Navigation Overview and the ADIRS

The navigation chapter is the aircraft's sensory system. Attitude, heading, altitude, speed, position, the weather ahead, the terrain below, the traffic around — every number the crew and the flight-control computers trust originates from a sensor in ATA-34. The auto-flight chapter (ATA-22) covers the brain: how the flight-management and guidance computers compute and steer. This chapter covers the senses: where the numbers come from, how good they are, and what happens when a sensor fails.

This first article lays out the whole-chapter map, then builds the skeleton of the single largest sensor set on the aircraft — the Air Data and Inertial Reference System (ADIRS): three ADIRUs, one mode-selector panel, two dozen probes, two switching rotary selectors, and a "what do we keep on emergency power only" trade-off philosophy that drives half the abnormal chapter.

1. How ATA-34 is organised — four families

The maintenance manual states the chapter's purpose plainly. Per AMM 34-00-00:

The aircraft navigation systems provide the crew with the data required for flight within the most appropriate safety requirements.

It then divides the chapter into four families, and the division is worth memorising because it is organised by where the information comes from:

ATA-34 Navigation
├─ 34-10/20  ADIRS + standby            → this article + air data / inertial / GPS / ISIS
│   ├─ 3 × ADIRU (ADR + IR) + MSU + 8 × ADM
│   ├─ probes: 3 pitot / 6 static / 3 AOA / 2 TAT
│   └─ standby: ISIS + standby compass
├─ 34-30  landing & taxi aids           → ILS / landing sensors
│   └─ 2 × MMR (ILS + GPS [+ GLS provision])
├─ 34-40  self-contained (no ground station)
│   ├─ 2 × weather radar (+ predictive windshear)
│   ├─ 2 × radio altimeter
│   └─ TCAS + EGPWS
└─ 34-50  externally-referenced (ground station / satellite)
    ├─ 2 × DME, 2 × VOR (marker in VOR 1), 2 × ADF
    ├─ 2 × ATC Mode S transponders (ADS-B Out)
    └─ 2 × GPS (inside the MMRs)

• ADIRS relies on the aircraft's own probes and gyros.
• Self-contained systems rely on transmitters the aircraft carries (radar energy, radio-altimeter beams) illuminating the outside world.
• Externally-referenced systems rely on signals transmitted from the outside world (ground stations, satellites, other aircraft's transponders).

That taxonomy directly decides who is still trustworthy in a failure. Lose every ground station in flight and the ADIRS and the radar are still alive; jam the GPS and the inertial platform still dead-reckons. It is the foundation of the entire GPS-interference and position-fault logic later in the chapter.

2. ADIRS — one system feeds the whole aircraft

Per FCOM DSC-34-10-10-10:

The Air Data and Inertial Reference System (ADIRS) supplies temperature, anemometric, barometric and inertial parameters to the EFIS system (PFD and ND) and to other user systems (FMGEC, FADEC, PRIM, SEC, FWC, SFCC, ATC, GPWS, CMC, CPC).

Count that user list: flight management (FMGEC), the engines (FADEC), the flight controls (PRIM/SEC), the warning system (FWC), the slat/flap computers (SFCC), the transponder (ATC), ground-proximity warning (GPWS), maintenance (CMC), cabin pressure (CPC). No other single system on the aircraft has that many downstream customers. That is exactly why an ADR or IR fault is never "an instrument failed" — it is a partial failure of the aircraft's senses, and its consequences fan out across almost every other system.

The redundancy level is engineered. Per AMM 34-10-00:

This configuration provides for triple redundant information for all inertial and air data functions. Each channel is isolated from the others and provides independent information as defined by ARINC 738.

"Isolated" is not a figure of speech. As the following sections show, the isolation is realised in probe allocation, in electrical supply, and even in physical mounting location.

3. Two half-brains inside one ADIRU

Per FCOM DSC-34-10-10-10:

Each ADIRU is divided in two parts, either of which can work separately in case of failure in the other:

• ADR (Air Data Reference): barometric altitude, speed, Mach, angle of attack, temperature, overspeed warning.
• IR (Inertial Reference): attitude, flight-path vector, track, heading, acceleration, angular rate, ground speed, vertical speed, position.

That separability is the first rule of the abnormal chapter: an ADR fault can be isolated by pushing only the ADR pushbutton, and the IR keeps supplying attitude — and vice versa. The method matters: use the ADR/IR pushbuttons, not the rotary selector. Turning the rotary to OFF de-energises the whole ADIRU, taking the ADR and the IR down together.

