Arming Plane

Arming your plane

Before you can fly your plane you need to arm it. Arming the aircraft before flight has two purposes:

  • prevent the motor from turning when the pilot is not ready to fly (a safety feature)
  • prevent takeoff before the autopilot is fully configured and ready to fly

In past releases of APM:Plane arming was optional, and the requirement to arm (controlled by the ARMING_REQUIRED parameter) was disabled by default. This was changed for the 3.3.0 release to require arming by default.

The key thing that arming does is to enable the motor. You will not be able to start the motor (ie. control the throttle) until the aircraft is armed.

Note: If you have AHRS_EKF_USE enabled (you are using the EKF) then it is particularly important that you have arming enabled with arming checks enabled. Flying EKF without arming checks may cause a crash.

Configuring Arming

There are three parameters which control how arming works:

  • ARMING_REQUIRE: this controls whether an arming step is required. The default is 1, meaning that arming is required before takeoff. If set to 0 then arming is not required (the plane starts off armed).
  • ARMING_CHECK: this controls what checks the autopilot does before arming is allowed. The default is 1, meaning all checks are done. Most users should leave it at 1, as the arming checks are important to ensure the autopilot is ready.
  • ARMING_RUDDER: This parameter allows you to configure rudder based arming/disarming. The default is 1, meaning you are able to arm with right rudder. If you set this to 2 you can also disarm with left rudder. If you set this to 0 then you will only be able to arm/disarm via a ground station.

How to Arm

When you are ready to fly you can ask Plane to arm. This can be done in two ways:

  • Rudder Arming. Hold the rudder stick fully to the right and the throttle stick fully down for 2 seconds.
  • GCS Arming. Press the arming button on your ground station

How to Disarm

Since APM:Plane 3.4.0 it is possible to disarm using the transmitter. This is done holding throttle at minimum and rudder to the left for 2 seconds. In ArduPlane this condition could be accidentally triggered by pilots while flying so there are additional requirements prior to disarm:

  • You need to allow rudder disarming by changing ARMING_RUDDER parameter to 2 (ArmOrDisarm).
  • The flight controller needs to make sure that you are not actually flying. There is an algorithm for this that uses the airspeed sensor readings. So you need this source available and giving values lower enough (in a windy day you might not be able to disarm even landed if the plane thinks you are still flying)

You can also disarm without using the transmitter with one of the following methods:

  • use a ground station to issue a disarm command
  • use the safety switch on your aircraft (on Pixhawk)
  • after an auto-landing the plane will automatically disarm after 20 seconds if still on the ground (controlled by LAND_DISARMDELAY parameter)

Visual and Audible signals

APM:Plane will provide visual and audio clues to the arming state if your autopilot has LEDs and a buzzer. The clues are:

  • if the autopilot is disarmed, but is ready to arm then the large 3-colour LED will be flashing green
  • if the autopilot is armed and ready to fly the large 3-colour LED is solid green
  • when the autopilot is ready to arm it will play a “ready to arm” sound on the buzzer
  • when the autopilot is armed or disarmed it will play the corresponding sound

See the sounds page to listen to what the buzzer sounds like for each state.

Arming Checks

Before allowing arming the autopilot checks a set of conditions. All conditions must pass for arming to be allowed. If any condition fails then a message explaining what failed is set to the GCS.

The checks performed are:

  • Safety switch. The PX4 safety switch must be set to the safety-off state before arming is allowed. This is either done by pressing the safety switch for 2 seconds until it stops flashing, or you can disable the use of the safety switch by setting BRD_SAFETY_ENABLE=0
  • Barometer check. The barometer must be healthy (getting good data)
  • Inertial Sensor Checks. The accelerometers and gyroscopes must all be healthy and all be calibrated. If you have more than one accel or gyro then they need to be consistent with each other.
  • AHRS checks. The AHRS (attitude heading reference system) needs to be initialized and ready. Note that if you have the EKF enabled this may take up to 30 seconds after boot.
  • Compass checks. All compasses must be configured and calibrated, and need to be consistent with each other (if you have more than one compass)
  • GPS Checks. You need to have a 3D GPS fix.
  • Battery checks. The battery voltage must be above the failsafe voltage (if configured)
  • Airspeed checks. If you have configured an airspeed sensor then the sensor needs to be working.
  • Logging checks. The logging subsystem needs to be working (ie. a microSD must be fitted and working on PX4)
  • RC Control checks. You need to not be in RC failsafe

