Safety Configuration (Failsafes)

PX4 has a number of safety features to protect and recover your vehicle if something goes wrong:

  • Failsafes allow you to specify areas and conditions under which you can safely fly, and the action that will be performed if a failsafe is triggered (for example, landing, holding position, or returning to a specified point). The most important failsafe settings are configured in the QGroundControl Safety Setup page. Others must be configured via parameters.
  • Safety switches on the remote control can be used to immediately stop motors or return the vehicle in the event of a problem.

Failsafe Actions

Each failsafe defines its own set of actions. Some of the more common failsafe actions are:

Action Description
None/Disabled No action (the failsafe will be ignored).
Warning A warning message will be sent to QGroundControl.
Hold mode The vehicle will enter Hold mode. For multicopters this means the vehicle will hover, while for fixed/wing the vehicle will circle.
Return mode The vehicle will enter Return mode. Return behaviour can be set in the Return Home Settings (below).
Land mode The vehicle will enter Land mode, and lands immediately.
Flight termination Turns off all controllers and sets all PWM outputs to their failsafe values (e.g. PWM_MAIN_FAILn, PWM_AUX_FAILn). The failsafe outputs can be used to deploy a parachute, landing gear or perform another operation. For a fixed-wing vehicle this might allow you to glide the vehicle to safety.
Lockdown Kills the motors (sets them to disarmed). This is the same as using the kill switch.

It is possible to recover from a failsafe action (if the cause is fixed) by switching modes. For example, in the case where RC Loss failsafe causes the vehicle to enter Return mode, if RC is recovered you can change to Position mode and continue flying.

If a failsafe occurs while the vehicle is responding to another failsafe (e.g. Low battery while in Return mode due to RC Loss), the specified failsafe action for the second trigger is ignored. Instead the action is determined by separate system level and vehicle specific code. This might result in the vehicle being changed to a manual mode so the user can directly manage recovery.

QGroundControl Safety Setup

The QGroundControl Safety Setup page is accessed by clicking the QGroundControl Gear icon (Vehicle Setup - top toolbar) and then Safety in the sidebar). This includes the most important failsafe settings (battery, RC loss etc.) and the settings for the return actions Return and Land.

Safety Setup (QGC)

Low Battery Failsafe

The low battery failsafe is triggered when the battery capacity drops below one (or more warning) level values.

Safety - Battery (QGC)

The most common configuration is to set the values and action as above (with Warn > Failsafe > Emergency). With this configuration the failsafe will trigger warning, then return, and finally landing if capacity drops below the respective levels.

It is also possible to set the Failsafe Action to warn, return, or land when the Battery Failsafe Level failsafe level is reached.

The settings and underlying parameters are shown below.

Setting Parameter Description
Failsafe Action COM_LOW_BAT_ACT Warn, Return, or Land based when capacity drops below Battery Failsafe Level, OR Warn, then return, then land based on each of the level settings below.
Battery Warn Level BAT_LOW_THR Percentage capacity for warnings (or other actions).
Battery Failsafe Level BAT_CRIT_THR Percentage capacity for Return action (or other actions if a single action selected).
Battery Emergency Level BAT_EMERGEN_THR Percentage capacity for triggering Land (immediately) action.

RC Loss Failsafe

The RC Loss failsafe is triggered if the RC transmitter link is lost.

Safety - RC Loss (QGC)

PX4 and the receiver may also need to be configured in order to detect RC loss: Radio Setup > RC Loss Detection.

The settings and underlying parameters are shown below.

Setting Parameter Description
RC Loss Timeout COM_RC_LOSS_T Amount of time after losing the RC connection before the failsafe will trigger.
Failsafe Action NAV_RCL_ACT Disabled, Loiter, Return, Land, Terminate, Lockdown.

The Data Link Loss failsafe is triggered if a telemetry link (connection to ground station) is lost when flying a mission.

Safety - Data Link Loss (QGC)

The settings and underlying parameters are shown below.

Setting Parameter Description
Data Link Loss Timeout COM_DL_LOSS_T Amount of time after losing the data connection before the failsafe will trigger.
Failsafe Action NAV_DLL_ACT Disabled, Hold mode, Return mode, Land mode, Terminate, Lockdown.

Geofence Failsafe

The Geofence Failsafe is a "virtual" cylinder centered around the home position. If the vehicle moves outside the radius or above the altitude the specified Failsafe Action will trigger.

Safety - Geofence (QGC)

PX4 separately supports more complicated GeoFence geometries with multiple arbitrary polygonal and circular inclusion and exclusion areas: Flying > GeoFence.

The settings and underlying geofence parameters are shown below.

Setting Parameter Description
Action on breach GF_ACTION None, Warning, Hold mode, Return mode, Terminate.
Max Radius GF_MAX_HOR_DIST Horizontal radius of geofence cylinder. Geofence disabled if 0.
Max Altitude GF_MAX_VER_DIST Height of geofence cylinder. Geofence disabled if 0.

