in lieu of procedure turn (HILPT) and as an intermediate fix. Because our track would cross RARYO at 90 degrees or less, the pilot loaded the approach and quickly deleted the HILPT, a reasonable choice given the setup. ATC obviously intended to clear us straight-in unless we requested the course reversal.

What the moving map then displayed, however, puzzled the pilot. Instead of a track that anticipated the turn to join the course from the east, the MFD showed a path that would overfly the fix and make a sweeping turn to capture the leg to the final approach fix from the west.

It being a quiet evening, we left the flight plan alone and watched as the autopilot smoothly aimed us at RARYO and then followed the slalom path drawn by the magenta line to join the final approach course. The controller also may have watched our track, but he didn’t deduct style points when we canceled IFR.

Understanding why the GPS didn’t plot a graceful intercept from the east requires closely examining the fixes in the flight plan, paying particular attention to flyover and flyby waypoints.

Most of the time, the only flyover waypoint for an approach is the missed approach point, to ensure that you don’t turn before you reach that fix. But as we’ve seen, some fixes serve two purposes, and on a chart, those fixes appear only as flyby waypoints. The Aeronautical Information Manual provides background, buried deep in paragraph 1−1−17. That text explains that: “Fly-by waypoints connect…two segments by allowing the aircraft to turn prior to the current waypoint in order to roll out on course to the next waypoint. This is known as turn anticipation.”

But the AIM also notes that, “Some waypoints may have dual use; for example, as a fly-by waypoint when used as an IF for a NoPT route and as a fly-over waypoint when the same waypoint is also used as an IAF/IF hold-in-lieu of PT. Since the waypoint can only be charted one way, when this situation occurs, the fly-by waypoint symbol will be charted in all uses of the waypoint.” The Aeronautical Chart User’s Guide adds: “Waypoints designated as a holding fix are shown as fly-by, without the circle around the symbol. However, [if] the holding fix/waypoint is also designated in all other parts of the procedure unrelated to holding with a fly-over function, then the holding fix/waypoint will be charted as a fly-over point.”

The key to setting up for an approach that includes dual-purpose waypoints is noting how fixes and procedure legs appear in the flight plan page of your navigator, not just on the chart. In the list of waypoints for the approach at PAE, the first instance of RARYO, the hold entry point, is shown and encoded as an IAF marked as an RNAV fix symbol with a circle. After the hold, RARYO is labeled as the hold exit and is shown as a triangle, sans circle.

To fly a smooth intercept from the east, instead of deleting the HILPT, the pilot should have activated the leg from the hold exit instance of RARYO to the final approach fix, USDAW. That leg begins at RARYO coded as a flyby waypoint, and the GPS can anticipate the turn.

Now, we were in no danger of hitting an obstacle during the S-turn. When course changes are required, procedure designers accommodate a range of aircraft performance and account for winds aloft. Moreover, some systems don’t provide turn anticipation. If you’re in doubt about how your navigator will perform, you can switch the autopilot to Heading mode or take control to fly smooth intercepts manually.

The more likely problem in such scenarios is distraction. Puzzling over the display or fiddling with the autopilot or GPS can lead to a pilot deviation, failure to complete a checklist, or even loss of control. So the next time you load an approach, inspect the road signs carefully and brief accordingly. Details matter.

Bruce Williams is a CFI. Find him at youtube.com/@BruceAirFlying and bruceair.wordpress.com