How it works the A320 RA Radio Altimeter?

General The RA system is a radio system, which determines the distance between the A/C and the terrain. The system is used during climb, approach and landing phase. The A320 uses two independent RA systems.

Indication All DMC receives radio height data from both RA transceiver. In normal case, RA1 data is displayed on Capts PFD and RA2 data on F/Os PFD. If RA1 (2) fails, the PFD indication automatically switches over to the offside RA.

Users The RA data is sent to the GPWS for different warning activations.

The FWCs receive RA information to create the altitude call–outs below 2500 ft during approach and warnings in case of failure. These announcements come through the cockpit loudspeakers, even if the speakers are turned off. Note: The reference altitude for callouts is the radio height for precision approaches (DH), and baro altitude (MDA/MDH) for non precision approaches. Pin programming enables Operators to select the required callouts. If the aircraft remains at a height that is in the detection zone for a height callout, the corresponding message is repeated at regular intervals.

RETARD ANNOUNCEMENT The loudspeaker announces RETARD at:

• ‐ 20 ft, or

• ‐ at 10 ft if autothrust is active and one autopilot is in LAND mode.

(The TCAS Computer needs radio height to modify the trigger levels of traffic and collision warnings.) The FMGCs and ELACs use radio height for navigation and automatic flight purposes.

Antenna Each transceiver works with one transmission and one reception antenna. Both antennas are identical.

Warnings and Flags A faulty RA system results in the following cockpit effects:

• Flags on PFD

• Master Caution Lights on the glareshield

• Aural Warning (Single Chime)

• NAV RA 1 (2,1+2) FAULT on the upper ECAM display.

Normal Indication on PFD Radio Altitude This indication is on the bottom of the attitude sphere for height less than or equal 2500 ft. The dimension and the color of the digits change in relation to the height (RA) and the decision height (DH) as follows:

• – 400 ft < RA < 2500 ft

• – DH+100 ft < RA < 400 ft

• – RA < DH+100 ft

The resolution of the display is also a function of height:

• – RA > 50 ft

• – 5 ft < RA < 50 FT

• – RA < 5 ft

Decision Height

The DH value is shown on the right top corner of the PFD (FMA indication) as soon as the radio altimeter operates. The DH is entered on APP page of the MCDU and is transmit by the FMGCs if ILS approach is selected. When the radio height is lower then the DH, a amber DH warning message (flashing first, the steady) comes into view at the bottom of the attitude sphere.

Rising Runway Indication Below 300 ft, the height is shown by the distance between the horizon line and the limit of the sector 2. The limit of the sector 2 moves up as the aircraft is in the descent phase. The distance between these two lines is proportional to the ground height (sensitivity 5 ft/mm). As it moves up, the limit line erases the graduations on the pitch scale.

Red Ribbon When the aircraft is below 500 ft height, a red ribbon comes into view on the bottom and at the right of the altitude scale and moves up as the aircraft is in the descent phase. When the aircraft has touched the ground, the top of the ribbon is at the middle of the altitude window.

As you see this system is very important for the aircraft operation, later in another article we will discuss about the Abnormal Operation from a Procedure view in order to handle a Double RA Failure Inflight.

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