387 Vectors to Final and glide path behavior on RNAV (GPS) approaches + GA News

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9d ago 56:43

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Max talks about common gotchas with RNAV GPS approaches, especially when using vectors to final (VTF). The conversation picks up where Episode 383 left off, with listeners asking how vectors to final (VTF) influences glidepath capture on WAAS approaches.

He explains that under normal circumstances—flying the approach via own navigation from an initial approach fix (IAF)—the GPS glidepath doesn’t turn magenta or allow autopilot coupling until the FAF becomes the active waypoint. The magenta color indicates that the GPS receiver has completed a signal quality evaluation, verifying that the WAAS signal is good enough for LPV minimums. Until this point, the glidepath indicator remains a hollow white diamond, and the autopilot will not descend.

However, activating VTF triggers this signal check immediately, regardless of the aircraft’s location. If the aircraft’s track is within approximately 100–110° of the final approach course, the glidepath indicator turns magenta and autopilot coupling becomes possible—even miles from the airport. This can be risky if the aircraft is outside the protected segment of the published procedure, especially in mountainous terrain where blindly following the glidepath can be hazardous.

Max notes that activating VTF early, particularly from a downwind leg, may display the “SUSP” annunciator. He urges pilots not to press the SUSP button in this scenario, as it can inadvertently sequence legs inappropriately or disable the approach altogether. The SUSP annunciator will clear automatically during the final base-to-final turn, provided it's within 45° of the final approach course.

The episode then shifts to a common gotcha with ILS and RNAV approaches when being vectored in close. If a controller brings an aircraft in at an altitude that puts it above the glidepath—and the vertical deviation indicator centers before the lateral CDI centers—the autopilot will capture lateral guidance but not vertical guidance. Max describes this firsthand from a recent Vision Jet flight into Orange County, where his awareness helped avoid a missed capture. If the autopilot fails to capture the glidepath due to this timing issue, pilots must use vertical speed (VS) mode to manually descend and re-intercept it.

Responding to listener Matt’s question, Max discusses the use of VNAV (Vertical Navigation) mode as another way to descend to the glidepath for early coupling. He clarifies that while VNAV is a separate topic, it’s related because it can assist with managing step-down altitudes more precisely. If VNAV is properly configured—usually in conjunction with the APPR key—it can manage the descent and transition into the glidepath at the FAF. However, Max warns of several pitfalls: pilots often forget to dial in a lower altitude, which prevents descent; older G1000 systems may ignore VNAV commands if the top of descent (TOD) is more than five minutes away; and using VNAV may not align with ATC altitude restrictions.

He concludes that while VNAV has become more reliable due to Garmin software improvements, it introduces complexity and requires careful verification of ATC clearances and altitude settings.

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