Chapter 8 - Autothrottle System

Question 1

Describe the A/T system operation?

Answer 1

FMC sends a command to the A/T servo to move the throttle levers

Rate feedback is supplied back to FMC by A/T servo about the movement of the levers

Throttle resolver angle (TRA) transducers send throttle position data to the electronic engine computers (EEC)

EECs send data to the fuel control units to do a coarse adjustment of the engines

Engines provide an engine pressure ratio (EPR) feedback to the FMC through the engine interface units (EIU)

FMC sends a trim signal along with other data to the EEC to balance the thrust of the engines

Question 2

What are the two thrust management functions of the FMC?

Answer 2

• Autothrottle
• Thrust limit

Question 3

What are the three activities done by the FMC in A/T control?

Answer 3

• Engage logic
• Control laws
• Mode logic

Question 4

When will A/T disconnect message not be sent to the EICAS, in conditions where A/T goes from an engage status to off?

Answer 4

On the ground with thrust reverser applied

Question 5

What will the engage logic circuit do if conditions allow A/T engagement?

Answer 5

Excitation power set to the servomotor

Question 6

Describe a A/T disconnect message?

Answer 6

• Level B message
• Causes master caution annunciation

Question 7

What are the three circuits for A/T control?

Answer 7

• Speed
• Vertical speed
• Thrust

Question 8

When will the control law not change engine thrust, even where there is a deviation between required vertical speed and actual vertical speed?

Answer 8

If the vertical speed is above that needed, the control law does not reduce thrust

Question 9

How does the VNAV speed mode work?

Answer 9

Uses TAS supplied by the FMC performance function compared with ADC airspeed, and an airspeed error is generated

Control law calculates a throttle command with this error

Question 10

What is the thrust limit protection for?

Answer 10

Maintained to make sure engines do not overboost

Question 11

In the thrust circuit, what will the control law not allow?

Answer 11

The engines to go over the V_mo/M_mo limit

Question 12

What modes does A/T work with AP?

Answer 12

• Flight level change
• VNAV
• Go around
• Take-off
• Flare

Question 13

Describe the operation of flight level change?

Answer 13

A/T calculates the vertical speed to capture the mode control panel altitude in 125 seconds

Question 14

What does the first push of the GA mode button do?

Answer 14

Controls vertical speed if go-around (GA) is the active mode for either the AP or flight director

Question 15

What does the second push of the GA mode button do?

Answer 15

Throttles will go to the thrust limit unless the A/T first gets the flap placard speed

Question 16

What are the two limits produced by thrust limit calculation?

Answer 16

• Maximum thrust
• Thrust limit

Question 17

How does maximum thrust differ from thrust limit?

Answer 17

Maximum thrust is not reduced by any mode or derate selection

Question 18

What modes change the thrust limit?

Answer 18

• Take-off (TO)
• Go-around (GA)
• Continuous (CON)
• Climb (CLB)
• Cruise (CRZ)
• Fixed and variable derates for TO and CLB

Question 19

What data does EICAS thrust display show about FMC engine/thrust performance?

Answer 19

• TAT
• Thrust limit mode
• Selected temperature
• Thrust limit
• Maximum thrust limit
• Target N1

Question 20

When are derates used?

Answer 20

Airplane load and runway length do not require full take-off or climb thrust