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I'm asking this question having watched a documentary about Northwest Flight 85 in 2002, during which a 747-400 experienced a lower rudder hardover at an angle of 17 degrees to the left.

The pilots were able to recover by taking over manual control and using the ailerons, the upper rudder and asymmetrical engine thrust to oppose the forces being generated from the lower rudder hardover.

During the doco, I believe the pilot in control immediately turned off the autopilot as he realised it wasn't going to be able to recover from the initial roll.

I just wondered if an autopilot would be able to recover from such an event - i.e. is it technically possible? It seems like the sort of thing an autopilot could achieve easier than a pilot, assuming it was programmed correctly.

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Normal autopilots don't. They expect all systems to work correctly, and in case of failure of a single control surface they will stubbornly command actions which are no longer possible. For them, thrust is only for controlling the vertical speed, and the application of asymmetric thrust for yaw control is not possible.

There are experimental autopilots which compare the result of their actions with a number of pre-programmed scenarios and select for their next commands the scenario which agrees best with reality. These really do what you assumed, but the specific scenario must have been considered before so the specific control laws are part of the code. Computers small enough to be carried on airplanes became powerful enough to do this in real time maybe 20 to 25 years ago, but I am not aware that regular autopilots of airliners have this capability even today. It is, however, used for unmanned vehicles.

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    $\begingroup$ I would add that in most planes the autopilot only commands elevators and ailerons and separate yaw damper maintains coordination using the rudder and is usually limited so it can't correct big deviation like this. It keeps the systems simple, but in case of failure the interaction may cause undesired results. In this case the autopilot would counter rudder with ailerons inducing large forward slip and loss of airspeed as there would not be enough thrust for that at cruise level. $\endgroup$
    – Jan Hudec
    Aug 13, 2014 at 6:45
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    $\begingroup$ military autopilots will be able to recover losing a flight surface, the fly by wire system in modern fighters is designed to do this even in manual flight $\endgroup$
    – ratchet freak
    Aug 13, 2014 at 7:37
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Not all autopilots have this capability, but it usually specifically added for airplanes that have split rudders and are highly susceptible to Dutch rolls without active yaw damping (T-tailed, large sweep, significant dihedral). This should be supplemented with adequate indication of the situation to the pilot, and newer glass cockpits are even introducing displays of corrective actions and restricted flight envelopes, where the pilot can have a reduced workload and airframe stress will be minimized.

That said, hardovers are probably the most difficult potential error to design against, and in some cases would be catastrophic if they occurred, regardless of any attempts at correction. If this is the case, it is part of the aircraft manufacturer's job to prove that the probability of such hardovers is statistically impossible (usually defined as less than one expected occurrence per billion hours of flight).

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