Throttle Arming in Plane¶
A software safety feature that requires the throttle to be explicitly armed by a pilot is available for Plane. It was first introduced in version 2.77 and since 3.3.0 it is enabled by default. This is especially applicable to electric planes. The idea is to prevent an unexpected throttle up when someone’s hand is near a propeller.
This feature in no way removes the need to respect the prop! When the plane is powered, ALWAYS avoid placing hands in the vicinity of the propellor, even when the throttle is disarmed. If all is not well with the autopilot electronics or software there is always a slight possibility that signal could unintentionally reach the motor. Even though this is unlikely (and made even less likely by safety features such as this) it only takes one time to chew up a finger or hand!
Parameters governing throttle arming are introduced in the ARMING section of the parameters wiki page. This page discusses throttle arming in greater detail and walks through the typical procedures for using this safety feature.
A Safety Note About Arming¶
A safety feature of PX4 / Pixhawk is that all servo output is kept at at minimum values until the safety button is pushed. This has the nice effect of (usually) disabling electric motors and it is quite possible to use only this safety feature for propeller safety. However:
- This feature is not available for the APM.
- Generally during pre-flight checks it is desirable to verify servo operation (e.g., to check ailerons are deflecting in the expected direction). This can more safely be done with the throttle disabled. When the button is pushed all servos are enabled, including the motor – unless this additional software throttle safety is used.
A note about APM1 and channel 8¶
The APM1 has a hardware multiplexor on channel 8 which forces pass-thru of the first 4 channels if channel 8 is above 1750. This means the arming code is bypassed if you use channel 8 for your flight mode switch. So if you want to use arming on an APM1 then you should change FLTMODE_CH to another channel (say channel 5) and setup your transmitter/receiver to put the mode switch on that other channel.
IMPORTANT: RC Transmitter Calibration¶
It is essential that your RC radio transmitter be calibrated correctly before continuing. Please see the Calibrate your RC input wiki page if you don’t know how to calibrate your radio.
During RC calibration throttle ups are possible, even with safeties enabled and you should therefore remove the propeller or move the plane to a safe area and secure it.
Depending on the ground control software you use, the RC3_MIN value may be continuously changing during calibration. Also, if the RC3_MIN value is inadvertently set incorrectly the plane may throttle up after a bad calibration (e.g., if the low stick setting was not captured).
Note that if you have RCMAP_THROTTLE set to something other than 3, then the RCn_MIN value used will be the one for the channel you have selected as the throttle channel.
When calibrating your RC input you should also be careful to set the minimum value of the throttle (usually RC3_MIN) to the minimum value when in normal flight control. Don’t set it to the value used by your transmitter when in throttle failsafe or you won’t be able to arm using the rudder as the APM will think you are at a non-zero throttle level.
Simplest Solution: Use Only ARMING_REQUIRE¶
The simplest way to use the throttle arming feature is to require the user to request the throttle to arm. ARMING_REQUIRE has three possible values, 0, 1 and 2 (the default value is 1, enabled). They have the following effect:
- ARMING_REQUIRE=0: No effect. Throttle arming safety is not employed.
- ARMING_REQUIRE=1: Before the user arms throttle, send minimum PWM to the throttle channel (which is usually channel 3). Therefore the RC3_MIN PWM value is sent on the servo output rail.
- ARMING_REQUIRE=2: Before the user arms throttle, send no PWM signal to the throttle channel (usually channel 3). Some ESCs are not happy with this and will continuously beep, some will not. Some users may prefer this setting as it ensures no signal is sent to the ESC when disarmed.
When the ARMING_REQUIRE parameter is set to 1 or 2 (enabled) and ARMING_RUDDER is not to 0 (disabled), all that is required to arm the throttle is to either:
Arm the throttle via the ground control software.
Arm the throttle by applying full right rudder input for several seconds.
To arm throttle in Mission Planner, use the Flight Data screen, then select the Actions tab. This provides an “Arm/Disarm” button that can be used to arm and disarm the throttle.
Alternatively, apply full right rudder to your RC transmitter for several seconds to arm the throttle. Whichever method you choose should result in a message from your ground control software stating that throttle arming was successful. Mission planner displays a message in the artificial horizon on the Flight Data screen:
Increased Safety: Perform System Checks Before Arming Throttle¶
By default the ARMING_CHECK parameter is set to 1 so the flight controller performs system health checks before arming throttle when a user attempts to arm. You can enable/disable any check with a bitmask. See wiki documentation on the ARMING_CHECK parameter for more information.
One thing to be aware of if you typically do not fly with a ground control station: it will be difficult to determine why your autopilot is not arming if you are not connected to a ground control station when arming. The ARMING_CHECK parameter should probably be left at 0 when at the an airfield without a ground control station.
The following are possible system health messages that may return if ARMING_CHECK is enabled and the autopilot rejects a request to arm the throttle:
- Message: “Hardware Safety Switch.” Solution: push the hardware safety switch on the PX4 or Pixhawk (does not apply to APM).
- Message: “Battery failsafe on.” Solution: Ensure your battery is charged. If it is, ensure your battery failsafe values are set correctly. For more information on failsafes, see the Failsafe Functions wiki page.
- Message: “Radio failsafe on.” Solution: Ensure that the RC transmitter is able to communicate with the RC receiver. For more information on this failsafe, see the documentation on the Throttle Failsafe.
- Message: “Bad GPS Pos.” Solution: Need to get a 3D fix with the GPS receiver. After ensuring your GPS receiver is functioning properly, ensure nothing aboard the plane or in the immediate environment is interfering with GPS satellite signals.
- Message: “No GPS detected. Solution: ensure your GPS receiver is functioning.
- Message: “No compass detected.” Solution: If you do not intend to use a compass, then change the COMPASS_USE parameter to 0. If you do, then you’ll need to ensure your compass is installed and healthy.
- Message: “Compass not calibrated.” Solution: Calibrate compass. In Mission Planner this is accomplished in the Initial Setup screen, menu item Manadtory Hardware > Compass.
- Message: “Compass not healthy.” Solution: Ensure you do not have the compass installed near something that can induce a magnetic field, such as the motor. You also may try re-calibrating the compass.
- Message: “Baro not healthy.” Solution: Try rebooting the autopilot. If this fails you will have to diagnose what the problem is with the barometer.
Disabling Rudder Arming¶
Some pilots will not want to use the rudder arming capability. To only allow throttle arming via the ground control software set the ARMING_RUDDER parameter to 0. The autopilot will no longer arm throttle when full right rudder is applied on the RC transmitter.