Physically, the ADIRU is a 4-MCU box housing a three-axis laser-gyro / three-axis accelerometer inertial cluster, an air-data computing board, and integrated GPS/inertial hybridisation (GPIRS — the subject of a later article). Its mounting tolerance is measured in arc-minutes: pitch, roll, and azimuth mounting are each held to about ±12 arc-minutes — attitude-reference accuracy begins at the bolt. There is also a physical-segregation detail that is easy to miss. Per AMM 34-12-00:

The ADIRU 3 is installed in a different zone (Ref. Para. Component Location) for better physical segregation.

ADIRU 1 and 2 sit on the same avionics rack; ADIRU 3 is mounted separately, so that a single localised fire, water ingress, or shock cannot take all three. The "standby" identity of channel 3 begins at its mounting location.

4. Probes and ADMs — the allocation table

Four sensor types feed the system: 3 pitot, 6 static (a left/right pair per channel), 3 AOA, 2 TAT, plus 8 Air Data Modules (ADMs, pressure-to-digital converters). Commit the allocation to memory. Per FCOM DSC-34-10-10-10:

ADIRU 1 is supplied by CAPT probes, ADIRU 2 is supplied by F/O probes, ADIRU 3 is supplied by STBY probes and CAPT TAT

There are only two TAT probes, so ADIRU 3 borrows the captain's. This is the first of several cross-connections (ADR 3 shares probes with the standby instruments; TAT is shared with channel 1) that the crew relies on when diagnosing unreliable airspeed — knowing which indications should agree by design is half of finding the odd one out.

Mounting angles are worth noting, because location decides exposure to icing, spray, and airflow disturbance: the static ports sit 52°30′ below the fuselage reference line; pitot 1/2 at 42°, pitot 3 at 60°. The AOA sensors are self-heated wind vanes; at 100 kt their accuracy is ±0.3° — the same figure that underpins angle-of-attack protection in the flight-control chapter.

The ADM installation has an elegant purpose. Per AMM 34-11-00:

The ADMs are remotely mounted near and above the level of the pitot and static probes, this in order to make the ADM pneumatic plumbing self draining when the aircraft is stationary on the ground.

Water runs downhill, so the digitiser boxes sit high — a single plumbing choice that defends against the most dangerous adversary in the unreliable-airspeed chapter (trapped water producing a false speed). Each ADIRU powers the ADMs on its own side. Per AMM 34-11-00:

Each ADIRU supplies the power for the ADM of its side (CAPT, F/O, STBY).

The TAT sensor is a platinum-resistance element (500 Ω at 0 °C); air enters a scoop, passes a calibrated bleed orifice, then flows over the sealed sensing element. Its heating carries a ground rule. Per AMM 34-11-00:

The heating element must not be energized on the ground.

With no airflow on the ground, heating would drive the TAT to a false high reading — the ATA-34 counterpart of the "no TAT heat on the ground" rule from the ice-and-rain chapter.

5. Supply philosophy — normal three, emergency "keep two IRs, heat one side"

Normally each ADIRU draws 115 V AC (ADIRU 1 is fed from the static-inverter bus), with the 28 V DC hot battery bus as back-up and 26 V AC providing the AOA resolver excitation reference. One small action happens on every power-up. Per AMM 34-11-00:

At the beginning of each power cycle the ADIRU switches from the main to the back-up power to test the electrical generation.

That is the mechanism behind the ON BAT light briefly illuminating for a few seconds at every power-up — not a fault, but the ADIRU self-testing its battery path.

After a total loss of main generation (emergency electrical configuration), the trade-off is written into the AMM in one sentence. Per AMM 34-11-00:

The principle of the distribution is to supply two ADIRUs (1 and 3) but to have only one ADR function available to save power consumption: to achieve this, the sensors of one side are not heated.

Unpack it: attitude (IR) is kept in duplicate; speed and altitude (ADR) are kept singly. The IR is the flight controls' lifeline — without attitude there is no control law — while the ADR's power hog is probe heating (a single pitot draws 281 VA, an AOA 250 VA), so "heat one fewer probe set" is the cheapest possible saving. By default (AIR DATA at NORM) channel 1 is kept: ADR 1 + IR 1 available, only pitot 1 and AOA 1 heated; ADR 3 fails for lack of 26 V AC but IR 3 stays available. ADIRU 2 is on a timer. Per AMM 34-11-00:

28VDC HOT BUS 702PP supplies 28VDC to the ADIRU but Time Delay Opening (TDO) relay 12FP will stop this supply after five minutes in the emergency configuration.

