Unexpected Disarm in ACRO mode when Throttle at zero during GCS/RC Failsafe while airborne

[Lqs66]

Bug report

Issue details

In ACRO mode, if the vehicle is airborne and the throttle is at the minimum position, triggering an GCS/RC Failsafe causes the copter to immediately disarm in mid-air.

While the "Disarm on low throttle" logic is expected on the ground, it should not be triggered when the flight controller detects the vehicle is still in flight (Airborne), especially during a failsafe event where a controlled RTL or Land is expected.

See line 472:

ardupilot/ArduCopter/events.cpp

Lines 463 to 482 in cef6eb7

	bool Copter::should_disarm_on_failsafe() {
	if (ap.in_arming_delay) {
	return true;
	}
	switch (flightmode->mode_number()) {
	case Mode::Number::STABILIZE:
	case Mode::Number::ACRO:
	// if throttle is zero OR vehicle is landed disarm motors
	return ap.throttle_zero || ap.land_complete;
	case Mode::Number::AUTO:
	case Mode::Number::AUTO_RTL:
	// if mission has not started AND vehicle is landed, disarm motors
	return !ap.auto_armed && ap.land_complete;
	default:
	// used for AltHold, Guided, Loiter, RTL, Circle, Drift, Sport, Flip, Autotune, PosHold
	// if landed disarm
	return ap.land_complete;
	}
	}

Steps to Reproduce:

1. Take off and switch to ACRO mode.
2. Lower the throttle stick to the minimum position.
3. param set FS_THR_VALUE -1 (trigger an RC Failsafe)/param set FS_GCS_TIMEOUT 0 (trigger a GCS Failsafe).
4. The copter disarms immediately and falls.
5. Expected Result: The copter should trigger the configured Failsafe action (e.g., RTL or Land) regardless of the throttle position, as long as it is airborne.

Version
Copter 4.4 -> main

Platform
[ ] All
[ ] AntennaTracker
[x] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Airframe type
sitl

Logs
RC Failsafe:
log_0_2025-12-27-12-45-46.zip

GCS Failsafe:
log_0_2026-1-7-16-26-53.bin.zip

[IamPete1]

At zero throttle in acro the vehicle is already falling. So your already having a crash before the failsafe happens. Have you actually hit this in real life?

[deepak61296]

I was able to reproduce this and found the issue. it's in the should_disarm_on_failsafe() function at line 472 in events.cpp.

the problem:

currently uses OR logic:

return ap.throttle_zero || ap.land_complete;

disarms if throttle is zero OR vehicle landed. so when airborne with zero throttle, failsafe just disarms you mid-air.

the fix:

change || to &&:

return ap.throttle_zero && ap.land_complete;

only disarms when throttle is zero AND actually landed.

tested in SITL:

took off to ~280m, zeroed throttle in ACRO, triggered failsafe.

before: immediately disarmed and fell
after: switched to RTL mode. still crashed because the copter was already falling hard in ACRO when RTL kicked in, but at least it tried to execute the failsafe action instead of just disarming.

triggering RTL is the right call - gives the copter a chance to recover. if caught earlier it should work fine.

opened PR #31806 for this. same fix applies to all failsafe types since they use this function.

---

[LupusTheCanine]

At zero throttle in acro the vehicle is already falling. So your already having a crash before the failsafe happens. Have you actually hit this in real life?

It is possible that failsafe will be triggered during acrobatics and, especially with airmode enabled, throttle may be briefly at a minimum when RC/GCS (battery FS probably shouldn't happen with 0 throttle 😅) failsafe triggers. Depending on situation recovery may be preferable course of action.

[IamPete1]

especially with airmode enabled,

throttle_zero is always false with airmode enabled.

Its very easy to come up with theoretical situations where we currently do the wrong thing. Its just as easy to come up with situations where the proposed fix would be the wrong thing. Before we fix something that is not broken I just want to make sure someone really hit this.

[LupusTheCanine]

Apparently somebody did. I think we can discuss the proposed fix in the PR theead #31806.

[LupusTheCanine]

@Lqs66 the behavior is documented
"If the copter is armed in Stabilize or Acro modes, and the throttle input is at minimum, the copter will immediately disarm."
here

@IamPete1 I think we should add a note about AirMode to the wiki. ArduPilot/ardupilot_wiki#7321

[tpwrules]

I think we've had some previous discussion on this topic but I can't find it...

[rmackay9]

I had a quick look at the log and it is from SITL so I agree with @IamPete1 that it's very possible this is not a real issue. I also think the issue won't happen if AirMode is being used

[peterbarker]

Apparently somebody did. I think we can discuss the proposed fix in the PR theead #31806.

Who? Flight log required....

This code has been like this since 2016.

Given it's very much in the finger-choppy bits of the code we really do need to be careful with changes in this area.

[Lqs66]

I’ve identified a similar issue in the GCS Failsafe behavior specifically for Acro mode : ).