AIR DATA INERTIAL REFERENCE SYSTEM (ADIRS)

An Air Data Inertial Reference Unit (ADIRU) is a key component of the integrated Air Data Inertial Reference System (ADIRS), which supplies air data (airspeed, angle of attack and altitude) and inertial reference (position and attitude) information to the pilots’ electronic flight instrument system displays as well as other systems on the aircraft such as the engines, autopilot, aircraft flight control system and landing gear systems.

An ADIRU acts as a single, fault tolerant, Independent source of navigational data for both pilots of an aircraft. It may be complemented by a Secondary Attitude Air Data Reference Unit (SAARU), as in the Boeing 777 design.

An ADIRS consists of:

1. Up to 3 fault tolerant ADIRUs located in the aircraft electronic rack

2. An associated control and display unit (CDU) in the cockpit.

3. Remotely mounted Air Data Modules (ADMs).

Each ADIRS consists of:

Three Air Data/Inertial Reference Units (ADIRU)

• A Mode Selector Unit (MSU)

• 3 pitot probes

• 6 static probes

• 8 Air Data Modules (ADM) connected to the pitot and static probes

• 2 Total Air Temperature (TAT) sensors

• 3 Angle of Attack (AOA) sensors

Each of the ADIRUs has two parts:

• The Air Data Reference (ADR) part which supplies air data parameters

• The Inertial Reference (IR) portion which supplies attitude and navigation parameters.

ADIRS block diagram

POWER SUPPLY

1. ADIRU - 28VDC.

2. AOA Sensor - 115VAC & 26VAC.

3. Pitot probe: 115VAC.

4. TAT Sensor: 115VAC.

5. Static probe: 28VDC.

6. ADM: 13.5VAC from ADIRU.     

   
   

   In other words:

   1. 26VAC - AOA Sensor

   2. 115VAC - AOA sensor, Pitot probe, TAT Sensor

   3. 13.5VAC - ADIRU

   4. 28VDC - ADIRU & Static probe           

Probe and sensor Location

The TAT sensors and the angle of attack sensors are directly connected to the ADIRUs. The pitot probes and the static probes are connected to eight Air Data Modules (ADM) which convert pressure data before sending them to the ADIRUs.

Static Probes

Each of the three systems comprises two static probes which are linked to each ADR portion of the ADIRUs through five ADMs.

The probe is protected from icing with a 28 VDC heater circuit.

In normal configuration, the Captain Primary Flight Display (PFD) and the Navigation Display (ND) show the data computed by the ADIRU 1.

The First Officer PFD and ND show the data computed by the ADIRU 2.

The data from the ADIRU 3 can be displayed by action on the AIR DATA selector switch or on the ATT HDG selector switch located on the SWITCHING panel.

These selector switches, which define a priority order for the ADIRUs, are also used by several other aircraft systems

ADM

The term Air Data Module (ADM) refers to any remotely-located LRU which senses pressure information and transmits it to the ADIRU in ARINC 429 format.

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.

The ADM comprises the following components:

(a) Transducer board - This device provides two signals (Pressure Time Pulse (PTP) and Temperature Time Pulse (TTP)) used by the processor board.

(b) Processor board - The processor board uses the PTP and TTP signals sent by the transducer board and the transducer modeling coefficients stored in the NVM to compute the pressure.

(c) EMI/lightning board -

• power supply: the ADM is supplied by the associated ADIRU with +/- 13.5 VAC.

• EMI filterering.

• Lightning protection

• filtering of input discretes.

(2) Modes of operation

The ADM has three modes of operation:

• initialization

• normal operation

• Memory Access/Calibration (MA/C).

(a) Initialization mode

The ADM automatically enters the initialization mode immediately after power is applied to the device or after a Watchdog Timer is activated due to an ADM failure.

The ADM performs several tasks while in the initialization mode:

• initializes software variables and hardware interfaces,

• performs various self-monitoring tests to determine if the ADM operates properly,

• reads the fixed Program Pin discretes.

The ARINC 429 bus is disabled during the initialization mode and does not output any data on the ARINC 429 bus.

The initialization mode is completed within 900 ms (maximum) after valid power is applied to the ADM. Upon completion of the initialization mode, the ADM automatically enters the normal operation mode.

(b) Normal operation mode

The ADM enters the normal operation mode after the completion of the initialization mode. It remains in the normal operation mode until either power is removed from the ADM, or an internal failure activates the Watchdog Timer, or an internal BITE detects an unsafe condition, at which time a failure warning is annunciated or ARINC 429 transmissions cease.

The ADM performs the following functions in the normal operation mode:

• measures the pressure and temperature outputs from the transducer,

• uses filtering or averaging techniques consistent with the noise, resolution, and dynamic frequency response requirements

• computes the Applied Pressure based upon the pressure and temperature signals,

• filters the Computed Pressure,

• formats the ARINC 429 word to be transmitted,

• transmits the ARINC 429 Computed Pressure word,

• formats and transmits two ADM discrete words (labels 270, 271),

• reads and debounces the MA/C discretes,

• performs the BITE functions,

• formats and transmits ADM software and equipment I.D. words (labels 040,377).

(c) Memory access/calibration mode

The Memory access/calibration (MA/C) mode is used for purposes of transducer calibration and fault analysis. The MA/C mode requires external test equipment which interfaces to the ADM via a RS-232C bus. This mode can be considered as an auxiliary mode which is accessible in a repair shop environment only.

Total Air Temperature(TAT) Sensor

The aircraft is equipped with two TAT sensors with two sensing elements each. The sensing elements of the sensor have variable resistances. The TAT sensor 1 is linked to the ADR portion of the ADIRUs 1 and 3, the TAT sensor 2 is linked to the ADR portion of the ADIRU 2.

The TAT sensors are set at 2.33 m from the nose and at 0.60 m of the aircraft axis below the fuselage. The TAT sensor 1 is located on the left side and the TAT sensor 2 on the right side.

The air flow enters the scoop of the sensor, goes through a calibrated choke and flows over the hermetically-sealed platinum resistance sensing element where the temperature is measured. The speed of the flow over the element is controlled by the choke in the element tube.

The ADR portion is designed to operate with 500 ohms (at 0 deg. C) temperature sensor unit corresponding to the basic Callender - Van Dusen equation. To improve the accuracy of the sensor, a network of precision resistors is used. This technique is identified by the term Precision Calibration Interchangeability (PCI).

These sensors are heated with 115 VAC through the probe heating system. The heating element must not be energized on the ground.

The heating element is implanted in the scoop and strut and keeps the probe free of ice under the most severe icing conditions.

The No 3 ADIRU is a redundant unit that may be selected to supply data to either the commander’s or the co-pilot’s displays in the event of a partial or complete failure of either the No 1 or No 2 ADIRU. There is no cross-channel redundancy between the Nos 1 and 2 ADIRUs, as No 3 ADIRU is the only alternate source of air and inertial reference data.

An inertial reference (IR) fault in ADIRU No 1 or 2 will cause a loss of attitude and navigation information on their associated primary flight display (PFD) and navigation display (ND) screens.

An air data reference (ADR) fault will cause the loss of airspeed and altitude information on the affected display. In either case the information can only be restored by selecting the No 3 ADIRU.

Each ADIRU comprises an ADR and an inertial reference (IR) component.