383 Why Your RNAV Glide Path Won't Capture—and How to Fix It + GA News

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. If you're getting value from this show, please support the show via PayPal, Venmo, Zelle or Patreon. Support the Show by buying a Lightspeed ANR Headsets Max has been using only Lightspeed headsets for nearly 25 years! I love their tradeup program that let's you trade in an older Lightspeed headset for a newer model. Start with one of the links below, and Lightspeed will pay a referral fee to support Aviation News Talk. Lightspeed Delta Zulu Headset $1199 Lightspeed Zulu 3 Headset $899Lightspeed Sierra Headset $699 My Review on the Lightspeed Delta Zulu Send us your feedback or comments via email If you have a question you'd like answered on the show, let listeners hear you ask the question, by recording your listener question using your phone. News Stories ForeFlight integrates FAA weather cameras FAA Lifts Some Restrictions on Experimental Aircraft Duffy Will Allow Air Traffic Controllers to Work Past 56 ATC 'hotline' between Pentagon and Reagan Washington National Airport broken since 2022 Pilot seriously injured when he forgets to remove wheel chocks Overrun Accident Traced to Pilot Fatigue, Other Factors Pilot Sentenced Over Threat to Kill Prospective Employer Mentioned on the Show Buy Max Trescott's G3000 Book Call 800-247-6553 Lightspeed Delta Zulu Headset Giveaway Video of the Week: Garmin AutoLand in SR22/G7 Tom Turner's Flying Lessons Weekly Free Index to the first 282 episodes of Aviation New Talk So You Want To Learn to Fly or Buy a Cirrus seminars Online Version of the Seminar Coming Soon – Register for Notification Check out our recommended ADS-B receivers, and order one for yourself. Yes, we'll make a couple of dollars if you do. Get the Free Aviation News Talk app for iOS or Android. Check out Max's Online Courses: G1000 VFR, G1000 IFR, and Flying WAAS & GPS Approaches. Find them all at: https://www.pilotlearning.com/ Social Media Like Aviation News Talk podcast on Facebook Follow Max on Instagram Follow Max on Twitter Listen to all Aviation News Talk podcasts on YouTube or YouTube Premium "Go Around" song used by permission of Ken Dravis; you can buy his music at kendravis.com If you purchase a product through a link on our site, we may receive compensation.