Summary
- ACARS is a system developed in the 1970s to automate data transfers between pilots and airline operations.
- It started as a simple system called ‘OOOI,’ which tracked the time of pushback, liftoff, landing, and arrival at the gate.
- ACARS uses VHF, SATCOM, and HF as data transfer mediums, with VHF being the most common and cheapest method, SATCOM used for long-distance transfers, and HF as a backup option.
Using voice communication through High Frequency (HF) and Very High Frequency (VHF) channels has been the standard in aviation for a long time. Even today, voice communication through set frequencies by radio is common. This includes communication between pilots and air traffic controllers and, to a certain extent, between pilots flying in the air and the airline operations team on the ground.
Not so long ago, a system based on text messages was introduced primarily for communication between pilots and airline operations. This system is known as the Aircraft Communications Addressing and Reporting System, or ACARS for short.
What exactly is ACARS?
In the 1970s, airlines sought a more efficient way to track their flights to find precisely when their airplanes pushed back from the gate, got airborne, landed, and parked at the designated gate at the destination. Pilots used to transmit this data to the operations through their radios, but this was not the best method as, if pilots forgot to perform this task, the data would not be transmitted, with the airline losing any data.
Photo:Jaromir Chalabala | Shutterstock
The solution to this problem was tackled by a telecommunication company named Aeronautical Radio, Incorporated (ARINC). ARINC developed an automated datalink communication system that could automatically transfer data from the aircraft to the airline operations without any pilot input. This system was based on VHF. Because ACARS was initially developed by ARINC when it started, ACARS was known as ARINIC Communications Addressing and Reporting System (also ACARS).
Initially, the only transferred set of data was called OOOI:
- Out: This used sensors on aircraft doors to determine when the doors are closed, generating a pushback time.
- Off: As the aircraft gets airborne, the Weight on Wheel (WOW) sensors give out the time the aircraft lifts off the runway.
- On: When the aircraft touches the ground, the WOW switches compress, providing a landing time.
- In: At the gate, when the door opens, the door sensors provide the system with the time the aircraft comes to a stop at the gate.
Photo: Delta Air Lines
The ACARS soon developed into a highly sophisticated system that could transfer more data between pilots and airline operations, such as free texting, direct flight plan uplinks, weather reports, position reports, etc. The ACARS, which started with VHF as a means of data transfer, can now use satcom and even HF as a transfer mechanism. This makes it possible to keep connected even when out of VHF reach, which is line-of-sight limited. However, VHF is the cheapest, and thus, whenever VHF is available, the aircraft system uses it over SATCOM and HF.
Later, another company emerged, this time in France, named Société Internationale de Télécommunications Aéronautiques (SITA), which, like ARINC, became a datalink service provider. Today, ARINC (owned now by Collins Aerospace) and SITA remain the two primary service providers for ACARS operations.
How did ACARS become a part of the air traffic system?
In the 1980s, the ICAO was working on a program called Future Air Navigation Systems (FANS). The program aimed to implement a CNS/ATM (Communication, Navigation, Surveillance / Air Traffic Management) concept. Like ACARS, it was based on datalink communication to increase airspace capacity. The development of such a system requires a lot of time and a lot of infrastructure, which is not readily available in all parts of the world.
However, the airlines and ICAO realized that the current ACARS network could be used for air traffic communications until the new ATN (Aeronautical Telecommunication Network) comes into effect, even though it would not fulfill all the requirements and protocols of ATN.
Photo:Gorodenkoff | Shutterstock
To make this possible, Boeing developed the FANS 1, which can be used on the ACARS network. Airbus soon followed by designing FANS A. These two systems are essentially the same thing and are most of the time referred to as FANS 1/A. An improved version of it is known as FANS 1/A+. This new version included improvements such as a message latency monitor to detect old messages that may no longer be applicable.
CPDLC
Controller Pilot Data Link Communications (CPDLC) involves any datalink communication between a pilot and an air traffic controller. CPDLC helps increase airspace capacity and efficiency by using text as a communication medium between pilots and controllers.
The main limitation of voice communication using VHF is that all stations or aircraft handled by a particular controller are on one single frequency, and only one person at a time can transmit on that frequency. This reduces communication efficiency as pilots wait in a sort of “queue” to pass their message. The other problem is that if a pilot misses a transmission or feels that he misread or misheard it, it can take time to clarify the information or clearance from the controller, as other pilots may be transmitting at that point.
CPDLC solves this issue as the text sent to the pilot from the controller can be viewed in plain words in the co*ckpit displays, and any doubts can be cleared by texting back to the controllers. There is no waiting involved.
Today, CPDLC is rarely used in continental airspace, and voice remains the primary means of communication between pilots and controllers. However, CPLDC is widely used in oceanic airspace where VHF is unavailable. When crossing large oceans or bodies of water, pilots can log on to the respective login address, which depends on the Flight Information Region (FIR). These addresses are four-letter codes, which are very similar to airport ICAO codes. For example, the login address for Mumbai FIR is VABF.
