The document discusses the Air Data Inertial Reference System (ADIRS) on the Boeing 737 NG. The ADIRS contains two air data inertial reference units (ADIRUs) that each have an air data computer and inertial reference system. The ADIRS provides flight data like position, speed, altitude and attitude to other aircraft systems. It aligns using the aircraft's position, earth's rotation, and gravity to calculate latitude but not longitude.

1. B 737 NG Ground School.
See the aircraft study guide at www.theorycentre.com
The information contained here is for training purposes only. It is of a general nature it is
unamended and does not relate to any individual aircraft. The FCOM must be consulted for
up to date information on any particular aircraft.

2. B737-800
FLIGHT MANAGEMENT SYSTEMS.
INERTIAL REFERENCE SYSTEM

3. Navigation systems include the flight management system (FMS); global
positioning system (GPS); air data inertial reference system (ADIRS); radio
navigation systems (ADF, DME, ILS, marker beacons, and VOR); transponder;
and weather radar.

4. Inertial Reference System.
ADIRU.

5. Air Data Inertial Reference System (ADIRS)
The ADIRS produces flight data such as position, speed, altitude and attitude for
the flight displays, flight management computers, flight controls, engine controls
and all other systems requiring inertial and air data.
The major components of the ADIRS are:
Two air data inertial reference units (ADIRUs)

6. Air Data Inertial Reference System (ADIRS)
The ADIRS produces flight data such as position, speed, altitude and attitude for
the flight displays, flight management computers, flight controls, engine controls
and all other systems requiring inertial and air data.
The major components of the ADIRS are:
Four air data modules (ADMs)

7. Air Data Inertial Reference System (ADIRS)
The ADIRS produces flight data such as position, speed, altitude and attitude for
the flight displays, flight management computers, flight controls, engine controls
and all other systems requiring inertial and air data.
The major components of the ADIRS are:
One inertial system display unit (ISDU)
One dual mode select unit (MSU)

8. Air Data Inertial Reference System (ADIRS)
The ADIRS produces flight data such as position, speed, altitude and attitude for
the flight displays, flight management computers, flight controls, engine controls
and all other systems requiring inertial and air data.
The major components of the ADIRS are:
Six static ports
Three pitot probes
Two alpha vanes
One total air temperature probe

9. Inertial system display unit (ISDU)
Dual mode select unit (MSU)
Aft Overhead panel.

10. VHF NAV Radio IRS Source select switches
and FMC source select switch.
Forward Overhead panel.

11. There are two air data Inertial Reference Units.

12. The ADIRU has two systems in one box.
The Air Data Computer (AD)
Inertial Reference System (IRS)
There are two air data Inertial Reference Units.

13. AIR DATA

14. AIR DATA
4 Air Data Modules Convert Atmospheric data into Digital signals.

15. AIR DATA
4 Air Data Modules Convert Atmospheric data into Digital signals.
The ADM’s send digital information to the ADIRU.
The ADIRU send that information to the DEU’s for the CDS and other
aircraft systems.

16. Inertial Reference System.
3x
Accelerometers
3 X Ring
Laser Gyros

17. RING LASER GYRO
A ring laser gyroscope (RLG) Uses two counter-propagating light
beams to detect rotation.

18. YAW
ROLL
PITCH
Accelerometers

19. The idea behind the ring laser gyroscope actually dates back to 1913, when a
French physicist, Georges Sagnac, experimented with rays of light moving in
opposite directions around a circular cavity on a turntable. Sagnac showed that
when he rotated the turntable, the light traveling with the rotation arrived at a
target slightly after the light traveling against the rotation. He believed he had
proven the existence of ether in space. In fact, he was demonstrating a
property of light that came to be understood much better with the invention
of the laser in the 1950s.
A laser (light amplification by stimulated emission of radiation) operates by
exciting atoms in a plasma to release electromagnetic energy, or photons, in a
cavity. Each end of the cavity reflects the energy back and forth, and it forms a
standing wave pattern. The wave frequency—its pattern of peaks and
troughs—is determined in part by the length of the cavity.
“If you had a linear laser and the light bounced back and forth between two
mirrors at either end, and if you [increased] the spacing between those two
mirrors slightly, you would actually stretch the wavelength of the light in the
cavity,
“What causes the light to stretch? The fact that it had to go farther. Because
when it comes back, it has to come back exactly the same way it left,
Sagnac’s counter-rotating beams of light are analogous to beams in a linear
cavity. If the turntable rotates clockwise, the beam traveling clockwise has
farther to go to catch its starting point; the path of the counterclockwise beam
is shorter.

