Don't get me wrong, airplanes with no autopilot can always be trimmed - a poor person's autopilot! On the ailerons, elevators and rudder (the three flight control surfaces) there are often "trim tabs." Using wheels inside of the cockpit, a pilot can trim their airplane so that it flies straight and level. That's fine as long as you want to just fly straight and level. What about turns, climbs and descents, etc? More sophisticated airplanes (or at least avionics packages) can practically do an entire flight from start to finish. Unfortunately, it can cause a pilot to become lazy. It takes good control touches to level out at an altitude and adjust the power simultaneously so passengers don't even realize it's being done. Also, pilots must monitor the autopilot - since it's only as good as what's entered in there (just like any computer: garbage in, garbage out).
Ultimately, what the airplane does is ultimately the responsibility of the pilot in command (PIC). If the autopilot isn't programmed correctly, the airplane could fly the incorrect routing. If this creates a problem with air traffic control, the pilot could get "violated:" like getting a traffic ticket for pilots. It's no excuse to say that it's the autopilot's fault. The PIC must operate the airplane safely and correctly, and that includes the operation of the autopilot.
For example, say a pilot isn't able to complete an instrument approach into an airport and she has to execute the missed approach. Since she studied the missed approach in case this happened, she knows that she must fly straight ahead until 1000 feet above ground level (agl) and then turn left to a 300 degree heading, then climb to 2000ft agl. If she didn't program in a climb to 2000ft, the airplane may level off at 1000ft. As soon as the pilot realizes that the airplane isn't climbing to 2000ft, she MUST correct this - whether it is to quickly reprogram the autopilot or to disengage the autopilot to do what she is required to do. This is something that is looked for on checkrides: does the pilot have an awareness of where she is, where she's going and is she ready to make corrections immediately (some people will look at the autopilot in confusion, saying, "what's it doing?" until it's too late - hopefully busting the checkride will be the only consequence). Yes, it is said that pilots are almost now just "systems managers!"
I saw a great example in AvWebFlash:
Overheard in IFR Magazine's "On the Air"
I was doing practice approaches when I heard approach control contact another aircraft on a similar mission:
Controller:"N1234, what are your intentions after this approach?"
N1234:"I'll be missed approach, and I'd like vectors to the ILS 36."
Controller:"Missed approach instructions: Climb to 2800 and direct to the VOR."
[... sometime later ...]
N1234:"Approach, N1234 is missed approach direct to the VOR. Request vectors for the ILS 36."
Control:"Turn right to 1-1-0 degrees, vector for the ILS. Maintain 2800 feet. I'm showing you at 3400 feet."
N1234:"Descending to 2800. I'm sorry. I'm having a discussion with the auto-pilot as to which of us is actually pilot-in-command."
John Steinervia e-mail
Autopilot is great - although I have flown airplanes without an autopilot, nothing beats cruise control on long flights!