American Airlines launched transatlantic service yesterday using a Piper PA-28-201T, or at least that’s what our receivers saw when flight AA755 departed Paris for Philadelphia yesterday. In reality, the issue lies in a miscoded Mode S transponder.

What’s in a name?

To track each flight we use an aircraft’s Mode S address (ICAO 24-bit address), usually represented as the ‘Mode S hex code’ on our site and in our apps. This address is unique to an aircraft worldwide. In the case of American’s Piper, a one bit error in the Mode S address turns the Airbus A330-200 registered N284AY into a Piper registered N2837A. Here’s how:

N284AY is assigned hex address A2DCEE, which when entered in binary is 101000101101110011101110.
N2837A is assigned hex address A2DC6E, which when entered in binary is 101000101101110001101110.

Spot the difference? A single bit flip changes the A330 into a Piper.

101000101101110011101110
101000101101110001101110

Air traffic control is using the octal ‘squawk code’ and callsign to control the flight, so an incorrect Mode S address doesn’t have an effect on the flight in this case, but it does it make interesting to watch. As the ADS-B mandate nears, however, the FAA is trying to ensure all transponder codes are accurate.