ACARS

Further, the hourly rates for flight and cabin crew salaries depended on whether the aircraft was airborne or not, and if on the ground whether it was at the gate or not.

The original avionics standard was ARINC 597, which defined an ACARS Management Unit consisting of discrete inputs for the doors, parking brake and weight on wheels sensors to automatically determine the flight phase and generate and send as telex messages.

It also contained a MSK modem, which was used to transmit the reports over existing VHF voice radios.

On-board ACARS equipment[4] consists of end systems with a router, which routes messages through the air-ground subnetwork.

Ground equipment is made up of a network of radio transceivers managed by a central site computer called AFEPS (Arinc Front End Processor System), which handles and routes messages.

Generally, ground ACARS units are either government agencies[clarification needed][dubious – discuss] such as the Federal Aviation Administration, an airline operations headquarters, or, for small airlines or general aviation, a third-party subscription service.

Ground system provision is the responsibility of either a participating air navigation service provider (ANSP) or an aircraft operator.

[7] These OOOI events are detected using input from aircraft sensors mounted on doors, parking brakes, and struts.

ACARS messages may be sent using a choice of communication methods, such as VHF or HF, either direct to ground or via satellite, using minimum-shift keying (MSK) modulation.

VHF communication is line-of-sight propagation and the typical range is up to 200 nautical miles at high altitudes.

[13] On 24 February 2021, the ACARS unit of a South African Airways flight from Johannesburg's OR Tambo International Airport to Brussels sent an ACARS message about an “alpha floor event”, which was activated when the Airbus A340-600's envelope protection system activated to override the pilots to prevent the plane from stalling on take-off.

[14] On 19 February 2023, there were numerous ACARS reports of a large white balloon near Hawaii in the aircraft lanes.