Configuring Loss of Control Protection Parameters
1. Principle
The Failsafe function is a critical safeguard for UAV flight safety. When the UAV loses RC signal during flight due to signal interference or RC transmitter failure, the flight controller will automatically execute corresponding protection actions (such as Return to Home or Landing) based on pre-set failsafe parameters, avoiding UAV loss of control or accidents.
Failsafe on ArduPilot flight controllers is usually divided into two stages:
Short-term Protection (FS_SHORT): RC signal is lost for a short time; the flight controller executes preset actions to wait for signal recovery.
Long-term Protection (FS_LONG): RC signal is continuously lost; the flight controller automatically executes preset safety protection actions (such as Return to Home, Landing, etc.).
2. Equipment Preparation
1) Flight Controller: Any of the SpeedyBee F405 WING APP / SpeedyBee F405 WING MINI / CoreWing F405 WING V2 flight controllers applies to this tutorial.
2) Assembled Aircraft: Includes RC transmitter, receiver, GPS, compass, VTX (video transmitter), and FPV equipment (goggles/camera).
3) Ground Control Station: Mission Planner GCS.
3. Specific Operations
3.1 Failsafe Parameter Configuration
Step 1: Enable Failsafe Parameters
Modify the THR_FAILSAFE parameter to 1 to enable the failsafe function.
Enter the Configuration/Tuning page;
Enter the full parameter list;
Search for THR_FAILSAFE;
Set the value of THR_FAILSAFE to 1 to enable the failsafe function;
Click Write Parameters.
Step 2: Determine Failsafe Trigger Throttle Value
First, you need to set the throttle value that triggers failsafe; this value must be lower than the RC transmitter's minimum throttle output value:
Enter the Configuration/Tuning page;
Enter the full parameter list;
Search for THR_FS_VALUE;
Set the value of THR_FS_VALUE to
950;Click Write Parameters.
Important
Note: Set this value lower than the RC transmitter's minimum throttle output (e.g., if the RC minimum throttle is 1000, it is recommended to set it to 950).
3.2 Short-term Failsafe Configuration (FS_SHORT)
Short-term failsafe executes when the RC signal is briefly lost during flight; the purpose is to provide a brief opportunity for the RC signal to recover.
| Parameter Setting | Description |
|---|---|
| FS_SHORT_TIMEOUT = 1.5 | When the RC signal is lost for more than 1.5 seconds, short-term protection mode is activated. |
| FS_SHORT_ACTN = 1 | Enter Circle mode (hover at the current location) to wait for RC signal recovery. |
Enter the Configuration/Tuning page;
Enter the full parameter list;
Search for FS_SHORT;
Modify the above parameters;
Click Write Parameters.
Important
✅ Recommended short-term failsafe timeout (FS_SHORT_TIMEOUT) is 1~2 seconds to avoid frequent Return to Home caused by brief interference.
3.3 Long-term Failsafe Configuration (FS_LONG)
Important
Return to Home mode requires GPS; please confirm whether the aircraft is equipped with a GPS module!!
| Parameter Setting | Description |
|---|---|
| FS_LONG_TIMEOUT = 5 | After short-term protection mode is activated, if the RC signal is not recovered within 5 seconds, long-term protection actions will be initiated. |
| FS_LONG_ACTN = 1 | Automatically activate Return to Home (RTL) mode; the aircraft will hover or land above the HOME point. |
Enter the Configuration/Tuning page;
Enter the full parameter list;
Search for FS_LONG;
Modify the above parameters;
Click Write Parameters.
Important
✅ RC signal recovery during long-term protection: By default, the aircraft will not immediately return to the original mode; the RC transmitter must be used to manually switch flight modes. Once switched, normal RC control is restored.
✅ Note: If other failsafes occur simultaneously (such as low battery protection), the flight controller will prioritize executing the higher-priority protection action (e.g., low battery protection).
✅ Recommended long-term failsafe timeout (FS_LONG_TIMEOUT) is 3~5 seconds to ensure the UAV can execute safety actions in a timely manner.
✅ Long-term protection action is recommended to always be set to Return to Home (RTL) to ensure UAV safety.
3.4 Verify Failsafe Function
✅ Ground Verification:
Connect the RC transmitter and ground control station to ensure parameters are written correctly.
Monitor the RC signal status in real-time via the ground station or OSD screen.
Manually turn off the RC transmitter or cut the RC signal to observe if the flight controller automatically enters short-term protection.
Wait for longer than the short-term protection time to check if it automatically enters long-term protection (RTL mode).
✅ Actual Flight Verification:
Perform actual flight testing in a safe area and turn off the RC transmitter during flight.
Observe whether the UAV automatically activates RTL and safely returns to hover or land above the HOME point.
4. Q&A
Q: Why does the flight controller not automatically return to the original mode after the RC signal is recovered?
A: After the flight controller enters long-term protection by default, it will not automatically return to the original mode. You need to manually switch the flight mode once via the RC transmitter to restore normal control.
Q: What happens if other protections (such as low battery) are triggered during long-term protection?
A: The flight controller will execute the protection action with higher priority. For example, if low battery protection is triggered during RC failsafe protection, it will switch from failsafe mode to low battery protection mode.
Q: Why does the failsafe function not work even though the parameters are set?
A: Check if the RC transmitter's minimum throttle is less than THR_FS_VALUE to ensure the flight controller can correctly identify the RC signal loss status. Check if the parameters were successfully written to the flight controller; parameters must be saved and the flight controller restarted after modification.
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