So ADR 2 fails immediately, IR 2 survives five minutes — a short handover window — then channel 2 goes silent. If the AIR DATA selector is at CAPT ON 3, per AMM 34-11-00:

The CAPT ON 3 position of the AIR DATA selector switch agrees with the selection of ADR 3.

Relays re-route the heating supply to the channel-3 probes: ADR 3 + IR 3 available, ADR 1 fails, IR 1 survives. In other words, on emergency power that one rotary selector decides which side's airspeed lives. Combined with the electrical chapter's emergency "NAVIGATE" survivors (ND 1, FMGC 1, radar, RMP 1, VOR 1, DME 1, ILS 1/GPS 1), it defines the complete emergency-navigation inventory.

6. Switching — the two-rotary user network

The centre pedestal carries a pair of three-position rotary selectors: AIR DATA and ATT HDG, each with positions CAPT ON 3 / NORM / F/O ON 3. Normal allocation, per FCOM DSC-34-10-10-20:

NORM : ADIRU1 supplies data to PFD1, ND1, DDRMI and ATC1. ADIRU2 supplies data to PFD2, ND2 and ATC2.

CAPT ON 3 : ADR3 or IR3 replaces ADR1 or IR1.

Note the wording: the AIR DATA selector switches the ADR family, the ATT HDG selector switches the IR family, and the two are independent — an ADR 1 fault is answered with AIR DATA, an IR 1 fault with ATT HDG, and only half a brain need be swapped. The selectors' customers are not only the displays: the AIR DATA selector also notifies the DMCs and the FMGECs, and the ATT HDG selector additionally notifies the weather-radar antenna stabilisation (which IR to use). Turning a selector re-sources not just the PFD in front of you but the radar antenna's attitude reference as well.

Note: DDRMI (the electromechanical radio-magnetic indicator) is not fitted in the configuration covered here; DDRMI references in the manual are optional-equipment markers only.

Whenever either selector leaves NORM, a green ADIRS SWTG memo appears on ECAM — a quiet "you are now in a non-standard configuration" reminder.

7. The mode-selector panel (MSU) and its lights

The overhead panel is the Mode Selector Unit (MSU): each of the three channels has an IR mode rotary (OFF / NAV / ATT), an IR pushbutton, and an ADR pushbutton, plus one shared ON BAT annunciator. Per AMM 34-12-00:

An ON BAT annunciator is shared by the three channels.

ON BAT light — illuminates for the first few seconds of a full alignment (the power-up battery self-test of §5): normal. It does not appear on a fast alignment. Per FCOM DSC-34-10-10-20:

The ON BAT light does not come on in the case of a fast alignment.

If it stays on on the ground, the ADIRU really is running on the battery — an external horn and the service-panel amber light will call for attention.

IR FAULT light — flashing and steady are two different illnesses. Per FCOM DSC-34-10-10-20:

When IR 1(2)(3) FAULT light flashes, the attitude and heading information may be recovered in ATT mode.

When IR 1(2)(3) FAULT light is steady, the IR 1(2)(3) is lost.

Flashing = navigation is dead but attitude is recoverable (select ATT); steady = the whole IR is scrap. The mode rotary also carries a one-way street. Per FCOM DSC-34-10-10-20:

During the flight, if the IR mode selector is set from NAV to ATT or NAV to OFF, the NAV mode is lost for the remainder of the flight.

Alignment can only be done on the ground, so moving out of NAV in flight is spilt water — which is why the IR-fault procedure sequences "select ATT only if the IR still offers ATT, push OFF only if it is completely dead." In ATT mode only attitude and heading remain, and heading must be entered manually via the MCDU and re-checked roughly every ten minutes.

The alignment memo family: IRS IN ALIGN XXX (green, XXX = minutes remaining); on alignment failure the green text flashes. One exam favourite, per FCOM DSC-34-10-10-20:

IRS IN ALIGN : This memo appears in amber if engines are running.

Amber means you are aligning with an engine running — a violation of alignment discipline (alignment requires the aircraft stationary and not during engine start/run) — so investigate.

8. Bus fatalism — who hangs on whom decides who dies together

Each ADR has 8 low-speed ARINC 429 output buses (buses 5–8 are dedicated to the FADECs, fuse-isolated within the bus — because the ADIRU is a common point to every engine). Per AMM 34-12-00:

As each ADIRU is a common point to all engines installed on the aircraft, special care is taken to make impossible any electrical disturbance to propagate through the ADIRU/engine interface to other inputs/outputs dedicated to engines.