20. 2 opposing beams

21. Reflected from mirrors

22. Rotation moves mirrors close to Green
beam and further from Blue beam

25. Receiver collects Green Beam before
Blue. The light frequency is shifted.

26. In a ring laser gyroscope, the two counter-rotating beams are
channelled to a photo detector. If the vehicle is not rotating, the
beams remain in phase. If rotation is occurring about one axis, one
beam continuously changes phase with respect to the other. A diode
translates that moving interference pattern into digital pulses, each
pulse representing an angle of rotation typically .0005 degree per
pulse. The rate at which the pulses are produced is also a measure of
the rate of rotation.

27. During alignment senses EARTH GRAVITY and EARTHS ROTATION ONLY

28. During alignment senses EARTH GRAVITY and EARTHS ROTATION ONLY
It can only calculate its own LATITUDE!
It is NOT able to calculate LONGITUDE! Am I in Beijing 40⁰ N or Madrid 40⁰ N
Earths rotational velocity gives position North or South of the equator.
(Velocity at the poles (theoretically) Zero. At the equator around 900 miles
and hour.

29. Compares last LONGITUDE stored in memory ( Last
Position) with manually entered Longitude.
If there is more than 1° Error
The CDU Scratch pad Message <ENTER IRS POSITION.>
Is asking for conformation of the manually entered
position.

30. Senses EARTH ROTATION VELOCITY and calculates own LATTITUDE.
Compares own Latitude with manually entered latitude.
If more than 1° error CDU Scratch pad message
<ENTER IRS POSITION>
Again asking for confirmation that the manually entered position is
correct.

31. The final check during alignment is the stored
position of the origin airport entered in the CDU
and the IRS present position.

32. CDU MESSAGE “VERIFY POSITION” ONLY ON THE GROUND!
Indicates ORIGIN AIRPORT POSITION entered in the
CDU and IRS POSITION DIFFER by more than 4 nm.

33. Inertial Reference System (IRS) Mode Selector
OFF –
• alignment is lost
• all electrical power is removed from the system after a 30 second shutdown cycle.
ALIGN –
• rotating the selector from OFF to ALIGN initiates the alignment cycle
• rotating the selector from NAV to ALIGN automatically updates alignment and zeroes ground
speed error.
NAV (detent position) –
• system enters the NAV mode after completion of the alignment cycle and entry of present
position
• in NAV mode, all IRS information is available to airplane systems for normal operations.
ATT – provides only attitude and heading information:
• attitude information is invalid (attitude flag in view) until ALIGN light is extinguished
• heading information is invalid (heading flags in view) until the actual magnetic heading is
manually entered after the ALIGN light is extinguished

34. FAULT Light
Illuminated (amber) – a system fault affecting the related
IRS ATT and/or NAV modes has been detected.

35. ALIGN Light
Illuminated (white) –
• steady – the related IRS is operating in the ALIGN mode, the initial ATT
mode, or the shutdown cycle
• flashing – alignment cannot be completed due to IRS detection of:
• significant difference between previous and entered positions or an unreasonable
present position entry
• no present position entry.
Extinguished –
• IRS not in ALIGN mode
• with mode selector in NAV, alignment is complete, and all IRS information is available
• with mode selector in ATT, attitude information is available.
Heading information is available following entry of initial magnetic heading.

36. ON DC Light
Illuminated (amber) –
• the related IRS is operating on DC power from the switched hot battery
bus (AC power not normal)
• if on the ground, the ground–call horn in the nose wheel well sounds,
providing an alert that a battery drain condition exists
• momentary illumination is normal during alignment self–test.

37. DC FAIL Light
Illuminated (amber) –
• DC power for the related IRS is not normal
• if the other lights are extinguished, the IRS is operating normally on AC power.

38. IRS Alignment
An IRS must be aligned and initialized with airplane present position before it can enter the
navigation mode. The present position is normally entered through the FMC CDU. If the
present position cannot be entered through the FMC CDU, it may be entered through the ISDU
keyboard. The airplane must remain stationary during alignment.

39. IRS Alignment
An IRS must be aligned and initialized with airplane present position before it can enter the
navigation mode. The present position is normally entered through the FMC CDU. If the
present position cannot be entered through the FMC CDU, it may be entered through the ISDU
keyboard. The airplane must remain stationary during alignment.
Normal alignment between 78 degrees 15 minutes North or South is initiated by rotating the
MSU switch from OFF to NAV. The IRS performs a short power test, during which the ON DC
light illuminates. When the ON DC light extinguishes and the ALIGN light illuminates, the
alignment process begins. Airplane present position should be entered at this time. Alignment
time varies from five minutes at the equator to seventeen minutes at 78 degrees 15 minutes
North or South latitude.

