Fixed Wing Landing

PX4 enables autopilot-controlled fixed-wing (FW) landing in Missions, LAND mode, and RETURN TO LAUNCH (RTL) mode.

The landing logic has several phases, as shown below. In the first phase the vehicle will follow a fixed trajectory (FW_LND_ANG) towards the ground. At the flare landing altitude (FW_LND_FLALT) the vehicle will start to follow a flare path, and finally land at the touchdown point.

The flare landing altitude is relative to the altitude that the FW vehicle "thinks" is ground level. In LAND mode the ground altitude is not known and the vehicle will use assume it is at 0m (sea level). Often the ground level will be much higher than sea level, so the vehicle will land in the first phase (it will land on the ground before it reaches the flare altitude).

In a mission, RTL mode, or if the vehicle has a range sensor fitted then ground level can be accurately estimated and landing behaviour will be as shown in the preceding diagram.

Landing is further affected by the following parameters:

Parameter Description
FW_LND_ANG Landing slope angle prior to flaring
FW_LND_HVIRT Virtual horizontal line/altitude used to calculate the flare trajectory.
This represents the sub-ground altitude where the vehicle would reach the desired post-flare touchdown position, if it were to maintain its initial descent trajectory.
FW_LND_FLALT Landing flare altitude (relative to landing altitude)
FW_LND_TLALT Landing throttle limit altitude (relative landing altitude). The default value of -1.0 lets the system default to applying throttle limiting at 2/3 of the flare altitude.
FW_LND_HHDIST Landing heading hold horizontal distance
FW_LND_USETER Use terrain estimate (ground altitude from GPS) during landing. This is turned off by default and a waypoint or RTL altitude is normally used (or sea level for an arbitrary land position).
FW_LND_FL_PMIN Minimum pitch during flare. A positive sign means nose up Applied once FW_LND_TLALT is reached
FW_LND_FL_PMAX Maximum pitch during flare. A positive sign means nose up Applied once FW_LND_TLALT is reached
FW_LND_AIRSPD_SC Min. airspeed scaling factor for landing. Comment: Multiplying this factor with the minimum airspeed of the plane gives the target airspeed the landing approach. FW_AIRSPD_MIN x FW_LND_AIRSPD_SC
© PX4 Dev Team. License: CC BY 4.0           Updated: 2017-07-18 00:54:23

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