Throttle output when disarmed

When the plane is disarmed the throttle channel will not respond to pilot input. There are two possible behaviours you can configure:

  • ARMING_REQUIRE=1. When disarmed the minimum value for the throttle channel (normally RC3_MIN) will be sent to the throttle channel
  • ARMING_REQUIRE=2. When disarmed no pulses are sent to the throttle channel. Note that some ESCs will beep to complain that they are powered on without a control signal

Diagnosing failure to arm

It can be frustrating if your plane refuses to arm. To diagnose arming issues follow this guide

Check it is ready to arm

If your board has a “ready to arm” LED (the large LED in the middle of the board on a Pixhawk) then that LED should be flashing green when the board is ready to arm. If it is flashing yellow then that indicates that one of the arming checks is not passing.

Try arming

Try sending an arm command with your GCS. If arming is refused then a message will be sent from the autopilot to the GCS indicating why it is refusing to arm.

Rudder arming

If you are using right-rudder + zero-throttle to arm and you don’t get a message on your GCS giving a arming failure reason then it may be that your RC calibration is a bit off and the autopilot is not quite seeing zero throttle or isn’t quite seeing full right rudder.

Reasons for refusing to arm

When the autopilot refuses to arm it sends a STATUSTEXT MAVLink message to the GCS explaining why it is refusing. The possible reasons why the autopilot can refuse to arm are:

  • barometer not healthy. This is very rare. If it happens repeatedly then you may have a barometer hardware fault.
  • airspeed not healthy. If you have a airspeed sensor fitted and the autopilot is not getting an airspeed reading it will refuse to arm.
  • logging not available. If your microSD card has failed or is corrupt then logging won’t be available and you cannot arm.
  • gyros not healthy. If the gyros have failed the autopilot will refuse to arm. This is rare, and if it happens repeatedly then you may have a hardware failure.
  • gyros not calibrated. This happens when the automatic gyro calibration at startup didn’t converge. Try rebooting the autopilot with the plane held still.
  • accels not healthy. If the accelerometers have failed the autopilot will refuse to arm. Try recalibrating your accelerometers.
  • GPS accuracy errors. There are 4 types of GPS arming errors that can be reported. They are “GPS vert vel error”, “GPS speed error”, “GPS horiz error”, “GPS numsats”. Try moving your plane for better GPS reception or switching off any RF sources (such as a FPV transmitter) that may be interfering with your GPS.
  • Mag yaw error. This happens when your compass is badly out of alignment. Check your compass orientation and re-do your compass calibration or move your plane further away from any magnetic materials.
  • EKF warmup. This happens when the EKF is still warming up. Wait another 10 seconds and try again.
  • AHRS not healthy. This means the EKF is not healthy. If the error persists then try rebooting your board.
  • 3D accel cal needed. This happens when you have not done a 3D accelerometer calibration.
  • Inconsistent accelerometers. This happens when you have multiple IMUs (such as the Pixhawk which has two) and they are not consistent. This can be caused by temperature changes. If the error doesn’t clear itself after a minute you will need to redo your accelerometer calibration.
  • Inconsistent gyros. This happens when you have multiple gyros and they are not reporting consistent values. If the error does not clear itself after 30 seconds then you will need to reboot.
  • Limit errors. The arming checks some of your parameter settings to make sure they are in a reasonable range. The checks are “LIM_ROLL_CD too small”, “LIM_PITCH_MAX too small”, “LIM_PITCH_MIN too large”, “invalid THR_FS_VALUE”.
  • GPS n has not been fully configured. This happens with a uBlox GPS where the GPS driver is unable to fully configure the GPS for the settings that are being requested. This can be caused by a bad wire between the autopilot and GPS, or by a bad response from the GPS. If the message is about “GPS 0” then it is the first GPS. If it is “GPS 1” then it is the 2nd GPS. If you get a failure for the 2nd GPS and don’t have two GPS modules installed then set GPS_TYPE2 to zero to disable the 2nd GPS