40. IRS Alignment
An IRS must be aligned and initialized with airplane present position before it can enter the
navigation mode. The present position is normally entered through the FMC CDU. If the
present position cannot be entered through the FMC CDU, it may be entered through the ISDU
keyboard. The airplane must remain stationary during alignment.
Normal alignment between 78 degrees 15 minutes North or South is initiated by rotating the
MSU switch from OFF to NAV. The IRS performs a short power test, during which the ON DC
light illuminates. When the ON DC light extinguishes and the ALIGN light illuminates, the
alignment process begins. Airplane present position should be entered at this time. Alignment
time varies from five minutes at the equator to seventeen minutes at 78 degrees 15 minutes
North or South latitude.
Magnetic variation between 82 degrees north and 82 degrees south is stored in each IRS
memory. The data corresponding to the present position are combined with the true heading
to determine magnetic heading.

41. IRS Alignment
An IRS must be aligned and initialized with airplane present position before it can enter the
navigation mode. The present position is normally entered through the FMC CDU. If the
present position cannot be entered through the FMC CDU, it may be entered through the ISDU
keyboard. The airplane must remain stationary during alignment.
Normal alignment between 78 degrees 15 minutes North or South is initiated by rotating the
MSU switch from OFF to NAV. The IRS performs a short power test, during which the ON DC
light illuminates. When the ON DC light extinguishes and the ALIGN light illuminates, the
alignment process begins. Airplane present position should be entered at this time. Alignment
time varies from five minutes at the equator to seventeen minutes at 78 degrees 15 minutes
North or South latitude.
Magnetic variation between 82 degrees north and 82 degrees south is stored in each IRS
memory. The data corresponding to the present position are combined with the true heading
to determine magnetic heading.
If the latitude/longitude position is not within 4 NM of the origin airport, the CDU
scratchpad message VERIFY POSITION is displayed. If the entered
latitude/longitude position does not pass the IRS internal comparison tests, the
scratchpad message ENTER IRS POSITION is displayed.

42. IRS Alignment
The flashing ALIGN light alerts the crew that the position entered does not pass
one of the two internal comparison tests and should be checked for accuracy. If the
entered position does not agree with the last stored position, the first internal test
is failed, and the ALIGN light will flash. If the same position is re-entered, the IRS
will accept the position and continue the alignment process. A second internal
position test compares the entered latitude with the system-computed latitude. If
this test is failed, the ALIGN light will again flash. If two consecutive entries of
the same position do not pass the second internal position test, the FAULT light
will illuminate. If the test is passed, the IRS will proceed to complete the
alignment process and enter NAV mode.
During transit or through–flight stops with brief ground times, a thirty second fast
realignment and zeroing of ground speed error may be performed by selecting
ALIGN while the airplane is parked. Present position should be simultaneously
updated by manually entering latitude and longitude prior to selecting NAV.
Note: If the airplane is moved during alignment or fast realignment, the IRS
automatically begins the full alignment process.

43. NORMAL POWER SUPPLY
Left ADIRU AC Standby bus. Right ADIRU AC transfer bus 2

44. DC BACKUP SUPPLY
Both ADIRU’s supplied from the switched hot battery bus

45. DC BACKUP SUPPLY
Both ADIRU’s supplied from the switched hot battery bus
With the Aircraft on the ground. The GROUND CREW CALL HORN IN
THE NOSE WHEEL WELL will sound after a 20 second delay!
IRU on DC only!

46. DC BACKUP SUPPLY
Both ADIRU’s supplied from the switched hot battery bus
With the Aircraft on the ground. The GROUND CREW CALL HORN IN
THE NOSE WHEEL WELL will sound after a 20 second delay!
IRU on DC only!
Both AC and DC must be available when first switched ON.
There is a power supply test before alignment can begin.

47. IRU OUTPUTS

48. IRS MESSAGES ARE DISPLAYED IN THE CDU SCRATCHPAD

49. The ADIRU works only in True Heading.
It contains a DATA BASE OF MAGNETIC VARIATION FROM 82° N to 82° S
Magnetic variation is added or subtracted from true using the data
base.
The aircraft is limited to operations within 82° N to 82° S unless
equipped with a polar navigation suite.

