Aircraft Communications Addressing and Reporting System (ACARS) is a short message digital datalink protocol transmitted around 129-137 MHz in different regions. As well has assisting air traffic control, it is (I gather) how services like flightaware get their data.
For a quick test, I’m using the open-source rtl_acars_ng. It built and ran on MacOSX without trouble, connecting to an R820T2+RTL2832U dongle.
As always, all notes, schematics and code are in the Little Electronics & Arduino Projects repo on GitHub