Position & route reports
Coordinates, waypoints, estimated times, departure and destination data — when an aircraft chose to send them.
ACARS 101 • messages, not radar
What exactly was ACARS?
ACARS — the Aircraft Communications Addressing and Reporting System — is a digital communications system used by aircraft and ground organizations to exchange short operational messages. ACARSd primarily listened to VHF ACARS transmissions with an ordinary airband receiver connected to a computer sound card.
The important distinction
ACARS was not a flight-tracking protocol.
Modern ADS-B is built around frequent surveillance broadcasts: position, altitude, velocity and identity. ACARS served a different purpose. It carried operational communication. A message might contain a position — but it might instead contain an engine report, a clearance, maintenance status, a diversion request or a message between crew and operations.
That is why ACARSd could sometimes know remarkably much about what was happening aboard one aircraft while receiving almost nothing useful about another aircraft flying directly overhead.
WHO APPEARED?
Mostly commercial airliners.
The traffic received by ACARSd was dominated by airline aircraft. Business and executive jets could appear when suitably equipped. Small general-aviation aircraft, helicopters and military aircraft were largely absent from the VHF ACARS traffic seen by the project.
This describes the practical ACARSd monitoring experience, not a technical rule that excludes other aircraft categories from ACARS.
What could you see on ACARS?
Far more than a dot on a map. The reconstructed archive contains examples from all of these categories.
Clearances & oceanic routes
Oceanic clearances could expose NAT tracks, individually planned “RANDOM ROUTE” waypoints, flight level and Mach number.
Performance data
Oil pressure, fuel flow, temperatures, burner pressure and other aircraft/engine parameters appeared in maintenance and performance reports.
Faults & defects
Cabin pressure controllers, lighting, doors, equipment, maintenance status and fault reports were routinely transported.
Diversions & dispatch
Crews and operations could discuss alternate airports, fuel, weather, duty limits and other operational decisions.
Medical events
Requests for ambulances or medical assistance, use of oxygen or emergency medical equipment, and diversion planning sometimes appeared in readable messages.
Everyday airline operations
Cabin defects, loading issues, ground handling, equipment replacement and countless ordinary operational details.
Encoded or obscured payloads
Correctly decoded ACARS frames did not guarantee readable content. Some airline-specific payloads were transformed or otherwise encoded.
Real messages from the recovered archive.
These excerpts illustrate the range of material. They are presented as historical technical examples, not as a complete interpretation of each airline's proprietary message format.
| ACARS | ADS-B |
|---|---|
| Operational messaging system | Surveillance broadcast system |
| Messages occur when there is something to transmit | Aircraft state is broadcast repeatedly |
| Position appears only in some messages | Position is central to the service when available |
| Can carry text, clearances, faults, maintenance and performance data | Primarily identity, position, altitude, velocity and related surveillance data |
| A correct frame may still contain proprietary/encoded payloads | Standardized surveillance fields are designed for machine interpretation |
Sometimes ACARS told you too much
Readable radio did not mean public information.
VHF ACARS could be received by hobbyists, yet some messages contained information that, in retrospect, should never have been sent in readily readable form over such a channel. During the ACARSd years, sensitive operational information was occasionally observed and, in a very small number of cases, apparent payment-card information associated with onboard purchases.
No such personal or payment data is reproduced in this museum.
Developer recollection
“I don't want to wake the sleeping lion.”
Some German-airline payloads received during the ACARSd years were already unreadable after the ACARS frame itself had been decoded. Members of the monitoring community told KjM that this practice was connected to lessons following Hapag-Lloyd Flight 3378 in 2000. That causal link has not been independently established and is preserved here only as community oral history.
When somebody asked why he did not simply try to reverse the transformation, his answer was: “I don't want to wake the sleeping lion.”
Decoded does not always mean understood: ACARSd could correctly recover the radio frame, aircraft registration, flight number, label and payload bytes while an airline-specific payload itself remained proprietary, transformed or encoded.