QuadPlane Safety Issue: RTL Enters Fixed-Wing Mode Before Q_RTL When Inside RTL_RADIUS at Low Altitude

[sundar249]

When flying a QuadPlane (Hybrid VTOL) in AUTO mission mode, switching to RTL while already inside RTL_RADIUS causes the system to first enter fixed-wing RTL logic before transitioning to Q_RTL.

This creates a delay in VTOL motor takeover. At low altitude (~15 m AGL) this delay may result in insufficient height for safe VTOL stabilization, potentially causing ground impact before Q_RTL authority is established.

Observed sequence:

AUTO → RTL (FW logic active) → position check → Q_RTL

The aircraft briefly behaves as a fixed-wing RTL vehicle even though it is already near home where immediate VTOL control would be safer.

Version
ArduPlane 4.6.3

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

Airframe type
QuadPlane (Hybrid VTOL)

Hardware type
Pixhawk-class flight controller (Cube compatible)

Logs
Logs available and can be provided. Key indicators in logs:

MODE change: AUTO → RTL

Delay before second MODE change: RTL → QRTL

STATUSTEXT: "RTL" before VTOL entry

EV message showing Q-mode entry occurs after RTL logic

RCOU: VTOL motors idle until Q_RTL engages

Mode Transition Timeline

T0 – Aircraft in AUTO (FW)
T1 – Pilot switches to RTL
T2 – Mode = RTL (fixed-wing RTL active)
T3 – System detects aircraft inside RTL_RADIUS
T4 – Mode switches to Q_RTL
T5 – VTOL motors spool and stabilization begins

Risk Window: T1 → T4 (fixed-wing logic active at low altitude)

Parameter Interaction

RTL_RADIUS → Triggers the “already near home” logic

Q_RTL_MODE → Enables VTOL behavior during RTL

Q_RTL_ALT → Target VTOL altitude may not be reached before transition

Q_TRANSITION_MS → Adds transition delay

Q_ASSIST_SPEED → May delay VTOL dominance

Expected Behavior

If:

Vehicle = QuadPlane

RTL is triggered

Aircraft is already inside RTL_RADIUS

➡ System should immediately enter Q_RTL, skipping fixed-wing RTL logic.

Safety Impact

This affects emergency mission aborts near home at low altitude and represents a potential crash scenario due to delayed VTOL takeover.

Reporter: Sundar C

[IamPete1]

Can you share a log please.

[sundar249]

https://drive.google.com/file/d/1TJDGTese6_nX5Gr_z1MNgaQjiQJwMGPT/view?usp=sharing

[IamPete1]

This is due to the one second delay here:

ardupilot/ArduPlane/mode_rtl.cpp

Line 92 in 18ced5a

if ((AP_HAL::millis() - plane.last_mode_change_ms > 1000) && switch_QRTL()) {

For the last one in the log if you had been under your Q_RTL_ALT of 15 meters then it would have gone directly to QRTL without the one second in RTL because of this check:

ardupilot/ArduPlane/mode_rtl.cpp

Lines 27 to 36 in 18ced5a

	// if VTOL landing is expected and quadplane motors are active and within QRTL radius and under QRTL altitude then switch to QRTL
	const bool vtol_landing = (plane.quadplane.rtl_mode == QuadPlane::RTL_MODE::SWITCH_QRTL) || (plane.quadplane.rtl_mode == QuadPlane::RTL_MODE::VTOL_APPROACH_QRTL);
	if (vtol_landing && (quadplane.motors->get_desired_spool_state() == AP_Motors::DesiredSpoolState::THROTTLE_UNLIMITED)) {
	int32_t alt_cm;
	if ((plane.current_loc.get_distance(plane.next_WP_loc) < plane.mode_qrtl.get_VTOL_return_radius()) &&
	plane.current_loc.get_alt_cm(Location::AltFrame::ABOVE_HOME, alt_cm) && (alt_cm < plane.quadplane.qrtl_alt_m*100)) {
	plane.set_mode(plane.mode_qrtl, ModeReason::QRTL_INSTEAD_OF_RTL);
	return true;
	}
	}

So if your low with VTOL motors active it will already do what you want. In your use case you would be better to either set QRTL on your aux switch rather than RTL or use Q_RTL_MODE "QRTL Always".

[sundar249]

I kindly request consideration of one of the following improvements:

Remove the 1-second delay for QuadPlane RTL transitions.

Immediately evaluate QRTL when:

Vehicle is QuadPlane

RTL is triggered

Aircraft is already within RTL_RADIUS

Add a parameter to optionally disable the 1-second delay for VTOL platforms.

Bypass delay during failsafe-triggered RTL events.

Why This Matters

Low-altitude aborts near home are common during:

Battery failsafe events

Emergency mission termination

In these cases, immediate VTOL authority significantly improves safety margins.

This is not a tuning issue but a mode sequencing safety concern for hybrid platforms.

Thank you for reviewing this request and for your continued support of QuadPlane development.

Reporter:
Sundar C