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During a climb with the autopilot engaged in the V/S mode (holding of vertical speed), the auto-throttle:
  • A
    can be operated in the SPEED mode (holding of IAS).
  • B
    can be operated in the SPEED (holding of IAS ) or N1 (holding of N1) modes.
  • C
    is not available.
  • D
    can be operated in the N1 / EPR mode (holding of N1 or EPR).

Find in this table, the summary of the auto-throttle modes of operation and the respective flight phase:

Flight phase
Auto-throttle mode
Meaning
Ground / TO roll N1 hold auto-throttle commands full thrust for take-off.
Climb N1 hold auto-throttle commands full thrust for climb and the desired speed is maintained by the AP pitch channel.
Climb with V/S IAS hold auto-throttle continuously regulates the thrust output to maintain the required speed.
Cruise / ALT hold IAS/MACH hold auto-throttle continuously regulates the thrust output to maintain the required speed.
Approach IAS hold auto-throttle continuously regulates the thrust output to maintain the required speed.


On a large commercial aircraft, the auto-throttle system usually has the following modes of operation: SPEED/MACH mode (holding of IAS/Mach number) and THRUST mode (N1 or EPR hold). In fact, the autopilot pitch channel and the auto-throttle system work together. The operating mode of one will determine the operating mode of the other.

For instance, when the auto-throttle is in the THRUST mode it will either command full or idle thrust. The IAS/Mach hold mode is provided by the autopilot pitch channel in the climb/descent mode at a constant IAS or Mach number. The aircraft will basically use its kinetic energy to increase/decrease the rate of climb/descent. When take-off mode is engaged, the auto-throttle sets thrust to a pre-set, fixed thrust setting (N1 or EPR hold). Most of the times it is a computed flex thrust setting, but it can also be TOGA thrust if needed due to performance limitations.

If the auto-throttle system is operating in the SPEED/MACH mode, the pitch channel of the autopilot will work independently. The speed will not be maintained by changing the pitch angle of the airplane anymore - the engines will simply regulate the required amount of thrust for the engines to deliver in order to maintain the required speed. The IAS/Mach hold mode is provided by the auto-throttle in the altitude hold mode (ALT HOLD) or glide path mode.


Another scenario where the auto-throttle can be operated in the SPEED mode (holding of IAS) is when the aircraft is in a climb with the autopilot engaged in the V/S mode (holding of vertical speed). Since the V/S mode is commanding the autopilot pitch channel to achieve a given rate of climb/descent, the pitch channel cannot be used anymore to maintain the speed. Therefore, the desired speed will be maintained by the auto-throttle system.

In order to know in which mode the auto-throttle is engaged, the crew will check the FMA – Flight Mode Annunciator (=mode annunciator panel).

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