ACARS is a data transmission system, a method of communicating information to the dispatch center, maintenance, or to anybody who needs information from the airplane itself or from the cockpit.  Some parts are automatic and others are manual input. Lets say I'm flying toward a place where there's a thunderstorm en route, and my dispatcher may send me an ACARS text message saying, "There's a thunderstorm, consider rerouting." I'll send back a message saying, "I got it, thanks," and then I may notify the dispatch. ACARS messages can be sent by text, but some aircraft have the ability to uplink by voice as well. The system also allows us to pull up the weather.

Often the ACARS is co-located with the FMS, but on a different screen. It's just like using a different window on your computer screen.   We can enter information from the ACARS into the FMS, but that doesn't happen automatically. The crew may be able to download position reports through ACARS, and that function could be automated in the new 777s so that the FMS could be downloaded automatically, but I’m not quite sure.


Officials investigating the disappearance of a Malaysia Airlines Boeing 777-200ER are tight-lipped about Aircraft Communications Addressing and Reporting System (ACARS) data reported by the aircraft.  Malaysia’s Department of Civil Aviation is examining ACARS data from the missing aircraft, but senior DCA officials declined to comment on their findings, if any. They also declined to say when ACARS data from flight MH370 will be released – or even if it will be.

In response to a question posed by Flightglobal about the aircraft’s ACARS data, one of the officials cited the “sensitivity of the investigations.”  The comments were made after a ministry of transport press conference scheduled for 1000 was cancelled, apparently indefinitely. According to the Twitter account of acting transport minister Hishamuddin Hussein, he instead embarked on one of the search and rescue flights looking for the aircraft in the Straits of Malacca on Malaysia’s west coast.

Media reports have said that no ACARS data was received from the aircraft when its transponder disappeared from radar at 0130 local time on Saturday 8 March. The aircraft mysteriously vanished halfway between the Malaysian town of Kota Bahru and the southern tip of Vietnam as it operated the Kuala Lumpur-Beijing route.

"All Malaysia Airlines aircraft are equipped with continuous data monitoring system called ACARS which transmits data automatically," says MAS. "Nevertheless, there were no distress calls and no information was relayed."

Presumably the aircraft would have transmitted ACARS data prior to its disappearance, but an industry source familiar with ACARS says this would be entirely dependent on the level of service enshrined in MAS’s ACARS contract with one of two ACARS service providers, ARINC or SITA.  Use of ACARS among carriers varies widely. Users of the service can have data transmitted at widely varying intervals, such as every minute to every thirty minutes.

When an aircraft is flying over land, ACARS data is transmitted via VHF to ground stations, but over water the data is transmitted to satellites.  Typically ACARS data has a narrow focus, with engine performance being the most widely monitored aspect of aircraft health. The flight crew can also deactivate ACARS transmissions if they elect to do so.  Nonetheless, ACARS data proved crucial for gaining an early understanding of Air France flight AF447, which crashed 1 June 2009. Within three days of this aircraft’s disappearance investigators released ACARS data, revealing that the aircraft had transmitted a number of failure reports for various aircraft systems.   It is far from clear, however, if ACARS data will be of similar use in the MH370 case.


Malaysia’s Ministry of Transport has published an article detailing how Inmarsat helped to pinpoint the southern corridor flight path taken by MH370. It reports that Inmarsat informed them on 13 March that routine automatic communications between the Inmarsat satellite and MH370 could be used to determine several possible flight paths.  The UK Air Accidents Investigation Branch (AAIB) then presented Inmarsat’s most recent findings on 24 March, indicating the southern corridor.

Calculating ‘pings’ - The report states that the calculations were made using the automatic ‘pings’ sent to the satellite via the ground station and the aircraft after it vanished.  It explained that if the ground station does not hear from an aircraft for an hour it will transmit a ‘log on/log off’ message – a ‘ping’ – and the aircraft automatically returns a short message indicating that it is still logged on, a process described as a ‘handshake’.  The ground station log recorded six complete handshakes after ACARS, the aircraft’s operational communications system, stopped sending messages.

Inmarsat was then able to calculate the range of the aircraft from the satellite, and the time it took the signal to be sent and received, to generate two arcs of possible positions – a northern and a southern corridor.  

The report goes on to explain that Inmarsat developed a second innovative technique that took into account the velocity of the aircraft relative to the satellite and the resulting change in signal frequency, known as the Doppler Effect.  The Inmarsat technique analyses the difference between the frequency that the ground station expected to receive and the one actually measured, known as the Burst Frequency Offset.