Configuring RC Failsafe

RC failsafe is very important part of configuration, don't skip it, you will never know when you will have problems with RC. At least go to the last, “Testing” section and check if failsafe works as needed.

Autopilot must be able to detect RC signal loss to prevent flying away and crash. Some RC receivers with PPM and PWM output has build-in failsafe function, that allows to generate predefined output in case of signal loss. To let autopilot know that actually signal from transmitter is lost, failsafe values should be configured to some special value, that is out of normal operation range.

It's recommended to use value below normal minimum on throttle channel, e.g.:

  • min throttle: 1000
  • max throttle: 2000
  • failsafe threshold (configured on autopilot): 900
  • failsafe value (configured on transmitter): 800

Receivers / Transmitters

RC failsafe must be configured when using receivers that has failsafe function, including:

  • FrSky TFR4 (CPPM output)
  • FrSky D4R-II (CPPM output)
  • Futaba receivers with PWM output + PPM encoder
  • EzUHF
  • TODO: add other receivers

It should NOT be configured when using:

  • Futaba S.Bus receivers (S.Bus already has “signal lost” flag)
  • FrSky X8R, X6R, X4R-SB (S.Bus output)
  • TODO: add other receivers

If you are not sure, test if your autopilot can detect signal loss. See “Testing” section below.

Transmitter configuration

Configuration method depends on transmitter model.

Futaba T8FG

  1. Turn on transmitter
  2. Go to LINKAGE MENU → END POINT
  3. Change values for throttle channel from 3 THR 135 100 100 135 to 3 THR 135 100 135 135, i.e. set value for downmost position to -135%.
  4. Go to LINKAGE MENU → FAIL SAFE
  5. Turn throttle stick to downmost position and update failsafe value for THR channel (to set value press RTN button for 1 sec when failsafe value is highlighted), row should appear like 3 THR F/S OFF -135%
  6. Go to LINKAGE MENU → END POINT again
  7. Return values for throttle channel back to 3 THR 135 100 100 135

Autopilot configuration

To configure failsafe you need to know PWM values for throttle channel, failsafe function must be already configured on transmitter at this point.

  1. In qgroundcontrol go to Config → Radio Calibration section
  2. Turn on transmitter and set min throttle (downmost position), check value for throttle channel under green bar
  3. Turn transmitter off and check this value (failsafe) again, it should be different from min throttle value and out of normal range, i.e. you should not see green bar

RC failsafe configured on autopilot side with following parameters:

  • RC_FAILS_THR: failsafe threshold, set it between min throttle value and failsafe value, e.g. if min throttle value is 1000 and failsafe is 800, set RC_FAILS_THR = 900. Set to 0 to disable failsafe. Warning: RC_FAILS_THR will be silently ignored if it lies within the range [fs_ch_MIN, fs_ch_MAX]

Don't forget to save parameters and check how it works (next section).

Testing

To check that autopilot can detect signal loss do the following steps:

  1. Power on autopilot and keep transmitter switched off
  2. Run qgroundcontrol, connect to autopilot and open Messages window
  3. Switch transmitter on, you should see: “detected RC signal first time”
  4. Switch transmitter off. If you see: “CRITICAL: RC SIGNAL LOST” then failsafe works as needed, if not - then something goes wrong and you MUST configure failsafe before flight
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