Return Mode (VTOL)
The Return flight mode is used to fly a vehicle to safety on an unobstructed path to a safe destination, where it may either wait (hover or circle) or land.
VTOL vehicles use the Mission Landing/Rally Point return type by default. In this return type a vehicle ascends to a minimum safe altitude above obstructions (if needed), and then flies directly to a rally point or the start of a mission landing point (whichever is nearest), or the home position if neither rally points or mission landing pattern is defined. If the destination is a mission landing pattern, the vehicle will then follow the pattern to land. If the destination is a rally point or the home location, the vehicle will fly to that destination and land.
The vehicle will return using the flying mode (MC or FW) it was using at the point when return mode was triggered. Generally it will follow the same return mode behaviour of the corresponding vehicle type, but will always transition to MC mode (if needed) before landing.
VTOL supports the other PX4 return types, including home/rally point return, mission path and closest safe destination. The default type is recommended.
INFO
- Mode is automatic - no user intervention is required to control the vehicle.
- Mode requires a global 3d position estimate (from GPS or inferred from a local position).
- Flying vehicles can't switch to this mode without global position.
- Flying vehicles will failsafe if they lose the position estimate.
- Mode requires home position is set.
- Mode prevents arming (vehicle must be armed when switching to this mode).
- RC control switches can be used to change flight modes on any vehicle.
- RC stick movement is ignored.
Technical Summary
VTOL vehicles use the Mission Landing/Rally Point return type by default, and return using the flying mode (MC or FW) it was using at the point when return mode was triggered.
Fixed-wing Mode (FW) Return
If returning as a fixed-wing, the vehicle:
Ascends to a safe minimum return altitude defined by RTL_RETURN_ALT (safely above any expected obstacles). The vehicle maintains its initial altitude if that is higher than the minimum return altitude.
Flies via direct constant-altitude path to the destination, which will be the closest of the start of a mission landing pattern and any rally point, or the home location if no mission landing pattern or rally points are defined.
If the destination is a mission landing pattern it will follow the pattern to land.
A mission landing pattern for a VTOL vehicle consists of a MAV_CMD_DO_LAND_START, one or more position waypoints, and a MAV_CMD_NAV_VTOL_LAND.
If the destination is a rally point or home it will:
Fly to the selected VTOL approach loiter associated with that landing location (if any are defined) and use it to descend to the approach altitude. If several approach loiters are defined for that location, PX4 chooses the one that best matches the estimated wind at the landing point.
See VTOL Rally Point Approach Loiter below for information on how to define approach loiters (using MAV_CMD_NAV_LOITER_TO_ALT items in your rally plan).
Loiter/spiral down to the approach altitude, or to RTL_DESCEND_ALT above the destination if no approach altitude is defined.
Circle for a short time, as defined by RTL_LAND_DELAY.
Fly from the approach loiter to the return destination (the rally point or home location).
Transition to MC mode at the destination and land.
Note that NAV_FORCE_VT is ignored: the vehicle will always land as a multicopter for these destinations.
VTOL Rally Point Approach Loiters
VTOL rally point approach loiters are MAV_CMD_NAV_LOITER_TO_ALT items associated with a particular rally point (MAV_CMD_NAV_RALLY_POINT) in a Rally plan. They define options for where the VTOL can descend to the rally point approach altitude, and how it will approach the rally point. Several approach loiters can be defined for a rally point, and PX4 will choose the one that best matches the estimated wind at the landing point.
TIP
The MAV_CMD_NAV_LOITER_TO_ALT items that define approach loiters are associated with a Rally/safety point, and are hence part of the rally point plan when used in this way — not the mission plan. This behaviour is not defined in the MAVLink rally point plan specification.
When uploading VTOL approach loiters through MAVLink, upload them as rally/safe-point mission items (MAV_MISSION_TYPE_RALLY). The MAV_CMD_NAV_RALLY_POINT item must come first, followed by one or more MAV_CMD_NAV_LOITER_TO_ALT items that define the approach loiters for that rally point. The next MAV_CMD_NAV_RALLY_POINT starts a new landing-location block.
For each MAV_CMD_NAV_LOITER_TO_ALT item, x/y/z define the loiter center and approach altitude, and param2 defines the loiter radius used by RTL. If param2 is unset or zero, PX4 falls back to RTL_LOITER_RAD.
For example, a rally upload with one rally point and two possible approach loiters would use:
| Sequence | Command | Purpose | Key fields |
|---|---|---|---|
| 0 | MAV_CMD_NAV_RALLY_POINT | Landing location | x/y/z: rally latitude, longitude, altitude |
| 1 | MAV_CMD_NAV_LOITER_TO_ALT | First VTOL approach loiter | x/y/z: loiter latitude, longitude, altitude; param2: loiter radius |
| 2 | MAV_CMD_NAV_LOITER_TO_ALT | Second VTOL approach loiter | x/y/z: loiter latitude, longitude, altitude; param2: loiter radius |
Note that the approach loiter is not the back-transition point. If the selected approach loiter is far from the rally point or home location, the vehicle remains in fixed-wing mode after the loiter, flies to the destination at the approach altitude, and only then back-transitions for landing.
Multicopter Mode (MC) Return
If returning as a multicopter:
- The behaviour is the same except that the vehicle flies as a multicopter and respects multicopter settings.
- In particular, if landing on rally point or the home position the vehicle uses the RTL_CONE_ANG instead of just the RTL_RETURN_ALT for defining the minimum safe return altitude. For more information see the explanation of the "cone" in Return mode (Generic Vehicle) > Minimum Return Altitude.
Parameters
The RTL parameters are listed in Parameter Reference > Return Mode. If using a mission landing, only the RTL_RETURN_ALT and RTL_DESCEND_ALT are relevant. The others are relevant if the destination is a rally point or the home location.
| Parameter | Description |
|---|---|
| RTL_TYPE | Return type. |
| RTL_APPR_FORCE | [VTOL FW only] If set, PX4 only considers home or rally-point RTL destinations when a valid VTOL approach loiter is defined for that landing location. Mission landing patterns are unaffected. |
| RTL_RETURN_ALT | Return altitude in meters (default: 60m)If already above this value the vehicle will return at its current altitude. |
| RTL_CONE_ANG | Half-angle of the cone that defines the vehicle RTL return altitude. Values (in degrees): 0, 25, 45, 65, 80, 90. Note that 0 is "no cone" (always return at RTL_RETURN_ALT or higher), while 90 indicates that the vehicle must return at the current altitude or RTL_DESCEND_ALT (whichever is higher). |
| RTL_DESCEND_ALT | Minimum return altitude and altitude at which the vehicle will slow or stop its initial descent from a higher return altitude (default: 30m) |
| RTL_LAND_DELAY | Time to hover at RTL_DESCEND_ALT before landing (default: 0.5s) -by default this period is short so that the vehicle will simply slow and then land immediately. If set to -1 the system will loiter at RTL_DESCEND_ALT rather than landing. The delay is provided to allow you to configure time for landing gear to be deployed (triggered automatically). |
| RTL_LOITER_RAD | [Fixed-wing Only] The radius of the loiter circle (at RTL_LAND_DELAY. |
| MIS_TKO_LAND_REQ | Specify whether a mission landing or takeoff pattern is required. Generally fixed-wing vehicles set this to require a landing pattern but VTOL do not. |