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.

An American Airlines A330 displayed on Flightradar24 as a Piper due to a miscoded Mode S address

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.


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.

And for some, such errors even inspire creativity.

The famous transatlantic Piper XL330 (AA755)…on short final to CDG


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