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11. Navigation

11.1. Air Data and Inertial Reference System (ADIRS)
11.2. Radio navigation
11.3. Standby instruments
11.4. EGPWS
11.5. Radio altimeter

11.1. Air Data and Inertial Reference System (ADIRS)

There are three identical Air Data Intertial Reference Units (ADIRU). In general ADIRU 1 and ADIRU 2 supply on-side systems and ADIRU 3 is a hot spare. Each ADIRU combines an Air Data Reference (ADR) computer and a laser gyro based Inertial Reference (IR) system. The two sub-units are completely independent. The ADR part gathers data from aircraft probes and sensors, and provides the following data to other systems:

  • Airspeed and Mach number

  • Temperature

  • Barometric altitude

  • Angle of attack

  • Overspeed warnings

The IR part provides the following data:

  • Attitude

  • Heading

  • Aircraft position

  • Track

  • Acceleration

  • Ground speed

  • Flight Path Vector

The ADIRS control panel is located at the top left of the overhead panel. ADR controls are at the bottom of the panel and IR controls are at the top, with the rotary selectors affecting both sub-systems. The ON BAT light indicates that the IRs are being powered by the aircraft batteries. This light will illuminate for a short time during the self-test at the start of the alignment sequence. The ALIGN lights are illuminated steady during the alignment sequence, and should extinguish after approximately ten minutes when the sequence is complete. The lights will flash white under the following conditions:

  • IRU alignment fault

  • No present position entered on the INIT page of the MCDU after 10 minutes from the start of alignment sequence.

  • The entered position differs from the shutdown position differs by more than 1°.

A display, keypad and selectors are provided for viewing IR information directly on the ADIRS panel. The TEST position tests the lights on the keypad and provides a test pattern on the display. If HDG is selected on the display during alignment, a time to navigation (TTN) readout is shown.

IR realignment is usually uneccessary on turn arounds. If an IR has a residual ground speed >5kt (shown on one of the NDs for IR1 and 2 and on the ADIRS panel for IR3) a quick align should be carried out. This is acheived by selecting the OFF position on the IRS selectors then reselecting NAV within 5 seconds.

The ADIRS have a comparator feature and will alert the crew to differences in the attitudes displayed on the PFDs. The faulty data can be determined by cross checking with the standby attitude indicator and the third ADIRU can then be selected as required.

11.2. Radio navigation

FMGCs auto-tune VORs, ILSs and DMEs for position updating. The ADFs are only auto tuned under specific circumstances. Manual tuning of the aids is via the RAD NAV MCDU page. When aids are manually tuned, FMGC auto tuning continues in the background. If an ILS approach is selected the PFDs show the on-side ILS and the NDs show the off-side ILS.

If both FMGCs fail, radio aids may be tuned using the nav mode of RMP1 and RMP2. The RMPs tune their on-side VORs, DMEs and ADFs. The ILS frequency tuned on either RMP is sent to both ILSs. When a NAV key is pressed on an RMP, the RADIO NAV page blanks, showing only the titles. DME information will ot be displayed on the PFD for an ILS/DME tuned on an RMP.

Standby display of radio data is provided on the DDRMI. This combines a traditional RMI presentation with raw DME data for any selected VORs. The compass card displays the bearing supplied by ADIRU 1.

11.3. Standby instruments

A standby ASI, altimeter and attitude indicator are provided to the right of the captain's ND and a pull down standby compass is fitted on top of the windshield centre post. The standby attitude indicator is the only standby instrument requiring electrical power. It will operate for approximately five minutes after a total electrical failure.

11.4. EGPWS

The EPGWS normal inputs are:

  • RA1

  • ADIRS1

  • ILS1

  • FMGC1

  • LGCIU1

The basic GPWS has no forward looking capability - it mainly monitors RA1 for potentially hazerdous values or trends. The warnings and alerts provided are:

High descent rate at low level"Sink rate""Whoop whoop pull up"
Rising ground"Terrain, Terrain""Whoop whoop pull up"
Rate of descent during initial climb or go around"Don't sink"None
Gear and/or flaps retracted close to ground"Too low terrain", then "Too low gear" or if gear is down "Too low, flaps"None
Significantly below ILS glideslope"Glideslope""Glideslope" (louder)

The "Glideslope" alert and warning are accompanied by an amber G/S light on the GPWS warning light. All other alerts and warnings give a red GPWS warning.

Some basic GPWS warnings may be supressed using the GPWS panel on the left side of the overhead panel. The most common supression is GPWS FLAP 3, used for flap 3 landings. FLAP MODE OFF is used for landings below flap 3. G/S MODE inhibits glideslope warnings. The system can be completely deactivated by pressing the SYS button.

The EGPWS system provides a look ahead capability by comparing caution and warning terrain envelopes generated from a terrain database to FMGS position data and baro data from the Captain's altimeter. Both en-route terrain and runway clearance floor envelopes are provided. The extended functions are inhibited when navigation performance is LOW.

The terrain is displayed on the ND when the TERR ON ND putton on the center panel is pressed. Areas not included in the database are color coded magenta. Dotted red areas are more than 2000ft above the aircraft, dotted orange areas more than 1000ft above the aircraft and amber areas are between 1000ft above and 500ft below (250ft below with gear down) the aircraft.

The EGPWS derived en-route caution generates a "Terrain Ahead" aural alert combined with an GPWS red light, a TERR AHEAD amber message and conflicting terrain displayed solid yellow on the ND. The en-route warning generates "Terrain ahead, pull up" and the message and terrain are displayed in red in the ND. The clearance floor alert generates a "Too low terrain" aural alert. The terrain display is automatically shown in all cases {TODO: check true for CF case}.

The enhanced functions may be inhibited with the TERR button on the GPWS panel. A failure in the extended functions will cause an amber FAULT light on the SYS button, but this will not affect basic GPWS functions.

EPGWS warnings are overridden by stall or windshear warnings.

The system can be tested by pushing the GPWS/GP warning light.

11.5. Radio altimeter

There are no cockpit controls for the radio altimeters. They self test when AC power is first applied to the aircraft, then enter a standby mode. They become active at lift off and operate continuously until touchdown.

Rad alt data is displayed whenever below 2500ft radio height. This consists of color coded digits at the bottom of the attitude indicator, a red ground bar on the altimeter scale and a white ground bar on the attitude indicator. The ground bar merges with the horizon at touch down. Radio heights are also announced by a synthetic voice during approach.

If a single radio altimeter fails, data from the remaining one will be displayed on both screens.