Bitmask for when to allow negative reverse thrust (USE_REV_THRUST)

Description

This bitmask controls exactly when the autopilot is allowed to use reverse thrust (negative throttle) during autonomous flight modes. Reverse thrust is a powerful tool for steep descents and shortening landing rolls, but it can cause stalls or loss of control if engaged inappropriately (e.g., during normal cruise or loiter).

The Mathematics

The parameter is a bitmask where each bit represents a flight condition:

• Bit 0: Auto Always
• Bit 1: Auto Land Approach (Most Common)
• Bit 2: Auto Loiter to Alt
• Bit 3: Auto Loiter All
• Bit 4: Auto Waypoint
• Bit 5: Loiter
• Bit 6: RTL
• Bit 7: Circle
• Bit 8: Cruise
• Bit 9: FBWB
• Bit 10: Guided
• Bit 11: Auto Landing Pattern
• Bit 12: FBWA

The Engineer's View

Defined in ArduPlane/Parameters.cpp and used throughout the flight mode logic (e.g., ArduPlane/reverse_thrust.cpp).

• Safeguards: Reverse thrust usually requires THR_MIN to be negative.
• Default (2): USE_REVERSE_THRUST_AUTO_LAND_APPROACH. This is the safest default, allowing braking only when committed to a landing.

Tuning & Behavior

• Default Value: 2 (Auto Land Approach Only)
• 0: Never use reverse thrust in auto modes.
• 2 (Bit 1): Allow only during the landing sequence.
• 65 (Bit 0 + Bit 6): Allow in all Auto modes + RTL.
• Warning: Enabling reverse thrust in modes like FBWA or CRUISE can lead to mid-air stops if the pilot pulls back on the throttle stick. Use with caution.