Technology - The Online Black Box

The system performs a battery of tasks from monitoring in real time engine parameters, fuel status or any abnormal flight conditions to storing the data that is accessed by interface unit of ACARS

Aircraft communications addressing and reporting system (ACARS) is a ground-to-air and air-to-ground communication system that links an airliner with operations, maintenance, engineering, dispatch, catering and customer service of an airline. The system helps reduce general workload as well as enhances safety and efficiency. Non-voice digital data transmission technology, christened as ACARS, to link commercial aircraft with operators was first developed and experimented successfully in the 1980s by Aeronautical Radio Inc (ARNIC). Through this, routine messages such as weather reports can be automatically transmitted thereby reducing crew workload.

Today, messages are exchanged routinely using ACARS via very high frequency (VHF) and high frequency (HF) bands or SATCOM links with data link service providers (DSP) such as SITA and ARNIC which commission terrestrial communication networks with airlines or the air traffic control (ATC). DSPs uplink and downlink ACARS messages to and from aircraft to the concerned department of the airlines. ACARS messages are automatically transmitted from aircraft on ground or in flight in short bursts that last less than a second. There are about 200 characters per message and approximately 100 standard ACARS message formats. However, some operators prefer to use their own formats. Radio messages are automatically generated in complex sequence supported by avionics and onboard computers of a modern day aircraft equipped with ACARS. Contents of the messages have details such as event, time, aircraft systems performance data and other vital information such as door closing time, departure origin, cruise, landing at destination—all transmitted automatically to the concerned departments of the airline. For example, if information reflects a disparity in exhaust gas temperatures of the engines, the maintenance crew is alerted and advised where to look for the problem before the aircraft lands.

Modern-day ACARS are more sophisticated with far greater capability than those of the 1980s. Earlier, messages were limited to expected time of departure and arrival or delays, some rudimentary performance data and weather. Today’s ACARS perform more complex tasks—capture performance and activities data of the aircraft, from engine parameters to a hijack situation. Thanks to advancement of ACARS that allows data from sensors of different aircraft systems to be collated and processed. These systems perform a battery of tasks from monitoring in real time engine parameters, fuel status or any abnormal flight conditions to storing the data that is accessed by interface unit of ACARS. The data generated provides vital information to maintenance, dispatch and other personnel in different areas such as engine performance parameters, aircraft systems status, aircraft position and weather.

ACARS Messages

Alpha-numeric messages (see Diagram) transmitted by ACARS are highly structured containing address field, message label, downlink block identifier, message sequence number, carrier and flight number. Thus, messages are combination of two, three, four or five letters depending upon the nature of the message. Messages are decoded to make sense out of these alpha-numeric characters. There are no internationally accepted standards for message formats and each operator may choose its own formats. For example, a message transmitted by an aircraft on ground taxing for take-off, would appear as a string of alpha-numeric characters described on the computer screen of the dispatcher.

Likewise, over 100 different message formats—some short and some long, are routinely transmitted from aircraft.

Architecture

Introduced by ARINC in yesteryears, ACARS is composed of two main units, the ACARS management unit (AMU) and a control display unit (CDU). The AMU computer is designed to send and receive digital messages using the industry standard transmission protocols over VHF and HF band. On the ground, the ACARS is supported by a network of radio transceivers, managed by and connected to a central computer that receives or transmits the data link messages. These messages are then accessed by respective departments of the airlines on the network. When transmitted by an aircraft, the remote ground stations (RGS) receives the messages that are sent to a DSP’s main computer system. Messages can be accessed by airline operators from DSP.

For an aircraft to receive data transmission from ground networks over VHF and HF, an interface between ACARS MU and flight management system was introduced in the 1990s. Thus began two-way communication between aircraft and operator. This enabled the flight crew to get real-time data on prevailing weather and other information from operations. ACARS transmission can also be intercepted through a single side band equipped shortwave radio for transmissions in HF band over a dozen frequencies between 2,000-2,100 Khz. Decoding the transmissions into meaningful messages is possible by connecting the output of radio to the sound card of a computer with ACARS decoding software freely available from the Internet.

Application

Performance data of engines and aircraft operations are available for the ground crew as part of maintenance data. However, this option is impossible without an interface between ACARS MU and flight data acquisition and management system (FDAMS) that monitors all the parameters of an engine such as fuel flow, oil pressure, oil temperature, engine vibrations, RPM and so forth. Traditionally, FDAMS data was collected by engineers physically from the aircraft. This practice continues for aircraft where ACARS MU and FDAMS are not interfaced. ACARS MUs are also interfaced directly with a control display unit (CDU) in the cockpit giving the flexibility to flight crew to receive messages or place a request for latest weather reports or other ground support. These messages are usually displayed on the screen or printed out.