ACARS (aircraft messages)
A system that sends short text messages between airliners and their airlines: departure and arrival times, positions, weather and maintenance notes.
What does it sound like?
A short, shrill burst, like a squeaky modem, heard in AM.
Who uses it?
Airlines and aircraft.
Where and when can I hear it?
VHF airband, for example 131.550 MHz (worldwide) and 130.025, 129.125 or 131.725 MHz.
The science
ACARS sends data at 2400 bits per second as audio tones on an ordinary AM airband voice channel: minimum-shift keying (MSK) centered at 1800 Hz with a 1200 Hz shift, so the audio alternates between 1200 and 2400 Hz. Characters are 7-bit ASCII with an eighth bit for odd parity, and each block ends with a 16-bit block check sequence that lets the receiver detect corrupted data. A block carries a 7-character aircraft address, a two-character label saying what kind of message it is, and up to 220 characters of text.
Why use it?
Because the data rides on the aircraft's standard VHF AM radios, around 130 MHz in the ordinary airband, airlines could automate reports on equipment they already had. Much traffic that crews once passed by voice, such as data from aircraft sensors and flight plans, now moves as ACARS messages.
Lore
ACARS has been in use since 1978. Its first application was OOOI: automatically reporting the out-of-gate, off-ground, on-ground and into-gate times, sensed from doors, parking brake and weight on the wheels. Piedmont Airlines famously measured the value of those accurate times, especially for tracking pilots' monthly flight limits.
What do the decoded parts mean?
- Registration: The aircraft's tail number.
- Flight: The flight number.
- Label: A two-character code for the message type.
- Text: The message: often abbreviated times (out, off, on, in), positions or weather.
Easily confused with
- VDL Mode 2 (aircraft data link): VDL2 bursts are shorter and denser