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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Max tackles a common frustration among pilots: why RNAV (GPS) glide paths sometimes fail to couple with the autopilot. A recent Facebook post from a pilot flying a Cirrus SR22T triggered the discussion, as the poster encountered exactly this problem during an LPV approach, mistakenly attributing it to an expired database. Max explains that the real culprit was a fundamental misunderstanding of RNAV approach procedures.
Unlike an ILS glide slope, which is a physical radio beam transmitted from ground equipment, an RNAV glide path is a computed construct generated by the aircraft’s GPS navigator. It only becomes “real” for the autopilot to couple to when flown properly, following all step-down altitudes as published. A key indicator is the glide path diamond: it remains hollow and white until the aircraft crosses the fix before the Final Approach Fix (FAF), at which point it turns solid magenta, signaling that the autopilot can engage.
Max emphasizes that this behavior contrasts with ILS approaches, where pilots sometimes get away with intercepting the glide slope from above. Doing so on RNAV approaches, however, won’t work because the computed glide path isn’t extended infinitely; it’s activated only within a defined segment of the approach.
To verify this, Max conducted simulator flights on multiple RNAV approaches. He discovered a consistent pattern: the glide path diamond always turned magenta while crossing the fix immediately before the FAF—not at a set distance before or after a particular waypoint. This nuance is crucial for ensuring autopilot capture and a stabilized approach.
Drawing a parallel to AIM 5-4-5b, Max reinforces that both ILS and RNAV approaches are designed to be intercepted at specific altitudes. Deviating from these procedures risks automation surprises, or worse, capturing false glide slopes in the case of ILS. He recalls the Air India Express Flight 812 crash as a tragic example of why proper interception altitudes matter.
The key takeaway is a simple formula for reliably flying RNAV approaches: descend to each step-down fix altitude, and ensure you’re at the target altitude when crossing the fix before the FAF. When done correctly, the glide path diamond will turn magenta, allowing the autopilot to capture it and guide the descent.
Max’s detailed breakdown underscores why relying on social media for technical advice can be misleading. Instead, pilots should seek information from authoritative sources and understand the underlying principles behind automation behavior. By following published procedures and recognizing cues like the hollow versus solid glide path diamond, pilots can avoid surprises and maintain safe, stable approaches.
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236 Flying VOR approaches with GPS, Autopilot Gotchas + GA News
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Summary 236 Max answers a listener question about how to use GPS to fly a full VOR approach. He also talks about how to fly a VOR approach with a course change at VOR. He also gives an update on VOR+V approaches and some autopilot gotchas with these approach types and with back course approaches.
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So You Want To Learn to Fly or Buy a Cirrus seminars Online Version of the Seminar Coming Soon – Register for Notification
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[podcast src=”https://html5-player.libsyn.com/embed/episode/id/9273584/height/90/theme/custom/thumbnail/no/direction/forward/render-playlist/no/custom-color/88AA3C/” height=”90″ width=”100%” placement=”top” theme=”custom”]104 Learning Advanced Avionics and Upgrading Your Cockpit + GA News
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Summary 104 Max talks about how learning how to operate advanced avionics over the years has changed, and how to go about planning a major avionics upgrade for an airplane. He also talks about the new Garmin GPS 175 and GNX 375 GPS navigators, the Garmin G3X Touch displays, which can now be used in many certificated aircraft, Dynon’s SkyView products, which which can now be used in many certificated aircraft, the AeroCruze 100 digital autopilot and the low-cost xCruze 100 autopilot, and the Garmin GTX 335D and GTX 345D diversity transponders.
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