Call Reset

The control panel sends a ground to the reset buffer in the SELCAL decoder. When the reset buffer receives the ground, it supplies a signal to the microprocessor to reset the command to the output driver. When the output driver is reset, all indications go off.

Aural Warning

The SELCAL decoder unit supplies a ground discrete to the SELCAL aural warning relay. The energized relay sends 28V dc to the aural warning system through the REU. The aural warning system makes a single high/low chime that tells the

flight crew when a call comes in.

9.2. Назначение системы адресной связи.

Система адресной связи использует существующее радиосвязное оборудование для передачи с самолета на землю и обратно цифробуквенной информации в формате ACARS. Существующие системы адресной связи работают в ВЧ-диапазоне. Новое поколение этих систем будет использовать СВЧ-диапазон.

9.2.1. Аircraft communications addressing and reporting system (acars);

ARINC COMMUNICATIONS ADDRESSING AND REPORTING SYSTEM (ACARS)

ACARS is a solution to an ever increasing problem of an overcrowded VHF communication band. The domestic VHF system has all but run out of channels for operational control around some high density airports resulting in communications congestion and accompanying problems. Furthermore, much of the voice communication time between flight crew and flight operations control is concerned with routine messages involving aircraft status and departure/arrival times. The time required to orally report this information is extensive.

ACARS is an addressable digital data link system which permits exchange of data and messages between an aircraft and a ground based operations centre using the VHF communications band.

Downlink messages from the aircraft are transmitted in digital form to ARINC data link ground stations along most routes. The data is then relayed to a central ARINC data link control computer which is connected via land lines to the company communications control centre.

Fig. 9.5. ACARS GENERAL LAYOUT

Downlink messages from the aircraft may consist of position updates, weather data requests or details of aircraft system faults. Uplink messages from the ground network may consist of weather data, flight plan or other information for the flight crew. Downlink messages contain the aircraft's registration and flight number in order to identify the source of the message. Similarly uplink messages are addressed to a specific aircraft registration, the airborne processor ignoring messages which are not addressed to the aircraft to which the equipment is fitted.

Fig. 9.6. ACARS AIRCRAFT SYSTEM SCHEMATIC

Management Unit (MU)

The management unit provides control of airborne data and transmits downlink data to the ground through the VHF transceiver. It also accepts uplink data from the VHF transceiver and controls the distribution and display of data.

The MU provides monitoring of the aircraft OUT/OFF/ON/IN sensors and labels these as 0001 events. When a 0001 event occurs, the system generated GMT is automatically entered into the MU 0001 event memory and is available for recall and visual display any time after the event occurs. The 0001 times are automatically transmitted to the ground station at specified times in accordance with the system memory program. The 0001 logic is based on air/ground, parking brake and passenger door discretes.

The 0001 times are those at which the aircraft goes OUT of the gate at pushback, goes OFF the ground on take-off, and back ON the ground at landing, and back IN the gate.

ACARS Display Unit and Printer

The display unit is an interactive display unit that uses an infra-red LED matrix to detect finger pressure on the screen. The display unit provides an interface between the pilot and ACARS.

The printer provides automatic printouts of certain uplink messages and manually initiated printouts of uplink messages selected from the ACARS display unit.

Operation

A message may be initiated by crew action or may be generated as the result of a predetermined event or series of events.

The ACARS MU monitors the RF channel for traffic before the start of any transmission. In the event the channel is not clear, the system waits until the channel is clear before beginning any transmissions.

When the MU is first powered up it does not have data communications established with the ground network. This condition is known as "NOCOM". In order to establish a communication link the MU selects the first VHF frequency in its data frequency table and tunes its associated VHF transceiver to it. The MU then listens on the frequency for valid uplink messages. If, having listened for 2.1 minutes on a frequency and no uplink messages have been detected, the MU will remain in "NOCOM" and move on to the next frequency in its table.

The MU never transmits on a frequency unless it has received a valid ACARS uplink traffic on that frequency within 2.1 minutes.

On detecting uplink traffic the MU transmits a link test message in an attempt to establish contact. In the event that the MU receives no acknowledgement (ACK) from the ground station to this link test, the MU retransmits the message a total of six times. If no "ACK" reply is received after the sixth attempt no further attempts are made on that frequency. A "NOCOM" message is displayed on the ACARS display unit and the MU moves on to the next frequency in its table and repeats the process.

When valid uplink traffic is detected and the MU receives a link test "ACK" from the ground station the MU remains on that frequency and clears the "NOCOM" message.

Having established a good frequency for communications, the MU will continue to use the frequency until such time it fails to maintain contact. The MU will enter a "NOCOM" condition and the frequency selection process starts once again.

The MU detects ACARS messages for, and from, all ACARS equipped aircraft in the vicinity. All uplink messages are checked for address (aircraft registration) and validity. Uplinks addressed to other aircraft are ignored by the MU.