OPERATIONS MANUAL

Navigation Systems
Inertial Reference System:
The inertial reference system (IRS) comprises three inertial reference units—
labeled as IRU L, IRU C, IRU R—, and a control module on the overhead
panel containing three mode selectors.
Normally, IRU L is powered by AC bus 3, IRU C by AC bus 1, and IRU R
by AC bus 2. When AC power fails, the respective IRU is supplied by the
APU hot battery bus or—during APU start—by the main hot battery bus.
To conserve battery power, IRU C can operate on battery power only for
5 minutes; thereafter, power is automatically disconnected from IRU C.
Each IRU incorporates ring-laser gyros to sense rotational motion, accelerometers
to sense linear motion, and electronics that process the sensed data:
for example, based on the sensed motion directions and velocities, the IRU
continuously computes the present position. Minor errors accumulate during
the flight, but they typically remain within a small, allowable tolerance.
Normal operation is possible only if a position alignment has been performed
(NAV mode) and if a correct initial position has been set. The position
alignment begins when the crew turns the mode selectors from OFF to NAV.
The aircraft must be stationary until the alignment is completed. This will
take 10 minutes. During this time, the system will sense the earth’s rotation,
and accordingly determine the aircraft’s present true heading and latitude.
In polar regions, the tangential speed of the earth’s rotation is lower, causing
less accurate sensor inputs; therefore, a special high-latitude alignment is
necessary if the aircraft is parked north of 70°N or south of 70°S. In this case,
the crew sets the mode selectors from OFF to ALIGN, leaving them there for
17 minutes, then from ALIGN to NAV.
The FMC provides controls for IRU initialization and testing. During the
alignment, the IRU cannot detect the present longitude. The crew adds this
data by entering the present position on the FMC POS INIT page. The system
will test the entry and apply the following functions:
• If the distance between the entered position and the origin airport of the
active FMC route is greater than 20 nm, the FMC message
IRS POS/ORIGIN DISAGREE appears.
• If the latitude or the longitude of the entered position disagrees with
that of the last stored FMC position by more than 1°, the FMC message
ENTER IRS POS appears.
• If the sine of the entered latitude disagrees with that of the sensed latitude
by more than 0.15, the FMC message ENTER IRS POS appears.
• If the cosine of the entered latitude disagrees with that of the sensed
latitude by more than 0.012, the FMC message ENTER IRS POS appears.
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