Originally Posted by
parabellum
There are still people out there who will, as a first reaction to any problem, dump the autopilot and start handflying without proper analysis of the problem, the annoucement, "Disconnecting" usually comes a second or so after the buttons are pressed!
IIRC this turned out to be a major factor in the Kegworth accident. The decision to shut down the (working - incorrectly diagnosed as faulty) starboard engine occurred simultaneously with the decision to disengage A/P and A/THR. This had the effect not only of masking the very real problem in the port engine by reducing the fuel flow to near idle in preparation for the descent, but also markedly increasing the workload on the flight deck, so much so that when the Captain started troubleshooting - "Now, what indications did we actually get?" - he was interrupted by having to receive and reply to radio calls, the opportunity to correct the mistake passed and the rest is history.
Not being a pilot myself, I can't say - but most of the correspondence I've received over the years, as well as a considerable amount I've read on here, tends to indicate that modern automation is about as good as it can get, and while it is no substitute for analytical human minds on the flight deck, any assumption that an unexpected change in vertical or horizontal track must be due to a fault in the automatics (as opposed to inadvertent mis-programming) is likely to be mistaken. If you're not happy, then disconnect the FMS routing, but surely using a basic altitude hold while diagnosing the problem should be the next step. If that doesn't have the desired effect, then go hand-flying, but bear in mind that it will consume a lot of effort - possibly more than you anticipate.
To be clear, I'm not saying that handflying should be a method of last resort. If it's a good day, your airspace is relatively clear and you feel comfortable, nothing should stop you. However, if and when things head south one should give careful consideration to the increase in workload that handflying will put on the PNF, as well as yourself.