What are autothrust and manual thrust on an A320 (or other aircraft)? The accident report on the A319 G-EUOE incident in May 2013 mentions "manual thrust", and implies that using it is very unusual.

The autothrust system failed, requiring the use of manual thrust:


  The autothrust system cannot operate  in N1 mode and so was
  unavailable for the remainder of the flight, which was  flown using
  manual thrust


But the pilot hadn't used manual thrust outside a sim session for seven years (and the co-pilot had never used it outside the sim):


  The commander informed the AAIB that, excluding simulator training, he
  had  last flown the A320-series aircraft using manual thrust seven
  years before the accident


That implies that autothrust is always used in normal operations, and manual thrust is an emergency backup only. What exactly are autothrust and manual thrust and why is using manual thrust so unusual?
Manual thrust means that the engine thrust corresponds directly to the position of the thrust lever.

Autothrust means that a target speed is dialed into the system (using a knob that is usually located on the glare-shield) and the auto-thrust system automatically adjusts the engines to maintain that speed.

Now in an aircraft with classical controls the longitudinal stability means the aircraft tends to maintain speed with pitch and pilots basically need to use thrust to climb or descend and yoke to adjust speed, always making adjustments with the other control as well to avoid oscillations and compensate for the more complex parts of the power-pitch relationship.

Due to this, it is usually recommended to fly either fully manual or fully automatic (autopilot+autothrust).

But Airbus A320 and newer (A319 is just a variant of A320) changes that with the artificial neutral stability in normal control law. Even with autopilot off, the flight computer automatically adjusts trim so that the aircraft flies straight so that adding power makes the aircraft accelerate and reducing it makes it decelerate. However pushing the stick forward to descend will still make the aircraft accelerate and pulling it to climb will make it slow down.

Due to this, Aibrbus aircraft are easier to fly with autothrust always on (except for take-off roll; autothrust is disabled when the power levers are advanced above certain position, which is only done for take-off and in emergency where extra power is needed).