Danger Below MDA?

Recently, an airline began to receive notices that crews flying the RNAV (GPS) RWY 36 approach into Birmingham, Alabama (BHM) were receiving GPWS alerts while descending from the MDA to the runway. Since it wasnt an isolated incident, the airline suspected that the approach was flawed and notified the FAA, who flight checked the approach. The results of that inspection are educational to instrument-flying pilots at all levels.

Key Takeaways:

• Following the Vertical Descent Angle (VDA) below the Minimum Descent Altitude (MDA) on non-precision approaches (like RNAV GPS with LNAV minimums) can be dangerous and trigger GPWS alerts, as these VDAs are advisory and offer no guaranteed obstacle protection below the MDA.
• Unlike precision approaches (ILS/LPV) which have protected glidepaths, non-precision approaches lack implicit obstacle protection between the MDA and the runway, requiring pilots to understand that obstacle clearance surfaces below MDA primarily dictate visibility requirements, not clear flight paths.
• Pilots must differentiate between advisory VDAs and protected glidepaths, relying on visual cues and specific charted indications like Visual Glideslope Indicators (VGSI) for obstacle clearance during the visual segment of non-precision approaches, rather than continuous descent below MDA using the VDA.

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