50. For normal operation both IRS are in NAV mode. With the IRS
Transfer switch in NORMAL the left IRS is used by the left FMC
and is shared by both Flight Control computers.
The right IRS is used by the right FMC which works in
synchronisation with the left FMC
The captains attitude and Vertical speed come from the left IRS.
The F/O’s come from the right.

51. For normal operation both IRS are in NAV mode. With the IRS
Transfer switch in NORMAL the left IRS is used by the left FMC
and is shared by both Flight Control computers.
The right IRS is used by the right FMC which works in
synchronisation with the left FMC
Should either IRS fail, the IRS transfer switch is used to switch all
associated systems to the functioning IRS.

52. Data Displays
Two windows display data for the IRS selected with the system display selector
• type of data displayed is normally determined by the display selector
• keyboard entry of present position or magnetic heading overrides the selected display
• last digit of each window is for a decimal place (tenths).

53. Display Selector (DSPL SEL)
TEST (spring–loaded to TK/GS) –
• all lights in data displays and on the mode selector unit
momentarily illuminate, followed by a 10 second self–test
• use only during alignment.
TK/GS –
• left window displays true track (course)
• right window displays present ground speed (knots).

54. Display Selector (DSPL SEL)
PPOS –
• left window displays present latitude
• right window displays present longitude.
WIND –
• left window displays present inflight true wind direction
• right window displays present inflight wind speed (knots).

55. Display Selector (DSPL SEL)
HDG/STS –
• left window displays present true heading
• right window displays any applicable maintenance status codes

56. Display Selector (DSPL SEL)
HDG/STS –
• left window displays present true heading
• right window displays any applicable maintenance status codes
• during alignment, right window displays minutes remaining until
alignment is complete. For alignments greater than 15 minutes, the
window displays 15 until the time remaining reaches 14 minutes. The
display then counts down in one minute intervals.

57. Keyboard
Push –
• alpha keys:
• data displays are controlled by the keyboard when the N, S, E, W (latitude/longitude)
or H (heading) keys are pushed
• pushing an alpha key arms the keyboard for numeric entries.
• numeric keys:
• permit manual entry of present position when ALIGN light is illuminated
• permit manual entry of magnetic heading when either mode selector is in ATT.

58. System Display (SYS DSPL) Selector
L – selects left IRS for the data displays.
R – selects right IRS for the data displays.
Enter (ENT) Key
Illuminated (white) – N, S, E, W, or H entries are
being keyed.
Push – keyed data is entered into IRS following
completion of valid self–test for reasonableness.

59. Clear (CLR) Key
Illuminated (white) – an ENT attempt has failed (entry not
accepted by IRS).
Push – clears data display of any data not yet entered or
accepted. If illuminated, cue lights extinguish.

60. If the FAULT light on the IRS mode selector unit illuminates, this indicates:
a) A fault with the DC power supply has been detected.
b) Manual entry of an invalid present position.
c) IRS alignment for the respective IRS is lost.
d) A system fault affecting IRS ATT or NAV modes is detected.

61. If the FAULT light on the IRS mode selector unit illuminates, this indicates:
a) A fault with the DC power supply has been detected.
b) Manual entry of an invalid present position.
c) IRS alignment for the respective IRS is lost.
d) A system fault affecting IRS ATT or NAV modes is detected.
FAULT Light
Illuminated (amber) – a system fault affecting the related IRS
ATT and/or NAV
modes has been detected.

62. If you move the IRS mode selector to ATT during flight you will have
what information?

63. If you move the IRS mode selector to ATT during flight you will have
what information?
When selecting ATT there is a 2 second delay to
prevent accidental selection!

64. If you move the IRS mode selector to ATT during flight you will have
what information?

65. HOW is the Heading Manually entered?

66. There are two ways in which manual heading can be
entered.
1. Is on the POS INIT page
Only displayed when either ADIRU is in ATT mode.

67. 2. Is on the ISDU
Select HDG/STS
Select H for Heading.
Then Enter Magnetic Heading.

68. The IRS alignment time is;
a) 5 minutes at the equator increasing to 10 minutes at 70 degrees latitude.
b) 17 minutes at any latitude.
c) 10 minutes at the equator reducing to 5 minutes at 70 degrees latitude.
d) 10 minutes at the equator increasing to 15 minutes at 70 degrees latitude.

69. The IRS alignment time is;
a) 5 minutes at the equator increasing to 10 minutes at 70 degrees latitude.
b) 17 minutes at any latitude.
c) 10 minutes at the equator reducing to 5 minutes at 70 degrees latitude.
d) 10 minutes at the equator increasing to 15 minutes at 70 degrees latitude.