Once logged on, the aircraft also sends data to the ATC regarding its speed, position, altitude, track, heading, ground speed, etc. This is called Automatic Dependent Surveillance-Contract (ADS-C). With this, the controller or the Air Traffic Services Unit (ATSU) can automatically communicate with the aircraft to get the required data. This is done automatically with no pilot input.
Photo: FAA
Before CPDLC, pilots used High Frequency (HF) communications when crossing oceans. HF has an extended range when compared to VHF. However, HF has a lot of static and is a difficult way to transfer messages through voice.
How does the ACARS work?
As we have covered thus far, ACARS uses VHF, SATCOM, and HF as data transfer mediums.
VHF
VHF, again, is the most common and cheapest method of datalink transfer via ACARS. A third radio panel is dedicated to the VHF ACARS datalink in most transport aircraft. In the early days, VDL mode A was used, operating on the spectrum between 129-137 mHz (VHF). This allowed a data transfer speed of about 2.4 kbps. Due to the narrowband of frequencies allocated for datalink, it soon became constrained and reached its capacity in some parts of the world.
VDL mode 2 is the media to be used for ICAO’s ATN. It uses bits rather than characters to transfer data, which improves the efficiency and speed of data transfer. Because of its higher transfer rate of 31.5 kbps compared to only 2.4 kbps of VDL mode A, VDL mode 2 was adapted for ACARS usage.
When ACARS uses VHF, data from the aircraft is picked up by VHF antennas. These antennas then send the data to the service provider (ARINC or SITA), which then passes the message to the end user (airline operations or ATC). This can also be done in reverse, so airline operations or ATC can send data to the aircraft.
SATCOM
As VHF is limited to line of sight, it cannot be used for datalink when VHF antennae are far away from the aircraft. But, the aviation industry saw this problem was already solved in the maritime world using satellite communication, or SATCOM.
In 1979, a non-profit intergovernmental organization in liaison with the International Maritime Organization (IMO), the counterpart of the United Nations ICAO, launched several satellites into low earth orbit to provide satellite communications for the shipping industry. This was handled by an organization known as INMARSAT (International Maritime Satellite Organization). Soon after, ICAO became a part of it. Initially, 28 countries signed up for INMARSAT. However, in the mid-part of the 1990s, many of these countries no longer wanted to invest in improving the system. This led to the privatization of the company.
Photo: NASA and Boeing | Flickr
The ACARS data transfer via SATCOM works very similarly to VHF. The data from an aircraft is first collected by a satellite, which then transfers the data to a Ground Earth Station (GES). The GES sends this data to the data service provider (ARINC or SITA), where it gets transferred to the airline or ATC. Again, just as before, this can be performed in reverse direction.
It is not only INMARSAT that provides SATCOM datalink. There is also another company called Iridium delivering the same service.
HFDL
High-Frequency Data Link (HFDL) is used when VHF and SATCOM services are both unavailable. It uses HF to transfer data. Even though HF is one of the oldest voice communication methods used in the aviation industry, it was certified for datalink usage only at the start of the 2000s.
Photo: Airbus
HFDL is the slowest as it has a transmission speed of 1.8 kbps, and it is not uncommon for messages to be lost while being transferred. Interestingly, HFDL is the most expensive of the three datalink transfer methods.
How do pilots interact with ACARS?
ACARS is accessed through the aircraft’s Flight Management System (FMS) in most modern aircraft. The FMS has specific “pages” that are dedicated to ACARS operations.
Through ACARS, pilots can request things like the ATIS (Automatic Terminal Information Service), which provides information such as the current active runway, the weather at the airport, and other information necessary for the pilot to perform a takeoff or a landing. Traditionally, this information is provided through an ATIS VHF frequency, which the pilots can listen to.
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Photo: Air France
The ACARS can also be used to request departure clearance from ATC. This is called PDC (Pre-Departure Clearance).
Some aircraft, such as modern Airbus airliners, have a dedicated display called a DCDU (Data Communications Display Unit). There are two DCDUs, one for the captain and the other for the first officer. This DCDU comes into the picture only when communicating with the ATC - it is not used for communications between the pilot and the airline operations. When pilots log on to a CPDLC address, this is shown on the DCDU. Pilots write the required message from the keypad of the FMS, which is then displayed on the DCDU before being sent to ATC. The reply from ATC then comes on the DCDU with a ping to alert the pilots.
The pilots can then use the buttons on the DCDU itself to give an appropriate answer to the ATC. In modern Airbus aircraft, the DCDU is integrated into the FMS.
Photo:Kanat Rattananiyompan | Shutterstock
ACARS is also used to send data to the aircraft from airline operations. This includes data such as cruise and descent winds so that the FMS can correctly calculate the fuel and timings. It also sends the load sheets and amended flight plans to the pilots. This increases the efficiency of operations and reduces workload as pilots do not have to wait for a person to bring in a physical load sheet or amended flight plans to confirm and sign them.
ACARS can also automatically send aircraft and flight-related data to the relevant airline operational departments. This includes information such as aircraft system faults, flight timings, etc. Some airlines also record fuel usage, fuel invoices, and other relevant data from the ACARS. The pilots simply enter the data into the FMS before and after the flight, which is automatically sent to the airline operations. This dramatically reduces paper wastage and operational costs.
