Eli5: Why do pilots change altitude and speed manually even though the autopilot is on?
In: Engineering
Depending on the plane, they don’t.
If we’re talking about a your typical airliner like a 737 or A320, these planes come with autothrottle alongside autopilot. If the plane is flying level at FL220 (22,000 feet) and the pilots are instructed to climb to FL280 or something, they will turn a dial to set that altitude, and then activate that setting to tell the plane to climb.
If by “manually” you meant adjusting the dials, there is another way that is less manual called Vertical Navigation, or VNAV. When this is used, the plane will have a set altitude at a certain point along the route, and the autopilot will control when and how much it climbs or descends to reach that altitude at that point without the need for specific inputs at specific times, however the pilots typically will still have to dial in a top or bottom altitude, sort of as an approval for the plane to make that change.
There are a few other systems in play as well here. For instance, on climbs and descents there are typically points with certain altitude restrictions. If we’re descending from FL220 through 4 or 5 of these points down to, say, 4,000 feet where we intercept the runway localizer, the pilot may dial in 4,000 and instruct a descent, however, the plane will follow these restrictions, assuming they are programmed into the flight management system properly. So it will descent to maybe 16,000 then wait until it passes the point with the 16,000 restriction before continuing to descend down again to 10,000 and so on.
There’s a couple different methods that are used here with autopilot and altitude changes, but it’s not done manually if (full) autopilot is on. The act of trying to change altitude manually (via the yoke/stick) would disable the autopilot.
As the other commenter mentioned, there are other types of autopilot, available on airliners, but typically used on smaller planes. These are heading hold, and altitude hold. They do what they sound like, and can allow for manual control of what is not “held”.
Speed is a different thing. Autothrottle, if a plane has it, is separate from autopilot. If a plane doesn’t have autothrottle or it is not in use, then the pilot(s) will be required to adjust the throttle to maintain or change speed. Of course, speed is not just controlled by the throttle, a climb or descent also plays a role.
I guess I just kinda vomited information, so if that didn’t answer your question, feel free to follow up.
Latest Answers