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computers.
18 Details for the units on VH-QPA were as follows. Model name: LTN-101 Global Navigation Air
Data Inertial Reference Unit (GNADIRU). Part Number: 465020-0303-0316. ADIRU 1 Serial
Number 4167, ADIRU 2 Serial Number 4687, ADIRU 3 Serial Number 4663.
- 10 -
Figure 5: Simplified air and inertial data path
Air data inertial reference unit (ADIRU)
Each ADIRU was divided in two parts: the air data reference (ADR) part and the
inertial reference (IR) part. Each part could operate separately in the case of the
failure of the other part. Figure 6 shows ADIRU 1 from VH-QPA.
Figure 6: ADIRU 1 from VH-QPA
Inertial
reference
part
Air data
part
Flight
control
computers
(PRIMs
and SECs)
ADIRUs
Air data
sensors
- 11 -
The ADR part of the ADIRU supplied barometric altitude, speed, Mach, angle of
attack (AOA) and temperature information to other aircraft systems. It received air
data from the aircraft’s pitot probes, static pressure ports, AOA sensors, and total
air temperature probes. Air data modules converted pneumatic data from pitot and
static sources into electrical signals for the ADIRUs.
The IR part supplied attitude, flight path vector, track, heading, accelerations,
angular rates, ground speed, vertical speed and aircraft position information to other
systems. Two GPS receivers were connected to the IR part of the ADIRUs.
For most types of data, each ADIRU obtained its data from a different sensor. For
example, ADIRU 1, ADIRU 2 and ADIRU 3 each obtained AOA data from a
different AOA sensor. Each of the PRIMs monitored the outputs from each of the
ADIRUs. Figure 7 provides a simplistic representation of the relationship between
the sensors, ADIRUs and PRIMs using the AOA sensors as an example.
Figure 7: Angle of attack inputs to ADIRUs and PRIMs
Angle of attack sensors
Angle of attack19 (AOA) data was sourced from three AOA sensors (AOA 1, AOA
2, and AOA 3), installed on the forward fuselage. AOA 1 and 2 were installed on
the left and right sides of the fuselage respectively. AOA 3 was installed below
AOA 2. Figure 8 shows the AOA 2 and AOA 3 sensors of VH-QPA.
Each AOA sensor utilised two identical outputs (A and B for each sensor) for
redundancy. The relevant ADIRU checked the A and B signals to ensure that they
agreed. If they agreed, the data was passed on to other systems.
19 Angle of attack: the angle between the wing chord (centreline) and the airflow direction.
AOA PRIM 1
sensor 1
AOA
sensor 2
AOA
sensor 3
ADIRU 3
ADIRU 2
ADIRU 1
PRIM 2
PRIM 3
- 12 -
Figure 8: Right side AOA sensors (AOA 2 and AOA 3)
ADIRS control panel
An ADIRS control panel was located on the overhead panel in the flight deck
(Figure 9). The panel provided local fault indications for the parts of the system. If
there was a fault with the ADR part of an ADIRU (see item 1 in the figure), an
amber fault light illuminated. The relevant part could be deactivated by pressing the
push-button switch below the fault indication light. The IR part of the ADIRU
operated in the same manner (see item 2).
The IR rotary mode selector (see item 3 in Figure 9) allowed the flight crew to
select either the NAV position (supplied full inertial data to aircraft systems for
normal operation), ATT (supplied only attitude and heading information) or OFF
(ADIRU was not energised and ADR and IR information was not available).
Attitude heading and air data selectors
ADIRU 1 was connected to the captain’s displays and ADIRU 2 was connected to
the first officer’s displays. ADIRU 3 could be manually switched to either the
captain’s or first officer’s position in the event of a failure of ADIRU 1 or ADIRU
2. This was achieved using either the ATT HDG switch (for IR parameters) and/or
the AIR DATA switch (for ADR parameters). The switches were located on the
pedestal panel in the flight deck (see item 1 in Figure 10).
- 13 -
Figure 9: ADIRS control panel schematic
Figure 10: ATT HDG and AIR DATA switches
Meteorological information
The Bureau of Meteorology provided the following information regarding the
weather conditions prevailing at the location and time of the occurrence:
• A ridge extended over southern Western Australia with a surface trough
developing along the north and west coasts during the day.
• A sharpening upper level trough extended from the Great Australian Bight
through Perth and into the Indian Ocean.
• Some thunderstorm activity was recorded from about Karratha to just north of
Learmonth, with cloud tops to about flight level (FL) 330 (33,000 ft).
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