docs(safety): add altitude loss detection failsafe documentation (#27104)

docs/en/config/safety.md

@@ -310,6 +310,19 @@ Note that this check is _always enabled on takeoff_, irrespective of the `CBRK_F
 
 The failure detector is active in all vehicle types and modes, except for those where the vehicle is _expected_ to do flips (i.e. [Acro mode (MC)](../flight_modes_mc/acro.md), [Acro mode (FW)](../flight_modes_fw/acro.md), and [Manual (FW)](../flight_modes_fw/manual.md)).
 
+### Altitude Loss Trigger {#altitude-loss-trigger}
+
+<Badge type="tip" text="PX4 v1.18" /> <Badge type="tip" text="MC, VTOL only" />
+
+The failure detector can be configured to trigger if a rotary-wing vehicle loses too much altitude below its commanded setpoint while in an altitude-controlled flight mode (such as [Position mode](../flight_modes_mc/position.md) or [Altitude mode](../flight_modes_mc/altitude.md)).
+
+If the vehicle descends more than [FD_ALT_LOSS](#FD_ALT_LOSS) meters below the setpoint, [flight termination](../advanced_config/flight_termination.md) is triggered, which may deploy a [parachute](../peripherals/parachute.md).
+
+| Parameter                                                                                          | Description                                                                                                                           |
+| -------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------- |
+| <a id="FD_ALT_LOSS"></a>[FD_ALT_LOSS](../advanced_config/parameter_reference.md#FD_ALT_LOSS)       | Altitude loss threshold (m). Flight termination is triggered when the vehicle drops this far below the setpoint. Set to 0 to disable. |
+| <a id="FD_ALT_LOSS_T"></a>[FD_ALT_LOSS_T](../advanced_config/parameter_reference.md#FD_ALT_LOSS_T) | Time (s) the vehicle must remain below the threshold before flight termination is triggered.                                          |
+
 ### Attitude Trigger
 
 The failure detector can be configured to trigger if the vehicle attitude exceeds predefined pitch and roll values for longer than a specified time.

docs/en/peripherals/parachute.md

@@ -44,6 +44,7 @@ To enable flight termination:
 
 - Set [Safety](../config/safety.md) action to _Flight termination_ for checks where you want the parachute to trigger.
 - Set [Failure Detector](../config/safety.md#failure-detector) pitch angles, roll angles and time triggers for crash/flip detection, and disable the failure/IMU timeout circuit breaker (i.e. set [CBRK_FLIGHTTERM=0](../advanced_config/parameter_reference.md#CBRK_FLIGHTTERM)).
+- <Badge type="tip" text="PX4 v1.18" /> Set [FD_ALT_LOSS](../advanced_config/parameter_reference.md#FD_ALT_LOSS) to enable flight termination if a rotary-wing vehicle loses too much altitude below its setpoint (see [Altitude Loss Trigger](../config/safety.md#altitude-loss-trigger)).
 
 ::: info
 You can also configure an [external Automatic Trigger System (ATS)](../config/safety.md#external-automatic-trigger-system-ats) for failure detection.

docs/en/releases/main.md

@@ -50,6 +50,10 @@ Please continue reading for [upgrade instructions](#upgrade-guide).
 - Added new flight mode(s): [Altitude Cruise (MC)](../flight_modes_mc/altitude_cruise.md), Altitude Cruise (FW).
   For fixed-wing the mode behaves the same as Altitude mode but you can disable the manual control loss failsafe. ([PX4-Autopilot#25435: Add new flight mode: Altitude Cruise](https://github.com/PX4/PX4-Autopilot/pull/25435)).
 
+### Safety
+
+- Rotary-wing vehicles now support uncommanded altitude loss detection: if the vehicle descends more than [FD_ALT_LOSS](../advanced_config/parameter_reference.md#FD_ALT_LOSS) meters below its setpoint in altitude-controlled flight, flight termination (and parachute deployment) is triggered. See [Altitude Loss Trigger](../config/safety.md#altitude-loss-trigger). ([PX4-Autopilot#26837](https://github.com/PX4/PX4-Autopilot/pull/26837))
+
 ### Estimation
 
 - Added [EKF2_POS_LOCK](../advanced_config/parameter_reference.md#EKF2_POS_LOCK) to force constant position fusion while landed, useful for vehicles relying on dead-reckoning sensors (airspeed, optical flow) that provide no aiding on the ground.