Transponder error causes TCAS alert

The latest Risks Digest reports on a recent TCAS event caused by an altitude error in a transponder. The original details of the incident are found on what appears to be the British version of the NASA ASRS System, known as CHIRP, the Confidential Human Factors Incident Reporting Programme.

In the incident, an aircraft transponder was reporting spurious altitude information, which was known to ATC. The erroneous altitude however resulted in a TCAS Descent resolution advisory to a nearby aircraft. The reporter points out that had the error been different, the results could have been a climb and a TCAS caused collision. This report clearly illustrates the reason for those biennial transponder checks, and the reason turn off the altitude squawk when ATC recognizes an altitude error.