AutoPilot Hard Over?

[B737NG_Pilot]

What does auto pilot hard over protection mean?

[safetypee]

Some fail-passive autopilots can suffer a system failure which outputs the max rate pitch / roll demand to the controls (hard-over control input). These inputs are not normally the same as max manual control rate or limit, but this type of AP failure would be very surprising and potentially hazardous near the ground. Thus a segregated or independent external detector is installed to disengage the AP at a predefined point e.g. 1 deg/sec pitch rate or 30 deg roll angle, or a system which detects the duration-magnitude of an output deviation (sometimes used for trim protection).
These devices often define the minimum use height for the autopilot and thus this height must be respected. Also the AFM may require a hands-on follow-through when using automation below a certain altitude to provide a suitable response time e.g. less than 1000ft, 2 sec for pilot action vice 4 sec in the cruise.