What I want to know is this: what was the autopilot set to? doesn't it have a geographical fix, in which case you might expect it to say "Next exit ... ooh, make a legal u-turn ahead" like the one in my car does. Or is it just "heading 065, altitude FL370, speed 450, until something happens"?
And clearly, no one was listening to the radio for over an hour. This is WAY beyond a lapse, this was complete dereliction of duty, and they both should lose their licenses permanently.
AP can be the approach point, or top of descent, but was meant to be airport. It is pretty close to the same thing.
I am only familar with Boeing aircraft and you are right when the autopilot is set up to follow the GPS track or route, a light will go off on the forward panel to reset the mode control panel altitude so that the autopilot can start the descent. If you fly pass the top of descent, the airplane remains level and continues on the same track or heading. This had to be what happened because during preflight you program the computer to take you to the destination and once you fly past that, the computer will fly straight and level until you make a change to the autopilot or computer that is directing the autopilot.
Pilots can sometimes fly out of an range or radio contact before getting a new frequency. When this happens, you sometimes can hear other pilots talking because they are at high altitude, but you cannot hear Denver Center. If you keep flying for a long time on the old frequency, you will not be able to hear other pilots and everything is very quiet. It would be negligence to not check in with center when you fly out of their sector, but if all is quiet and nobody is thinking about how quiet the radios are, that is more likely, sleeping at the yoke.
The other possibility is that the pilots were on the wrong frequency, or using the speaker instead of headset and had the volume turned down to low and couldn't hear anyone calling them. One pilot flys while the other talks. But, both are suppose to listen and confirm all calls from ATC.
When pilots start approaching destination, they are suppose to get the weather, gate assignment and make special request for passengers, ie.. three wheel chairs, one minor child needs assistance, etc... It is a very busy time of the flight. To miss getting ready for the arrival, checking the weather, and everything else that is needed is gross. The best lawyers at ALPA will not be able to save these guys. They were with one of the best airlines in the world. Delta just bought NW and they will miss working for Delta.