Which ADIRU provides the DDRMI compass signal when the primary ADIRU is unavailable?

Which ADIRU provides the DDRMI compass signal when the primary ADIRU is unavailable?

• A. ADIRU 2
• B. ADIRU 3
• C. ADIRU 1
• D. IRS 1

Answer: (B)

This tests how the DDRMI heading signal is sourced when the main ADIRU fails. In the A320, the DDRMI—the cockpit magnetic heading readout—uses data from the ADIRUs. If the primary ADIRU is unavailable, the system switches to the backup ADIRU configured to feed the DDRMI. In this common setup, that backup source is the remaining ADIRU (the third unit). So the DDRMI continues to provide a compass heading even with the first ADIRU down. The other possibilities aren’t used for the DDRMI in this failure mode: the second ADIRU is tied to other systems, and IRS 1 is not the direct source for the DDRMI heading.

This tests how the DDRMI heading signal is sourced when the main ADIRU fails. In the A320, the DDRMI—the cockpit magnetic heading readout—uses data from the ADIRUs. If the primary ADIRU is unavailable, the system switches to the backup ADIRU configured to feed the DDRMI. In this common setup, that backup source is the remaining ADIRU (the third unit). So the DDRMI continues to provide a compass heading even with the first ADIRU down. The other possibilities aren’t used for the DDRMI in this failure mode: the second ADIRU is tied to other systems, and IRS 1 is not the direct source for the DDRMI heading.