Autopilots are great until like most things electronic they go wrong. Sods law says they will do this when you most need them. Although in the case of this accident it is likely the vacuum pump failure led the autopilot astray not all autopilots need a vac pump.
Worth checking out how yours works here:
Autopilots: ARC, Century, King, S-Tec
In the case of the accident aircraft here is further info:
Century III/IV/2000
These Century Autopilot systems work much like their counter parts, the ARC 400 series and the King KFC-150/200. Roll and pitch information is again derived in the horizon indicator and sent to the computer for processing.Century 2000 Like the other two axis systems we’ve talked about, the Century has an altitude hold module also that monitors static pressure but basic pitch is still picked off the horizon indicator.
The Century system can be driven from its own DG or just about any manufacturer’s HSI. It’s not uncommon to see a Century III with the heading system manufactured by King. In fact, Century even has two different HSI systems of their own; one is all electric and the other is vacuum AND electric. We will dive into HSI operation in our next session. In general, the Century autopilot isn’t a bad system; most of the problems we see with the Century are pitch oscillations and/or bad connections at the plastic connectors.
What Happens if My Turn Coordinator Fails? These autopilot systems do not get any information from the turn coordinator. The Century I single axis system uses the turn coordinator; we will discuss that system later.
What Happens if My Horizon Indicator Fails? The best you could hope for is the gyro would roll off to one side as it spooled down and disengage the autopilot. If you even suspect the horizon is failing, shut the autopilot off then. Fly the aircraft by hand, it’s not that hard.
What Happens if My Vacuum HSI or DG fails? The Century series autopilot will still keep the wings level and the pitch/altitude hold functions will still work as advertised. I’d recommend pulling out the turn knob, this will level the wings; from there you can turn this knob in the direction you wish to go and use the heading shown on the wet compass. If you turn this knob full in either direction the aircraft will bank in a standard rate turn. Most HSI indicators have a "Heading" warning flag. Century does sell an all electric HSI; of course this keeps working if the vacuum pump fails.
What Happens if I lose My Vacuum Pump? Unless the horizon gyro tumbles and disconnects the autopilot you’ve got a big problem; who knows what the aircraft will do. Monitor the vacuum warning system; at the first sign of a vacuum failure or problem, disengage the autopilot and be prepared to fly using partial panel. I bet at this point you’d wish you had that all electric HSIJ