diff --git a/plane/source/docs/rangefinder-autolanding.rst b/plane/source/docs/rangefinder-autolanding.rst index 5791f2b0c5b..670ee6e3758 100644 --- a/plane/source/docs/rangefinder-autolanding.rst +++ b/plane/source/docs/rangefinder-autolanding.rst @@ -37,8 +37,10 @@ A value for :ref:`LAND_FLARE_SEC` of 1.5 and :ref:`LAND_FLARE_AL Impact of Glide Slope Terrain variation ======================================= -If using rangefinder during landing, the rangefinder readings, once engaged (ie when getting good readings below the :ref:`RNGFND1_MAX` altitude on approach), will continuously adjust the glide slope target altitudes as it approaches. If the terrain is irregular, or there is a deep ditch before the runway threshold, using the rangefinder can actually be worse than just using the barometeric altitude. +If using rangefinder during landing, the rangefinder readings, once engaged, will continuously adjust the glide slope target altitudes as the aircraft approaches the landing point. Rangefinder engagement requires good readings below the :ref:`RNGFND1_MAX` height on approach and, if :ref:`RNGFND_LND_DIST` is used, the aircraft must also be within that horizontal distance of the landing point. -In these cases, you may wish to lower the :ref:`RNGFND1_MAX` altitude to a value closer to the expected vehicle altitude at the end of the runway so that it engages and is used shortly before the flare. +For approaches with irregularities or obstacles along the glide path, or in cases where the terrain height differs from the landing point, :ref:`RNGFND_LND_DIST` can be used so the rangefinder begins influencing the glide slope only when the aircraft is over terrain that should actually be used for landing corrections. It is especially useful for landings on platforms, small plateaus, or other scenarios where the relevant landing surface is reached only near the touchdown point. A value of zero disables this feature. + +Very small values for :ref:`RNGFND_LND_DIST` (near or below the average flare start distance) are generally not recommended unless the landing scenario requires them. They can force a large glide slope correction near the ground, may increase auto-abort frequency if :ref:`LAND_ABORT_DEG` is used, and provide no benefit if the rangefinder is engaged after the flare has already started. Alternatively, if the terrain is sloping toward or away from the runway, you may use the :ref:`TERRAIN_FOLLOW` parameter (bit 1(all modes) or 3(AUTO mode only) set to provide additional corrections to the rangefinder readings for the difference between landing point terrain altitude and current approach altitude above terrain to help prevent landing overshoots and undershoots. Be sure to carefully test for your particular situation and be ready to abort the autolanding if it does not look safe. \ No newline at end of file