Introduction

• A design deficiency with the Rockwell Collins Mode S TPR-901 transponder can cause it to transmit an incorrect Mode A code (0607), instead of the code entered by the crew, when it is interrogated by radar systems. As the incorrect code is not displayed in the cockpit, the crew are unaware of this problem.
• This deficiency can occur both on the ground and in the air, resulting in a loss of code callsign correlation and flight data on ATC radars. This increases air traffic controllers' workload, reduces their ability to carry out tasks efficiently and has the potential to cause a serious incident. The operation of the Airborne Collision Avoidance System (ACAS) safety net, however, is not affected by this issue.
• This problem was first identified at the end of 2004 and appears to be most prevalent within European airspace, although it has also been observed in airspace outside Europe (e.g. Australia).