70. The IRS alignment time is;
a) 5 minutes at the equator increasing to 10 minutes at 70 degrees latitude.
b) 17 minutes at any latitude.
c) 10 minutes at the equator reducing to 5 minutes at 70 degrees latitude.
d) 10 minutes at the equator increasing to 15 minutes at 70 degrees latitude.
Normal alignment between 78 degrees 15 minutes North or South
is initiated by rotating the MSU switch from OFF to NAV. The IRS
performs a short power test, during which the ON DC light
illuminates. When the ON DC light extinguishes and the ALIGN light
illuminates, the alignment process begins. Airplane present
position should be entered at this time. Alignment time varies from
five minutes at the equator to seventeen minutes at 78 degrees 15
minutes North or South latitude.

71. WHY does it take longer to ALIGN at HIGH LATITTUDES?
The IRS alignment time is;
a) 5 minutes at the equator increasing to 10 minutes at 70 degrees latitude.
b) 17 minutes at any latitude.
c) 10 minutes at the equator reducing to 5 minutes at 70 degrees latitude.
d) 10 minutes at the equator increasing to 15 minutes at 70 degrees latitude.

72. WHY does it take longer to ALIGN at HIGH LATITTUDES?
EARTHS ROTATIONAL VELOCITY IS SLOWER!
The IRS alignment time is;
a) 5 minutes at the equator increasing to 10 minutes at 70 degrees latitude.
b) 17 minutes at any latitude.
c) 10 minutes at the equator reducing to 5 minutes at 70 degrees latitude.
d) 10 minutes at the equator increasing to 15 minutes at 70 degrees latitude.
Normal alignment should complete within 10 minutes at most
airports.
If there is time pressure remember to select HDG/STS on the IRS
Display panel to see the remaining time for alignment.

73. The ON DC light on the left IRS mode selector unit
comes on. This indicates that the left IRS is powered
from which source?

74. The ON DC light on the left IRS mode selector unit
comes on. This indicates that the left IRS is powered
from which source?
The left IRS is operating on DC power from the
switched hot battery bus.

75. During transit or through flight with a brief ground time, a
fast realignment of the IRS will normally be completed in
what time?

76. During transit or through flight with a brief ground time, a
fast realignment of the IRS will normally be completed in
what time?

77. The right IRS is electrically powered from:
a) AC transfer bus 1 or the switched hot battery bus for a maximum of 5 minutes.
b) AC Transfer bus 2 or the switched hot battery bus for a maximum of 5 minutes.
c) AC transfer bus 1 or the hot battery bus for a maximum of 5 minutes.
d) AC transfer bus 2 or the hot battery bus for a maximum of 5 minutes.

78. The right IRS is electrically powered from:
a) AC transfer bus 1 or the switched hot battery bus for a maximum of 5 minutes.
b) AC Transfer bus 2 or the switched hot battery bus for a maximum of 5
minutes.
c) AC transfer bus 1 or the hot battery bus for a maximum of 5 minutes.
d) AC transfer bus 2 or the hot battery bus for a maximum of 5 minutes.

79. The right IRS is electrically powered from:
a) AC transfer bus 1 or the switched hot battery bus for a maximum of 5 minutes.
b) AC Transfer bus 2 or the switched hot battery bus for a maximum of 5
minutes.
c) AC transfer bus 1 or the hot battery bus for a maximum of 5 minutes.
d) AC transfer bus 2 or the hot battery bus for a maximum of 5 minutes.

80. DC BACKUP SUPPLY
Aircraft on the ground. What indications do you get for IRU on DC only?

81. DC BACKUP SUPPLY
Aircraft on the ground. What indications do you get for IRU on DC only?
GROUND CREW CALL HORN IN THE NOSE WHEEL WELL!

82. WHERE ARE IRS MESSAGES DISPLAYED?

83. WHERE ARE IRS MESSAGES DISPLAYED?
CDU Scratch pad

84. The FMC alerting message VERIFY POSITION indicates:
a) The aircraft is off of the flight plan course.
b) Entered position information is contradictory.
c) IRS position information is INVALID.
d) Left FCM disagrees with the right FMC

85. The FMC alerting message VERIFY POSITION indicates:
a) The aircraft is off of the flight plan course.
b) Entered position information is contradictory.
c) IRS position information is INVALID.
d) Left FCM disagrees with the right FMC
ORIGIN AIRPORT POSITION entered in the CDU
and IRS POSITION DIFFERENT by more than 4 nm.

86. The END of IRS
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