Setting GF_ACTION to terminate will kill the vehicle on violation of the fence. Due to the inherent danger of this, this function is disabled using CBRK_FLIGHTTERM, which needs to be reset to 0 to really shut down the system.

The following settings also apply, but are not displayed in the QGC UI.

Setting Parameter Description
Geofence altitude mode GF_ALTMODE Altitude reference used: 0 = WGS84, 1 = AMSL.
Geofence counter limit GF_COUNT Set how many subsequent position measurements outside of the fence are needed before geofence violation is triggered.
Geofence source GF_SOURCE Set whether position source is estimated global position or direct from the GPS device.
Circuit breaker for flight termination CBRK_FLIGHTTERM Enables/Disables flight termination action (disabled by default).

Return Mode Settings

Return is a common failsafe action that engages Return mode to return the vehicle to the home position. This section shows how to set the land/loiter behaviour after returning.

Safety - Return Home Settings (QGC)

The settings and underlying parameters are shown below:

Setting Parameter Description
Climb to altitude RTL_RETURN_ALT Vehicle ascend to this minimum height (if below it) for the return flight.
Return behaviour Choice list of Return then: Land, Loiter and do not land, or Loiter and land after a specified time.
Loiter Altitude RTL_DESCEND_ALT If return with loiter is selected you can also specify the altitude at which the vehicle hold.
Loiter Time RTL_LAND_DELAY If return with loiter then land is selected you can also specify how long the vehicle will hold.

The return behavour is defined by RTL_LAND_DELAY. If negative the vehicle will land immediately. Additional information can be found in Return mode.

Land Mode Settings

Land at the current position is a common failsafe action that engages Land Mode. This section shows how to control when and if the vehicle automatically disarms after landing. For Multicopters (only) you can additionally set the descent rate.

Safety - Land Mode Settings (QGC)

The settings and underlying parameters are shown below:

Setting Parameter Description
Disarm After COM_DISARM_LAND Select checkbox to specify that the vehicle will disarm after landing. The value must be non-zero but can be a fraction of a second.
Landing Descent Rate MPC_LAND_SPEED Rate of descent (MC only).

Other Failsafe Settings

This section contains information about failsafe settings that cannot be configured through the QGroundControl Safety Setup page.

Position (GPS) Loss Failsafe

The Position Loss Failsafe is triggered if the quality of the PX4 position estimate falls below acceptable levels (this might be caused by GPS loss) while in a mode that requires an acceptable position estimate.

The failure action is controlled by COM_POSCTL_NAVL, based on whether RC control is assumed to be available (and altitude information):

  • 0: Remote control available. Switch to Altitude mode if a height estimate is available, otherwise Stabilized mode.
  • 1: Remote control not available. Switch to Land mode if a height estimate is available, otherwise enter flight termination.

Fixed Wing vehicles additionally have a parameter (NAV_GPSF_LT) for defining how long they will loiter (circle) after losing position before attempting to land.

The relevant parameters for all vehicles shown below (also see GPS Failure navigation parameters):

Parameter Description
COM_POS_FS_DELAY Delay after loss of position before the failsafe is triggered.
COM_POSCTL_NAVL Position control navigation loss response during mission. Values: 0 - assume use of RC, 1 - Assume no RC.
CBRK_VELPOSERR Circuit breaker for position error check (disables error checks in all modes).

Parameters that only affect Fixed Wing vehicles:

Parameter Description
NAV_GPSF_LT Loiter time (waiting for GPS recovery before it goes into flight termination). Set to 0 to disable.
NAV_GPSF_P Fixed pitch angle while circling.
NAV_GPSF_R Fixed roll/bank angle while circling.
NAV_GPSF_TR Thrust while circling.

Offboard Loss Failsafe

The Offboard Loss Failsafe is triggered if the offboard link is lost while under Offboard control. Different failsafe behaviour can be specified based on whether or not there is also an RC connection available.

The relevant parameters are shown below:

Parameter Description
COM_OF_LOSS_T Delay after loss of offboard connection before the failsafe is triggered.
COM_OBL_ACT Failsafe action if no RC is available: Land mode, Hold mode, Return mode.
COM_OBL_RC_ACT Failsafe action if RC is available: Position mode, Altitude mode, Manual mode, Return mode, Land mode, Hold mode.

Mission Failsafe

The Mission Failsafe checks prevent a previous mission being started at a new takeoff location or if it is too big (distance between waypoints is too great). The failsafe action is that the mission will not be run.

The relevant parameters are shown below:

Parameter Description
MIS_DIST_1WP The mission will not be started if the current waypoint is more distant than this value from the home position. Disabled if value is 0 or less.
MIS_DIST_WPS The mission will not be started if any distance between two subsequent waypoints is greater than this value.

Traffic Avoidance Failsafe

The Traffic Avoidance Failsafe allows PX4 to respond to transponder data (e.g. from ADSB transponders) during missions.

