This mode holds height itself, and there is no height estimate

What ArduPilot says:

• need alt estimate

Armed in a mode where your stick drives the throttle (Stabilize, Acro), you asked for a mode that holds height by itself (AltHold, Loiter and every automatic mode), but the EKF has no valid height and climb rate. The change would hand the throttle to a controller that does not know the height, so it is refused. Between two modes that both hold height the check is not made.

What to do, most likely first

1. The EKF had not finished starting: the copter was armed and flown within seconds of power-on.

   CheckThe log shows “tilt alignment complete” only after take-off.

   FixAfter power-on leave the copter still until the ground station stops showing EKF and “Need Alt Estimate” pre-arm messages.

2. The height source is unhealthy or not the one fitted: the barometer failed, or EK3_SRC1_POSZ names a sensor that is not there.

   CheckRead EK3_SRC1_POSZ: 1 barometer, 2 rangefinder, 3 GPS, 4 beacon, 6 external navigation.

   FixSet EK3_SRC1_POSZ to 1 (barometer) unless you have a reason for another source, and look for “ERR BARO-4” in the log.

3. Heavy vibration made the EKF reject its height data.

   CheckVIBE.Clip counts up, or VIBE.VibeZ is above 30 m/s².

   FixFind and remove the vibration before flying modes that hold height.

Parameters to look at

Where to do it:Setup & Tuning GuideSetup & Tuning Guide