A very simple way to understand the difference between autopilot and fly-by-wire is to compare them to an autonomous car and its anti-skid system.
A more aeronautical way is to look at how a turbine engine is actually controlled: The pilot gives their orders using a throttle, but orders are overridden by the FADEC on the engine, which actually manages the engine controls. Even when the auto-throttle is on and orders come from a computer instead of a pilot, the FADEC still intercepts them. In both cases:
- The autonomous car computer and the auto-throttle are autopilot functions.
- The anti-skid and FADEC are FBW functions.
They live their lives independently of each other, exactly like the autopilot and the control surfaces FBW computers live independently of each other.
Autopilot is a pilot
Autopilot, or flight computer, is comparable to a substitute for the pilot manual inputs. What does the flight computer could be done by manual inputs. However why would you send manual orders to fly a holding pattern if a computer can do it automatically, managing wind, speed, bank angle limits, etc, itself?
For Airbus the autopilot is the Flight Management Guidance System (FMGS).
Flight-by-wire is a elaborate anti-skid system
FBW is in my opinion a very bad choice for the concept it covers, as it introduces the idea there are wires and electronics between the pilot hand or foot and the control surface actuator.
There are actually wires and electronics, but wires are not a requirement, this could be a mechanical linkage assisted by electronics. And actually in Airbus FBW there are both wires and mechanical linkages.
The point of a FBW system is the orders received, regardless of whether they come from the pilot or the flight computer, are "managed" by a flight control computer. In Airbus avionics, the crew and the FMGS (autopilot) send commands to redundant computers:
- 2x ELAC (elevator aileron computer) for elevators/THS and ailerons,
- 3x SEC (spoilers elevator computer) for elevators/THS and spoilers,
- 2x FAC (flight augmentation computer) for rudder,
Airbus fly-by-wire, Source
Computers send commands to actuators. ELAC, SEC and FAC have nothing to do with the FMGS, the computer used for autopilot functions. FGMS inputs are visible above at ELAC and FAC level.
To simplify: FBW is like an anti-skid system. The brakes on a car may receive a command which is too hard, and known to lead in certain conditions to skidding and a less efficient braking force than a smoother order. The brake computer (anti-skid) will then overrides driver inputs and translates the original order into a more efficient brake command.
The computer is able to react very quickly and to analyze actual braking in order to modulate the command. The role of this anti-skid system is independent of how the order was sent, by a human or a computer.
FBW computers are the anti-skid systems used to control rudder, elevators, ailerons and other control surfaces. They maintain the aircraft in its flight envelope, that is in the domain it is able to fly safely and efficiently, and where the control surfaces are operative (e.g. if the airspeed is too slow on the ailerons, they will be stalled, the flight envelope protection acts to prevent such possibility).
The way FBW work is based on ELAC and FAC being operational and receiving inputs from a lot of sensors, like the ADIRU (air data and inertial reference). If one of these computers fails or lacks inputs, then their roles is redistributed in a degraded way, and if nothing works, then control is returned to the pilot. This progressive "de-automation" is done by switching the "flight laws". Degraded laws are:
- Alternate law (two levels: With and without reduced protections),
- Direct law (no protection),
- Mechanical (mechanical linkage between controls and control surfaces).
Airbus flight control laws, source
See Is a Control Law Degradation in Airbus Planes displayed on the ECAM? for more.
Answers to your questions
Yes most of the time autopilot (Airbus FMGS) is active, and therefore send orders to control surfaces to fly according a pre-determined route. It manages track errors.
Yes the flight control computers (Airbus ELAC, FAC, SEC) always override pilot inputs. This is still true when the FMGS actually pilots the aircraft. (This is comparable to car drivers orders always overridden by the anti-skid, even when a computer drives an autonomous car.)
In case the flight control computers fail partially or totally, some degraded ways to override pilot inputs are used, in order to continue protecting the flight envelope as most as feasible. In last resort some functions are completely returned to the pilots: This is the mechanical linkage step. In that case a pilot is able to manually control rudder and elevators.
Note one axis (pitch, roll, yaw) can be under some law and another under another law. So it is incorrect to say an aircraft is under alternate law in most cases.