# 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).

To add one piece of information to Jan's answer, quoting from the A320 Flight Crew Training Manual, a detailed description of what exactly is autothrust:

OBJECTIVE

The A/THR computer (within the FG) interfaces directly with the engine computer, referred to as the FADEC. The A/THR sends to the FADEC the thrust targets that are needed to:

• Obtain and maintain a target speed, when in SPEED mode

• Obtain a specific thrust setting (e.g. CLB, IDLE), when in THRUST mode.

INTERFACE

When the A/THR is active, the thrust lever position determines the maximum thrust that the A/THR can command in SPEED or THRUST mode. Therefore, with A/THR active, thrust levers act as a thrust limiter or a thrust-rating panel. The A/THR computer does not drive back the thrust levers. The PF sets them to a specific detent on the thrust lever range.

The A/THR system provides cues that indicate the energy of the aircraft:

• Speed, acceleration, or deceleration, obtained by the speed trend vector

• N1, and N1 command on the N1 gauge.

All these cues are in the flight crew’s direct line of vision. In other words, the Thrust Lever Angle (TLA) should not be used to monitor correct A/THR operation. Neither should the thrust lever position of a conventional autothrottle, be considered a cue because, in many hazardous situations, the thrust lever position can be misleading (e.g. engine failure, thrust lever jammed).

The A/THR can only be active, when the thrust levers are between IDLE and the CLB detent. When the thrust levers are beyond the CLB detent, thrust is controlled manually to the thrust lever Angle, and the A/THR is armed . This means that the A/THR is ready to be re-activated, when the flight crew sets the thrust levers back to the CLB detent (or below).A/THR appears in blue on the FMA.

If thrust levers are set to IDLE, A/THR is set to off.

and:

AP, FD, A/THR SYSTEM INTEGRATION

There is a direct relationship between aircraft pitch control, and engine thrust control. This relationship is designed to manage the aircraft’s energy.

• If the AP/FD pitch mode controls a vertical trajectory (e.g. ALT, V/S, FPA, G/S): A/THR controls speed

• If the AP/FD pitch mode controls a speed (e.g. OP CLB, OP DES): A/THR controls thrust (THR CLB, THR IDLE)

• If no AP/FD pitch mode is engaged (i.e. AP is off and FD is off): A/THR controls speed

Therefore, any change in the AP/FD pitch mode is associated with a change in the A/THR mode.

On the other hand, manual thrust is a conventional and proportional interpretation of the position of the TLP: 0% throttle at IDLE, 100% at TOGA and linearly interpolated inbetween.

As for why the A/THR is seldomly not used, from the same manual (emphasis mine):

Use of A/THR is recommended during the entire flight. It may be used in most failures cases, including:

• Engine failure, even during autoland

• Abnormal configurations