Automation in modern transport category aircraft differs sharply from automation in light planes, both in design and philosophy. In most GA aircraft that have an autopilot - and many do not - it is often a simple type that can hold altitudes and headings. A higher end model might have the ability to hold climb or descent rates, capture altitudes, track VORs, and perhaps even shoot an ILS. Still, even capable autopilots are almost an afterthought, and are seldom well integrated with the rest of the aircraft's systems and avionics, even in newer glass cockpits. Most light plane pilots treat autopilots as a luxury that's nice to have at times, but easy to ignore when it's off. Rather few, in my experience, fully integrate autoflight capabilities into their procedures.
Modern airliners, however, treat automation systems as an integral part of the airplane, to be incorporated by varying degrees into almost every phase of flight. In these airplanes, it's never as simple as deciding whether to turn the autopilot on or off; it's moreso a matter of deciding what type of automation to use and how much of it for any given situation. This is as true of the Megawhacker as it is of the B777, and my airline's procedures reflect this.
Our Flight Operations Manual (FOM) defines three levels of automation. The lowest one, Level One, is hand flying without use of flight director or autopilot, commonly known as flying "raw data." This is the sort of flying GA pilots do every day without a second thought, but it is pretty rare at the airlines. It is generally done for proficiency purposes only, usually in good weather. Level One might also be used in situations where you need to maneuver rapidly, such as a TCAS resolution advisory or when the autopilot does something unexpected.
Automation Levels Two and Three are defined as "Basic Flight Director Command" and "Flight Management System Command," respectively. Interestingly, either level can be accomplished with or without the autopilot engaged; the automation policy makes no distinction between flight director and autopilot usage. In the Megawhacker, the autopilot merely couples to whatever flight director commands are active; you cannot use the autopilot without the flight director. The entire package is known as Auto Flight Control System (AFCS); you control the system through inputs to the Flight Guidance Control Panel, or FGCP.
This is the Megawhacker's FGCP. The three buttons with lighted carets on center-right indicate that autopilot and yaw damper are engaged, and that the AFCS is taking course guidance from the captain's-side HSI. The "Nav Source" knobs control whether those HSIs operate in VOR or LNAV mode (more on that later), the "Course" knobs control OBS selection on each HSI, and the "Hdg" knobs move the heading bug. The "Alt" knob moves the altitude selector on the pilots' primary flight displays (PFDs), and the buttons next to it select vertical modes. The pitch wheel controls those vertical modes, and the buttons to its right select lateral and approach modes.
This is a Primary Flight Display, or PFD. Above the attitude indicator it displays AFCS modes which were selected on the FGCP, lateral modes on the left and vertical modes on the right, with the altitude selector displayed just above the altitude tape. "But wait," you say, "there is no LNAV button on the FGCP!" Correct, LNAV is one of the modes selectable by pushing the NAV button. Others include VOR and LOC. Which mode actually gets selected depends on what's currently displayed on the HSI. Remember the "Nav Source" knob? We use it to decide if our HSIs get guidance from the FMS (LNAV mode ie "purple needles"), or from ground based navaids tuned in the radios (VOR mode ie "blue needles"). This is the difference between Level Two and Level Three Automation. In the picture above, the flight director is following course guidance given by the Flight Management System, which is following the flight plan route I programmed in before takeoff.
In this picture, you see that I have blue needles displayed on my HSI, although we're not receiving the localizer yet. I'm now in Heading Select mode, and the aircraft is turning to 050, where I set the heading bug. The airplane is descending in VS mode, and I've used the pitch wheel to select a descent rate of 1500 feet per minute. ATC has cleared us to descend to 11,000 feet, which is set on the altitude selector. Notice also that the vertical mode "Alt Sel" is displayed in white. Active modes are displayed in green, whereas white indicates an armed mode. The AFCS is armed to capture the selected altitude of 11,000 feet.
This picture is a good example of reverting from Level Three automation to Level Two. Moments before, I had been descending on a published arrival in LNAV mode, and I was actually using FMS-derived vertical navigation (VNAV) to descend. Then ATC told us to turn left heading 050 for vectors to the approach. I pushed VS to decouple VNAV and selected -1500 fpm, spun the heading bug to 050, pushed HDG, and then changed my Nav Source from purple needles to blue needles. I actually could've stayed with Level Three a while longer by inputing heading on the FMS while remaining in LNAV mode. I could stay on Level Three Automation all the way to the ground by requesting a GPS approach. I'll sometimes do this just for the practice. When I don't need the practice, though, it's just easier to go to Level Two and do it the "old fashioned way."
Note that all the inputs on the FGCP are the same whether the autopilot is engaged or I'm handflying with flight director. The only difference is that when I'm handflying, I'm supposed to command the PNF (pilot not flying) to make the inputs for me.
I think the average GA pilot would balk at all this gadgetry. "I'd rather just fly," you say. I can understand that - when I rent light planes, I find the utter lack of automation rather refreshing. But used properly, automation makes airline flying easier, safer, and more efficient. Highly automated flight decks are here to stay. That's why, as I've said before, you'll enjoy airline flying a lot more if you're a bit of a geek.