>> ACARS is an automated aircraft communication system that transmits a/c information, primarily maintenance information, to maintenance facilities like the airline, Boeing, Engine Manf, etc.
ACARS messages can be used for ATC purposes (Google 'FANS') as a replacement for verbal contact and Airline Operational Control (AOC
) purposes. AOC
messages can include position reports, maintenance reports, gate assignments, weather uplinks, weather downlinks, and many other purposes. To represent ACARS as a maintenance system would be incorrect. In the part of the world that MH370 was operating, its unlikely it was being used for ATC purposes.
>> ACARS is NOT a flight system - it is not needed for safe flight.
While it is not a flight control system, it certain parts of the world it can be used as a replacement for verbal messages and as a means of reporting position information to ATC over the ocean.
>> ACARS is a subscription service and costs money. All indications are the MH370 was subscribed only to engine health monitoring and data from that is sent to Rolls Royce.
ACARS billing is based on the quantity of data transferred, and the means by which it is transferred (SATCOM messages are more expensive that VHF messages), not by the message content. The 777 has a very configurable maintenance computer. Data reported and frequency of reporting is up to the airline.
>> ACARS communicates via VHF or SATCOM (and maybe Wifi at the gate). The communications channel depends on availability and is independent of the ACARS.
ACARS messages are never transferred over Wifi. Some airlines may use a separate Gatelink Wifi connection to transfer large datasets to/from the aircraft.
>> ACARS can be instructed not to use SATCOM or VHF from the Cockpit. This would effectively stop ACARS from sending data. Access to the EE bay is not required.
>> The Malaysian prime minister said (quote):
>> "We can say with a high degree of certainty that the Aircraft Communication Addressing and Reporting System
>> (ACARS) was disabled just before the reached the East coast of Peninsula Malaysia".
>> No explanation of how that determination could be made has been released.
ACARS can easily be turned off at any time by the flight crew from the cockpit. Note that this does not mean the radios have been powered off and housekeeping messages between the radios and the ground may still occur. So, if MH
had programmed their 777s to report engine or maintenance status every 10 minutes and those reports stopped, it would be possible to infer that the system was disabled at a certain time.