AINsight: Watch Out Below the Minimum Descent Altitude

Every instrument-rated pilot knows that you are not allowed to descend below the decision altitude on a precision approach or minimum descent altitude (MDA) on a non-precision approach unless certain elements of the airport environment are in sight.

A recent accident highlighted an issue that might catch pilots who aren’t aware that their avionics may provide an advisory glide path that goes below the MDA on a nonprecision lateral navigation (LNAV) GPS approach. This advisory glide path is designed to give vertical guidance from the final approach fix to the touchdown point on the runway. The idea is for the pilot to be able to fly a smooth, continuous descent instead of the so-called “dive and drive” maneuver, where the pilot descends quickly to the MDA, then levels off, then descends again when the runway environment is in sight.

Pilots need to know something crucial about advisory glide paths: they do not guarantee obstacle clearance.

I have to thank flight instructor and author Max Trescott for bringing this issue up on his excellent “Aviation News Talk” podcast shortly after the accident. Several pilots have contacted him after the podcast to express thanks for highlighting an issue that they weren’t aware of and that could save their lives.

This advisory glide path shows up on avionics as LNAV+V. For example, on a Garmin G1000 system, the magenta LNAV+V label pops up on the HSI while flying the approach, to let the pilot know what minimums are applicable for that approach. Your avionics may vary, but you should know how this works on your displays.

You should also try flying an LNAV+V approach during the day in clear weather and coupled to the autopilot, to see what happens when the airplane arrives at the MDA. I’m unaware of any avionics that will automatically level the airplane off at the MDA, unless the pilot has selected the MDA on the altitude select and the flight director is in vertical speed or flight level change mode and not approach mode.

The NTSB’s preliminary report on this accident, which occurred on the RNAV (GPS) Rwy 32 approach to Steamboat Springs (KSBS), Colorado, at night and with broken to overcast cloud layers near the MDA altitude, noted that the airplane was on autopilot when it crashed. The ADS-B flight path shows the airplane tracking precisely along that approach.

The advisory glide path for that approach intersects obstacles at the top of Emerald Mountain, which is approximately 8,250 feet high underneath the approach flight path. The NTSB report notes that the airplane impacted terrain at about 8,175 feet. The MDA at this point is 9,100 feet.

It is still too early in the investigation to understand what happened and why the pilot might have been following the advisory glide path below the MDA. We also don’t know whether the pilot was able to see the runway lights (he would have had to circle to land on Runway 14 because a straight-in or circle-to-land maneuver was prohibited for Runway 32 at night). Both the Runway 32 end identifier lights and PAPI were unusable, and notams had been issued to this effect.

But we do know that even when following an advisory glide path, it’s never legal to descend below the MDA until the runway environment is in sight. This is especially important in mountainous terrain.

The FAA seems to recognize that there may be inherent flaws in advisory glide paths, at least in the case of the KSBS RNAV (GPS) Rwy 32 approach, because six days after the accident, the agency issued a notam to make that approach “not authorized.” The FAA also told AIN, “We are reviewing information shown on charts produced by non-FAA charting providers. This notam will remain in effect until non-FAA charting providers have corrected any potential charting issues.”

The FAA did not answer AIN’s questions about whether the agency plans to warn pilots about the differences between precision GPS approaches and LNAV+V advisory glide paths, or whether the FAA plans to prohibit the use of and storage in avionics databases of +V advisory glide paths, and prohibit pilots' use of these +V procedures when coupled to the autopilot, especially in mountainous terrain.