IRS - INERTIAL REFERENCE SYSTEM

Question 1

How do you select the IRU mode selector from NAV?

Answer 1

• The selector must be pulled out

Note:FCOM, VOL 2; 11.10.9

Question 2

What is the function of the ALIGN position of the IRU Mode Selector?

Answer 2

• Initiates alignment
• Removes sensor errors when selected from NAV mode

Note:FCOM, VOL 2; 11.10.9

Question 3

How long does it take for full alignment of the IRS?

Answer 3

• 10 minutes

Note:FCOM, VOL 2; 11.20.3

Question 4

When is full alignment of the IRS required?

Answer 4

• When the time from the last full alignment to the next expected arrival time exceeds 18 hours.

Note:FCOM, VOL 2; 11.20.3

Question 5

How do you accomplish a fast alignment of the IRS?

Answer 5

• Airplane must be parked
• Position the selectors to ALIGN
• Enter present position
• Reposition the selectors to NAV
• Takes 30 seconds
• Fast alignment can be accomplished without entering present position.
  However, greater navigational accuracy is attained by 
  entering present position

Note:FCOM, VOL 2; 11.20.3

Question 6

What does the IRS ON Battery (BAT) Light mean, when displayed on the Overhead Maintenance Panel?

Answer 6

• IRS operating on backup electrical power (APU hot battery bus)

Note:FCOM, VOL 2; 11.10.10

Question 7

Explain what happens when alignment of the IRS is lost in flight.

Answer 7

• The navigation mode is inoperative for the remainder of the flight.
• Attitude information can be obtained by moving the selector to ATT.

Note:FCOM, VOL 2; 11.20.3

Question 8

What happens if the center IRU is operating on DC power?

Answer 8

• The center IRU operates on DC power for 5 minutes, then shuts down.

Note:FCOM, VOL 2; 11.20.4

Question 9

What are the sources of power for the IRSs?

Answer 9

AC power 1,2,3 – Primary, DC power (APU Hot Battery Bus) Backup

Question 10

How are the IRSs affected in the event of loss of Main AC power?

Answer 10

They revert to DC backup power (via the APU hot battery bus) Center only runs for 5 minutes.

Question 11

How do I fast align the IRSs

Answer 11

With the aircraft parked position the IRS selector switches to align, enter the present position and reset the switches to NAV. Aligns in 30 seconds

Question 12

What are 4 indications on the PFD that are from the IRU

Answer 12

Attitude
Ground Speed
Slip Skid Indicator
FPV

Question 13

What bus is the IRU on?

Answer 13

AC 3 - L
AC 1 - C
AC 2 - R

Question 14

IRS position update priority

Where can it be found

Answer 14

LOC and GPS
Loc and DME-DME
Loc and colocated VOR/DME
LOC
GPS
DME-DME
Colocated VOR/DME

11.31.8

Question 15

The FMC changes to LOC updating when

Answer 15

the airplane is less than 6,000’ above the localizer

the airplane is less than 20nm from the front course
or 12nm back course