Failsafe Setup (Safety)

ArduPilot handles low voltage, EKF failure, RC loss and ground station loss.

Battery failsafe

Normal failsafe

• Low action (BATTx_FS_LOW_ACT)

  • Triggers when total pack voltage drops below the low voltage or low capacity (mAh) threshold.
  • Recommended: RTL (Return to Launch).
  • This is the first warning stage. It sends the aircraft home under its own control, before an inattentive pilot lets the voltage keep falling.

• Low voltage threshold (BATTx_LOW_VOLT)

  • The voltage at which the low action fires. It is the whole-pack voltage, not a single-cell voltage.
  • Automatic: on the initial setup screen, enter the resting voltage of one cell and the system works out the pack threshold and overwrites the default.
  • Manual: set a value that suits your battery chemistry (LiPo, for example) and its specs.

• Critical voltage threshold (BATTx_CRT_VOLT)

  • The voltage at which the critical landing action fires. Again this is the pack voltage, not a cell voltage.
  • Important: this must sit below the low voltage threshold, and it must respect the minimum discharge voltage the battery maker specifies. Do not set it too low.
  • Automatic: on the initial setup screen, enter the resting voltage of one cell and the system works out the pack threshold and overwrites the default.
  • Manual: set a value that suits your battery chemistry and its specs.

• Critical action (BATTx_CRT_ACT)

  • Triggers when total pack voltage drops below the critical threshold.
  • Recommended: LAND.
  • This is the second, forced stage. The pack is close to its minimum safe cell voltage, so the aircraft has to land now — otherwise the battery can swell, be ruined, or catch fire.

Battery capacity failsafe (not recommended)

Smart batteries, or power modules accurate enough to match them, can also run the failsafe from remaining capacity.

• First set the pack capacity, BATTx_CAPACITY (this can be changed on the power module configuration screen).
• Low mAh threshold (BATTx_LOW_MAH)
  • Fires the low action (BATT_FS_LOW_ACT) when remaining capacity falls below this value.
• Critical mAh threshold (BATTx_LOW_MAH)
  • Fires the critical action (BATT_CRT_ACT) when remaining capacity falls below this value.

GCS failsafe

Fires when the telemetry link drops and the ground station loses contact with the aircraft.

• GCS failsafe (FS_GCS_ENABLE)
  • Options: disabled, RTL, RTL or continue the mission (dropped after 4.0 — see the failsafe options), smart RTL or RTL, smart RTL or land, land, auto land or RTL, Brake/land.
• Timeout (FS_GCS_TIMEOUT)
  • How long the link may stay down before the failsafe fires.
• Our advice: if you do not need live ground station control, and you have a transmitter, leave it off — or use the failsafe options. Only a link that was established and then dropped can trigger it; if you never connected a ground station, it never fires.

RC failsafe

RC failsafe works from the throttle PWM value or from the SBUS failsafe flag. For throttle failsafe you must set failsafe in the transmitter, so that when the link drops the receiver outputs a throttle PWM well below anything you use in normal flight.

• Throttle failsafe (FS_THR_ENABLE)

  • Options: disabled, always RTL, continue the mission in Auto, land.

• PWM threshold (FS_THR_VALUE)

  • Set it above the value the receiver outputs on loss, but clearly below what you use in normal control.

TIP

An SBUS receiver sends a failsafe flag on loss, so RC failsafe fires even without a PWM threshold. We recommend RTL, or the failsafe options with "continue the mission in Auto".

Failsafe options

ArduPilot 4.0 and later add an option mask for battery, RC and GCS failsafe, to cover special situations.

• Failsafe option mask
  • Options: continue the mission if in Auto when RC failsafe fires; continue if in Auto when GCS failsafe fires; continue if in Guided when RC failsafe fires; continue if already landing when any failsafe fires; continue if under pilot control when GCS failsafe fires.
  • Several options can be combined. They only add exceptions to the failsafe actions — set them to suit how you fly.