Television station broadcasting has in certain cases been found to interfere with ATC transponders and cause momentary TCAS OFF indications and a TCAS OFF EICAS advisory message. This phenomenon has occurred in limited locations in China, Japan, and Northern Italy. Boeing has reasons to believe that when an airplane (equipped with certain Collins Transponders) over flies a television transmission tower which is operating outside of its specified transmission limits, the transponder may experience a receiver or squitter fault due to a 1030-MHz carrier wave harmonic. The fault causes a TCAS OFF indication on the Navigation Display (ND) or horizontal situation indicator (HSI), as applicable, and a TCAS OFF advisory message (as installed) on EICAS-equipped airplanes. This interference phenomenon has been observed in cruise flight.
Separately, some transponders have also exhibited momentary TCAS OFF indications when operating in traffic congested areas. When attempting to reply to numerous interrogations, a transponder reply rate limit can be exceeded, resulting in replies being inhibited. When this occurs in these transponders, the squitter is also inhibited, causing the squitter monitor to fail momentarily. This condition can occur either in the air or on the ground.
These phenomena may be observable on 737NG, 747-400, 757, 767, and 777 model airplanes with Collins P/N 822-0336-001 (TPR900) and P/N 822-1338-002/003 (TPR901) ATC Transponders.
In either case, the transponder’s ATC communications may be unaffected, but TCAS is operating in standby mode while the TCAS OFF indication is displayed. The fault will clear when the condition no longer exists, or it may be cleared by selecting the other transponder when the fault occurs, and then switching back to the original transponder. If the EICAS message and/or the TCAS OFF ND/HSI indication blanks, it may be considered a nuisance and no further flight crew action is required. It is possible the situation may re-occur until the airplane has left the proximity of the offending transmission tower, and/or the congested airspace.
Boeing Flight Operations Review 30, 20th January 2009