IRS - Inertial Reference System *** Flashcards
How do you select the IRU mode selector from NAV?
• The selector must be pulled out
What is the function of the ALIGN position of the IRU Mode Selector?
- Initiates alignment
* Removes sensor errors when selected from NAV mode
How long does it take for full alignment of the IRS?
• aprox 10 minutes because is depend of the latitude of the present position
When is full alignment of the IRS required?
• When the time from the last full alignment to the next expected arrival time exceeds 18 hours.
How do you accomplish a fast alignment of the IRS?
• 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 a present position. However, greater navigational accuracy is attained by entering present position
What does the IRS ON Battery (BAT) Light mean, when displayed on the Overhead Maintenance Panel?
• IRS operating on backup electrical power (APU hot battery bus)
Explain what happens when the alignment of the IRS is lost in flight.
- The navigation mode is inoperative for the remainder of the flight.
- Attitude information can be obtained by moving the selector to ATT.
What happens if the center IRU is operating on DC power?
• The center IRU operates on DC power for 5 minutes, then shuts down.
What are the sources of power for the IRSs?
AC power 1,2,3 – Primary, DC power (APU Hot Battery Bus) Backup
How are the IRSs affected in the event of loss of Main AC power?
They revert to DC backup power (via the APU hot battery bus) Center only runs for 5 minutes.
Horn will sound on the landing gear
What are 4 indications on the PFD that are from the IRU
Attitude
Heading
V/S
Green A/S trend Arrow
What bus is the IRU on?
AC 3 - L
AC 1 - C
AC 2 - R
IRS position update priority
Where can it be found
LOC and GPS Loc and DME-DME Loc and colocated VOR/DME LOC GPS DME-DME Colocated VOR/DME
Fcom 2 11.31.8
The FMC changes to LOC updating when
the airplane is less than 6,000’ above the localizer navaid elevation
the airplane is less than 20nm from the front course
or 12nm back course
Can IRS fast alignment be accomplished without entering the present position?
YES, but greater nav accuracy is attained by doing so.
With regard to IRS, the primary mode of navigation is?
GPS Updating
What is some of IRS calculated information?
- Position
- Accel
- Track
- VS
- T/M HDG
- Wind
- ATT
- GS
IRS backup power source?
DC PWR via APU HOT BATTERY BUS
IRS alignment determines A/C relationship to?
True North
Why must A/C remain stationary for IRS alignment?
Ensures only motion detected is by Earth’s rotation
How is the IRS alignment process accomplished on the ground?
IRS switches are moved from OFF to NAV and crew enters IRS POS in FMC.
With regard to the IRS, what does it mean when FMC LAST POS changes to the new PRES POS on INIT PAGE?
The first IRU has entered navigation mode.
How do you know if all 3 IRUs have entered navigation mode?
- SET IRS POS disappears in FMC
- Memo message disappears on EICAS
- Appearance of HDG and ATT info on PFD
Most common reason to perform IRS fast realignment is?
Remove ATT, groundspeed, and track errors. It has no effect on true heading calculations.
When IRU ATT mode switch is selected, what must A/C do while IRU attitude alignment is detected?
Fly straight and level for 30 seconds.
When in IRU ATT mode, the system will not supply heading information until?
Current heading entered on FMC POS INIT page
What will IRS do if motion is detected during init?
Stops. Alignment continues and completes in approx 10 mins after motion has stopped.
When does IRS enter SPLIT IRS OPERATIONS?
When aircraft passes north of 84N or south of 84S
How long does a high latitude (POLAR) alignment take?
17 minutes
