Attitude Hold¶
Attitude Hold (ATTHOLD) holds the aircraft's commanded attitude when you release the sticks, rather than the aircraft continuing to fly whatever attitude it was last commanded (typical for a fixed-wing aircraft with no self-leveling). This gives a self-leveling-like behavior on demand, independent of full autopilot features.
Attitude Hold doesn't bound stick authority to any particular orientation -- it freezes and holds whatever attitude the aircraft happens to be in, at any orientation, not just vertical or level.
Each axis holds independently¶
Roll, pitch and yaw each track or freeze based on their own stick. An axis whose stick is inside Deadband (percent stick deflection) is held at the attitude it was in when the stick centered; an axis whose stick is outside the deadband gets full unmodified authority and continuously re-captures its target, so the next freeze is seamless whenever that stick settles again.
That means you can fly one axis while the others stay held:
- Work the ailerons alone for a clean axial or rifle roll while pitch and yaw stay locked.
- Ride the elevator through a high-alpha attitude while roll and yaw stay held, so torque roll can't creep in while you're not touching the ailerons.
Tuning¶
Gain sets how aggressively it corrects back to the held attitude, Deadband is the stick threshold above, and Max Rate caps how fast it's allowed to rotate the aircraft while doing so. These are on the Profiles tab.
Attitude Hold works at reduced strength (about a quarter) before the aircraft is airborne, so you can tilt the airframe by hand on the bench and see a real, gentler correction. See On the bench.
On the bench¶
Attitude Hold still works at reduced strength before the aircraft is airborne -- roughly a quarter of its in-flight authority, the same as Angle mode -- so tilting the airframe by hand shows a real, gentler correction. It's not fully live until liftoff.
If a safety mode (Failsafe, GPS Rescue, RTH, Loiter or Angle) takes over and later releases, Attitude Hold captures a fresh target from the aircraft's current attitude instead of resuming a stale one.
Letting go of a stick¶
An axis does not freeze the instant its stick returns to center. It keeps tracking until it has actually stopped rotating (under about 15 deg/s), or 0.4 seconds have passed, whichever comes first, and only then freezes. If it froze at the instant of release, it would pin the target to an attitude the aircraft was still rotating through, and the hold would pull it back past where you stopped. The 0.4 second cap stops a constant disturbance, such as torque roll, from keeping an axis in tracking forever.
I-term while holding¶
An axis that is holding a frozen target still lets its accumulated I-term drain, but slowly -- at about a tenth of the normal rate, a time constant of roughly 6 seconds at the default I-term decay time. That is slow enough that a real steady disturbance, such as torque roll, is still held (the hold simply re-grows the I-term it needs), but not zero, so stale I-term doesn't keep the surfaces parked off-center when there is no error and no motion. An axis that is tracking or settling drains its I-term at the normal rate, the same as in ordinary flight.
When a hold gives up¶
If a frozen axis has more than about 5° of error and has hardly moved (under 5 deg/s) for 3 seconds, the correction is not achieving anything -- the aircraft is pinned on the bench, or the surface has no authority. The axis then gives up and re-captures its target at the current attitude, so the surfaces do not stay pegged. There is no beep or indication when it happens.
The surface then returns to center over a few seconds, because any build-up of I-term is drained at the normal fast rate for 3 seconds after the re-capture. A small steady error, under about 5°, never triggers this, so a hold that is sagging slightly against a steady disturbance keeps holding.
Known limitation: hands-off flight counts as "landed"¶
The firmware decides whether the aircraft is airborne from stick activity and tilt, not from throttle or airspeed. With the sticks centered and the aircraft within about 26° of level, it is treated as landed, and the correction is cut to about a quarter (see On the bench). A level, hands-off aircraft in Attitude Hold, or in Angle mode, therefore holds with much less authority than one being flown. It returns to full authority as soon as you move a stick, or the tilt passes about 37°. Keep this in mind when you tune the Gain: judge it with a stick touched, not with hands off. The same applies to Angle mode.