Navigation

Question 1

When must a COMPLETE IRS alignment be performed?

Answer 1

• First flight of the day; or
• Crew change; or
• Departure apt located between 2ºN and 2ºS; or
• GPS not available and NAVAID coverage is poor on expected route; or
• GPS not available and flight time >3h.

Question 2

When must a FAST IRS alignment be performed?

Answer 2

• When a complete alignment is not necessary; and

- Difference between IRS position and FMGC position is AT OR ABOVE 5NM (mcdu pos monitor page)

Question 3

Checking NAV accuracy during CRZ (IF GPS PRIMARY LOST)

Answer 3

• If check is positive (error < 3NM): FM position is reliable: (use ND (ARC or NAV) and managed lateral guidance;
• If check is negative (error > 3NM): Fm position is not reliable. (Use raw data for navigation and monitor it).
  
  • If there is a significant mismatch between the display and the real position: Disengage MANAGED NAV mode and use raw data navigation (possibly switching to ROSE VOR, so as not to be misled by FM data).

Navigation accuracy must be monitored, particularly if:

• IRS only navigation; or
• PROG page displays LOW accuracy; or
• NAV ACCUR DOWNGRAD message appears on the MCDU

Question 4

Complete IRS alignment, what happens?

Answer 4

During a complete alignment, IRSs use the gravity and earth rotation to determine the aircraft attitude and true heading, and IRSs estimate a current aircraft latitude.

The IR mode selectors must be OFF for more than 5s. Then, the flight crew sets the IR mode selectors to the NAV mode.

Question 5

Fast IRS alignment, what happens?

Answer 5

During a fast alignement, IRSs reset the ground speed and some internal filters to 0, but IRSs do not estimate the aircraft position.
The flight crew sets the IR mode selectors to OFF, then back to NAV mode within 5 s