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Figure 17-1). A dual MSU (Mode Select Unit) installed in the cockpit provides control
functions for the two IRS systems. An optional third IRS including a single MSU may
also be installed.
Components and Operation
Inertial Reference Unit (IRU)
Each IRU uses three laser gyros and three linear accelerometers to sense aircraft
attitude and acceleration. After alignment is successfully completed, the IRU outputs
the following data for use by other aircraft systems:
• Attitude (pitch, roll, yaw)
• Heading (True and magnetic)
• Inertial position (latitude and longitude)
• Ground speed
• Wind direction and speed
• Three-axis body rate and acceleration (include an inertial vertical velocity
component to the VSI)
• Flight path data
NAVIGATION SYSTEMS
17-4 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
Each IRS receives true airspeed and altitude from the on-side air data system. In the
event of a failure of the on-side air data input, the cross-side input is automatically
selected.
Power Supply
Each IRU has two power sources, a 115-volt AC source and a backup 28-volt DC
source. Either source is sufficient for operation but both are required for initial startup.
Inside the IRU, the battery backup circuit automatically switches to the backup 28-volt
DC source when the normal 115-volt AC source is not available. During startup, the
battery backup circuit verifies that backup power is available by momentarily
switching off the 115-volt AC input. The IRS 1(2)(3) ON BATT advisory EICAS
message is displayed when an IRU is operating on DC power only.
115-volt, 400-Hz AC power is also supplied to the IRU cooling fans.
Mode Select Unit (MSU)
The mode select unit provides the necessary switches that select the modes of
operation of each IRS. All IRS-related annunciations are displayed as EICAS
messages or FMS CDU scratchpad messages.
Each MSU switch has three positions: OFF, NAV, and ATT. Power is applied to the
IRU whenever the respective switch is selected to the NAV or ATT position.
IRU # AC POWER BACKUP DC POWER
1 AC Essential Bus Battery Bus
2 AC Bus 2 DC Essential Bus
Battery Bus
3 AC Bus 1 Battery Bus
IRU Power Sources
Table 17-1
NAVIGATION SYSTEMS
For Training Purposes Only
Jan 04
17-5
P I LOT T R A I N I N G GU I D E
Figure 17-1 illustrates the dual MSU (for IRS 1 and 2) and the single MSU for the
optional third IRU.
Mode Select Unit
Figure 17-1
IRS Reversionary Mode Selectors
IRS reversionary mode selector switches are provided to control IRU outputs to the
pilot’s and copilot’s PFD and MFD displays. In the event of an IRU failure, the
selectors can be used to restore IRU data to the affected side’s displays.
With the reversionary switches at NORM, the PFD and MFD displays normally
receive data from their on-side IRS. In a dual-IRS installation, selecting the
reversionary selector to 1 or 2 results in both the pilot’s and copilot’s displays
receiving information from the selected IRU. In a triple-IRS installation, selecting
ALTN replaces the appropriate IRU data with IRU 3 data.
Single-source or cross-side IRS source annunciations are displayed on the affected
PFDs (see Figure 17-2).
Figure 17-3 shows the standard reversionary panel and optional reversionary panels
based on the indicated avionics configuration.
P604_17_002
OFF
NAV
ATT OFF
NAV
ATT
1 IRS 2
OFF
NAV
ATT
3 IRS
NAVIGATION SYSTEMS
17-6 For Training Purposes Only
Jan 04
P I LOT T R A I N I N G GU I D E
PFD IRS Source Annunciations
Figure 17-2
1.0
260
220
240
280
20
10
10
30
300
400
500
600
8000
TERM
MSG
FMS1
TRU
NO FLIGHT PLAN
TCAS
OFF
29.85 IN
2400M
GS
RA
LNV1
LOC 1
DR FLC
ALTS
FMS1
25
DTK 279
.3
HOKKE
NM
4
4
2
2
1
1
25
3.6
2500M
250
N
33
30
W
24
6
3
7
12000
VOR2
P604_17_067
IRS Source
Annunciation
IRS3
NAVIGATION SYSTEMS
For Training Purposes Only
May 03
17-7
P I LOT T R A I N I N G GU I D E
Source Selector Panel - IRS Reversionary Mode Selectors
Figure 17-3
L IRS
NORM ALTN
R IRS
NORM ALTN
L IRS R IRS FMS3
1 2
NORM
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