Narrative:

My airplane has a garmin G3X touch system fed by a GTN650. I've emailed garmin on this topic before; but they may need additional motivation. With the current design; it is almost impossible to fly a stable; legal vectors to final lpv (localizer performance with vertical guidance) approach.from this morning's email to them:back to vectors to final on an lpv approach. With the glide path coming alive only as the airplane passes the final approach fix; this pretty much guarantees an unstable approach. Here are several bad things that can happen:(1) suppose the airplane is at the FAF glide path crossing altitude with power to fly level; a perfect approach. Once the glide path comes alive at the FAF; the plane will overshoot the glide path because the velocity vector is still level; and neither an abrupt power reduction nor an abrupt pitch over (both destabilizing) will track the glide path as well as an intercept from below. (2) the airplane is slightly above the FAF glide path crossing altitude; either hand-flown or due to barometric altimetry or charting errors. In this case; the G3X will not capture the glide path; will not annunciate the failure to capture; and again; the pilot will have to capture the glide path from above; a destabilizing maneuver. (3) if the pilot anticipates glide path intercept by reducing power as the FAF gets close; this means that the pilot is flying an energy profile contrary to the vertical guidance; slowing in anticipation of intercept and then regaining speed on the glide path. That's pretty radical. (4) the only solution I see to having the automation be guaranteed to capture the lpv glidepath on vectors to final is to intercept the glide path below the intercept angle; such as by descending 100 feet below the intercept altitude. I've not looked at the GPS equivalent of the terps for guaranteed obstacle clearance; but deliberately flying one leg of the approach low is a tacky way to make up for the automation's shortcomings. I feel that this is a safety of flight issue -- that the automation will not let you fly a stable approach -- and will file an ASRS report on this.

Google
 

Original NASA ASRS Text

Title: Pilot reports that Localizer Performance with Vertical Guidance generated glideslopes in many cases on start at the FAF and that the transition from level flight vectors past the FAF to the artificial glideslope is not ideal.

Narrative: My airplane has a Garmin G3X touch system fed by a GTN650. I've emailed Garmin on this topic before; but they may need additional motivation. With the current design; it is almost impossible to fly a stable; legal vectors to final LPV (Localizer Performance with Vertical guidance) approach.From this morning's email to them:Back to vectors to final on an LPV approach. With the glide path coming alive only as the airplane passes the final approach fix; this pretty much guarantees an unstable approach. Here are several bad things that can happen:(1) Suppose the airplane is at the FAF glide path crossing altitude with power to fly level; a perfect approach. Once the glide path comes alive at the FAF; the plane will overshoot the glide path because the velocity vector is still level; and neither an abrupt power reduction nor an abrupt pitch over (both destabilizing) will track the glide path as well as an intercept from below. (2) The airplane is slightly above the FAF glide path crossing altitude; either hand-flown or due to barometric altimetry or charting errors. In this case; the G3X will not capture the glide path; will not annunciate the failure to capture; and again; the pilot will have to capture the glide path from above; a destabilizing maneuver. (3) If the pilot anticipates glide path intercept by reducing power as the FAF gets close; this means that the pilot is flying an energy profile contrary to the vertical guidance; slowing in anticipation of intercept and then regaining speed on the glide path. That's pretty radical. (4) The only solution I see to having the automation be guaranteed to capture the LPV glidepath on vectors to final is to intercept the glide path below the intercept angle; such as by descending 100 feet below the intercept altitude. I've not looked at the GPS equivalent of the TERPS for guaranteed obstacle clearance; but deliberately flying one leg of the approach low is a tacky way to make up for the automation's shortcomings. I feel that this is a safety of flight issue -- that the automation will not let you fly a stable approach -- and will file an ASRS report on this.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.