Each IR has 4 high-speed buses. The user table hides several "fates":

• GPWS is fed only from ADR 1 bus 4 and IR 1 bus 4 — so an ADR 1 or IR 1 fault cripples the EGPWS enhanced functions (TERR FAULT / SYS FAULT), which is the physical root of the "switch off the TERR and SYS pushbuttons" action later, and the origin of the MEL clause "ADR 1 inoperative → GPWS considered inoperative."
• ATC 1 is fed from ADR 1/ADR 3, ATC 2 from ADR 2/ADR 3 — the transponder altitude-source pairing that decides which transponder to use after a dual-ADR fault.
• Weather radar 1 uses IR 1/IR 3, radar 2 uses IR 2/IR 3 — the antenna-stabilisation source.
• ISIS takes one bus each from IR 1 and IR 3 — the standby instrument's air/ground backup source.
• The BSCU hangs on all three IRs (ground speed for braking and steering).

The ADR also outputs four LOW SPEED WARNING discretes (4 kt hysteresis): LSW 1 = 50/54 kt, LSW 2 = 260/264 kt, LSW 3 = 100/104 kt, LSW 4 = 280/284 kt. Per AMM 34-12-00:

The LOW SPEED WARNING 3 discrete is used in the Ram Air Turbine extension logic. The LOW SPEED WARNING 4 discrete is used for the landing gear safety valve.

The speed at which the RAT may auto-extend and the speed at which the landing-gear safety valve locks hydraulic pressure are both judgements written by the ADR. That is the other face of a "sensory system": it does not only show you numbers, it makes speed decisions for the whole aircraft.

One last detail sets up the alignment article: the ADIRU software loads in three parts — the operational program, data table 1 (SSEC / correction laws / monitoring thresholds), and data table 2 = magnetic-variation coefficients. Per AMM 34-12-00:

These coefficients are based on the national oceanic and atmospheric-association publications that are updated every five years.

The alarming limitation in LIM-NAV — that some latitudes are prohibited if one ADIRU carries a different magnetic-variation table — is exactly the case of this data table 2 being out of step across the three units.

9. Configuration baseline

Optional-equipment content in this material follows the configuration summary that some operators carry in the QRH operational data. Per QRH OPS.01A:

For awareness and for the specified aircraft, the following table provides the flight crew with a list of optional aircraft systems and functions related to aircraft flight operations.

The configuration covered here (determined by cross-checking the QRH optional table, the FCOM general options list, and AMM applicability) is:

Self-test

[!note]- Q1. On what principle does the AMM split ATA-34 into four families, and what does "self-contained" mean? By where the information originates. Self-contained = the aircraft carries its own transmitter that illuminates the outside world (radar, radio altimeter); it needs no ground station or satellite, so it survives loss of all external references.

[!note]- Q2. An ADR fails. What happens to the co-located IR, and how should the ADR be isolated? The IR keeps working (the two parts are separable). Isolate with the ADR pushbutton — never the rotary selector, which de-energises the whole ADIRU and takes the IR down too.

[!note]- Q3. Where is the "standby" identity of ADIRU 3 realised in hardware? Three places: it is fed by the standby probes (plus the captain's TAT); it is mounted in a separate zone for physical segregation; and on emergency power it is the channel kept alive alongside channel 1.

[!note]- Q4. On emergency power, which half survives by default, and what does CAPT ON 3 change? When does IR 2 finally lose power? Default: ADR 1 + IR 1 (channel-1 probes heated); IR 3 also alive, ADR 3 lost. CAPT ON 3 re-routes heating to channel 3, so ADR 3 + IR 3 live and ADR 1 is lost (IR 1 stays). IR 2 is cut by the TDO relay 12FP five minutes into the emergency configuration.

[!note]- Q5. The IR FAULT light flashes. What is recoverable? What if it is steady? And if you move the mode rotary out of NAV in flight? Flashing = attitude/heading recoverable in ATT mode. Steady = IR lost entirely. Moving out of NAV in flight loses NAV for the remainder of the flight (alignment is ground-only) — spilt water.

[!note]- Q6. Which two buses feed the GPWS, and which MEL clause does that explain? ADR 1 bus 4 and IR 1 bus 4. It explains "ADR 1 inoperative → GPWS considered inoperative."

Key takeaways

References

• FCOM DSC-34-10-10-10 / -20 — ADIRS description, user list, two-part split, probe allocation, panel, lights, switching, memos.
• AMM 34-00-00, 34-10-00, 34-11-00, 34-12-00 — four-family taxonomy, ARINC 738 triple redundancy, probes/ADMs, supply and emergency trade-off, ADIRU internals, bus user table, low-speed discretes, data tables.
• QRH OPS.01A — configuration summary (optional systems).

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.