The relevant parameters are shown below:

Parameter Description
NAV_TRAFF_AVOID Set the failsafe action: Disabled, Warn, Return mode, Land mode.

Adaptive QuadChute Failsafe

Failsafe for when a pusher motor fails (or airspeed sensor) and a VTOL vehicle can no longer achieve a desired altitude setpoint in fixed-wing mode. If triggered, the vehicle will transition to multicopter mode and enter failsafe Return mode.

The relevant parameters are shown below:

Parameter Description
VT_FW_ALT_ERR Maximum negative altitude error for fixed wing flight. If the altitude drops more than this value below the altitude setpoint the vehicle will transition back to MC mode and enter failsafe RTL.

Failure Detector

The failure detector allows a vehicle to take protective action(s) if it unexpectedly flips, or if it is notified by an external failure detection system.

During flight, the failure detector can be used to trigger flight termination if failure conditions are met, which may then launch a parachute or perform some other action.

Failure detection during flight is deactivated by default (enable by setting the parameter: CBRK_FLIGHTTERM=0).

During takeoff the failure detector attitude trigger invokes the lockdown action if the vehicle flips (lockdown kills the motors but, unlike flight termination, will not launch a parachute or perform other failure actions). Note that this check is always enabled on takeoff, irrespective of the CBRK_FLIGHTTERM parameter.

The failure detector is active in all vehicle types and modes, except for those where the vehicle is expected to do flips (i.e. Acro mode (MC), Acro mode (FW), Rattitude and Manual (FW)).

Attitude Trigger

The failure detector can be configured to trigger if the vehicle attitude exceeds predefined pitch and roll values for longer than a specified time.

The relevant parameters are shown below:

Parameter Description
CBRK_FLIGHTTERM Flight termination circuit breaker. Unset from 121212 (default) to enable flight termination due to FailureDetector or FMU loss.
FD_FAIL_P Maximum allowed pitch (in degrees).
FD_FAIL_R Maximum allowed roll (in degrees).
FD_FAIL_P_TTRI Time to exceed FD_FAIL_P for failure detection (default 0.3s).
FD_FAIL_R_TTRI Time to exceed FD_FAIL_R for failure detection (default 0.3s).

External Automatic Trigger System (ATS)

The failure detector, if enabled, can also be triggered by an external ATS system. The external trigger system must be connected to flight controller port AUX5 (or MAIN5 on boards that do not have AUX ports), and is configured using the parameters below.

External ATS is required by ASTM F3322-18. One example of an ATS device is the FruityChutes Sentinel Automatic Trigger System.

Parameter Description
FD_EXT_ATS_EN Enable PWM input on AUX5 or MAIN5 (depending on board) for engaging failsafe from an external automatic trigger system (ATS). Default: Disabled.
FD_EXT_ATS_TRIG The PWM threshold from external automatic trigger system for engaging failsafe. Default: 1900 ms.

Emergency Switches

Remote control switches can be configured (as part of QGroundControl Flight Mode Setup) to allow you to take rapid corrective action in the event of a problem or emergency; for example, to stop all motors, or activate Return mode.

This section lists the available emergency switches.

Kill Switch

A kill switch immediately stops all motor outputs (and if flying, the vehicle will start to fall)! The motors will restart if the switch is reverted within 5 seconds. After 5 seconds the vehicle will automatically disarm; you will need to arm it again in order to start the motors.

Arm/Disarm Switch

The arm/disarm switch is a direct replacement for the default stick-based arming/disarming mechanism (and serves the same purpose: making sure there is an intentional step involved before the motors start/stop). It might be used in preference to the default mechanism because:

  • Of a preference of a switch over a stick motion.
  • It avoids accidentally triggering arming/disarming in-air with a certain stick motion.
  • There is no delay (it reacts immediately).

The arm/disarm switch immediately disarms (stop) motors for those flight modes that support disarming in flight. This includes:

  • Manual mode
  • Acro mode
  • Stabilized
  • Rattitude

For modes that do not support disarming in flight, the switch is ignored during flight, but may be used after landing is detected. This includes Position mode and autonomous modes (e.g. Mission, Land etc.).

Auto disarm timeouts (e.g. via COM_DISARM_LAND) are independent of the arm/disarm switch - ie even if the switch is armed the timeouts will still work.

Return Switch

A return switch can be used to immediately engage Return mode.

Other Safety Settings

Auto-disarming Timeouts

You can set timeouts to automatically disarm a vehicle if it is too slow to takeoff, and/or after landing (disarming the vehicle removes power to the motors, so the propellers won't spin).

The relevant parameters are shown below:

Parameter Description
COM_DISARM_LAND Timeout for auto-disarm after landing.
COM_DISARM_PRFLT Timeout for auto disarm if vehicle is too slow to takeoff.

Further Information

© PX4 Dev Team. License: CC BY 4.0            Updated: 2024-02-13 22:49:01

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