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Purpose of the third EFIS symbol generator

Aviation Mechanic Airframe (AMA) practice question 8 of 20 on Aircraft Instrument Systems. ACS element AM.II.H.K15 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONAn electronic flight instrument system has independent dedicated pilot and copilot systems plus a third symbol generator. What is the third unit for?

  1. ATo take over if a primary unit fails.
  2. BTo drive the standby engine gauges.
  3. CTo supply data to the autothrottle system.
Show the answer and explanation

Correct answer: A. To take over if a primary unit fails.

  • A — correct A third, backup symbol generator is available to assume operation should one of the two primary units fail.
  • B — incorrect Standby engine indication is a separate combination gauge associated with crew alerting systems, not a function of the backup symbol generator.
  • C — incorrect Autothrottle functions are controlled by the thrust management computer, so supplying that system is not the purpose of the backup symbol generator.

BACKGROUND

An electronic flight instrument system, or EFIS, uses dedicated signal generators to drive two independent displays in the center of the basic T instrument arrangement. Cathode ray tubes replace the attitude indicator and directional gyro, presenting electronic attitude director indicator and electronic horizontal situation indicator information. Through a display controller the pilot selects the modes and screen features to be shown, and independent dedicated pilot and copilot systems are normal. A third, backup symbol generator is available to assume operation should one of the two primary units fail. Because the larger display area allows a vertical airspeed scale on one side, altitude and vertical speed scales on the other, and usually an inclinometer, all of the basic flight instruments can be depicted within the EFIS presentation.

STUDY TIP

For redundancy questions, identify how many primary channels exist and what the spare channel is meant to replace.

KEY POINTS

  • EFIS signal generators drive two independent pilot and copilot displays.
  • A third backup symbol generator takes over if a primary unit fails.
  • EFIS displays can show all of the basic T flight instrument information electronically.

SOURCES

  • sources/ama-airframe/ch10.txt "Electronic Flight Instrument System (EFIS)"
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MORE ON AIRCRAFT INSTRUMENT SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.