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.
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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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