Brazil Midair Collision Prompts NTSB Recommendations on TCAS

The midair collision of a Gol Airlines' B-737-800 and an Excelaire Legacy 600 over the Amazon jungle of Brazil last September has led the U.S. National Transportation Safety Board (NTSB) to make recommendations to the FAA for TCAS (traffic alert and collision avoidance system) and transponder changes to help ensure they are operating properly in flight.

The NTSB says preliminary findings from the accident investigation suggest that the Legacy's transponder and TCAS weren't operating immediately before the accident, which not only prevented its own TCAS from issuing alerts and warnings, but also made it "invisible" to the 737's TCAS.

The Legacy's last altitude clearance before the accident was FL (flight level) 370, which was confirmed by ATC during a frequency change just south of Brasilia, and which controllers continued to observe until the airplane was about 30 miles north-northwest of BRS.  At that point, the Legacy's transponder return was no longer observed, and remained undetected until shortly after the collision. According to ATC radar data, other aircraft in the vicinity produced normal transponder returns.

Data from the Legacy's FDR (flight data recorder) indicated that its pilots didn't perform any abnormal maneuvers during the flight.

The Legacy's CVR (cockpit voice recorder) data indicate that, during the crew's emergency descent to Cachimbo Air Base following the collision, the crew discussed whether or not the TCAS was on. ATC radar data indicate that the transponder return for the airplane was again visible to ATC radar less than 30 seconds after these comments.

Flight data recorder (FDR) and cockpit voice recorder (CVR) information from both airplanes revealed no indication of any TCAS alert on board either airplane (both airplanes were equipped with mode S transponders and TCAS II computer units), no evidence of pre-collision visual acquisition by either flight crew, and no evidence of evasive action by either crew.

According to the NTSB, for a TCAS to work properly, both it and the transponder must be turned on - not off or in "standby."  While a TCAS computer can fail by itself, such an occurrence would only affect its own ability to detect aircraft; TCAS on other aircraft will detect an aircraft with an inoperative TCAS as long as its transponder continues to operate.  But when an aircraft's transponder is inoperative, not only is its TCAS alerting information lost, but TCAS on other aircraft can't "see" it.

In this case, the only notification to the Legacy pilots regarding the loss of TCAS functionality was a small text message on the pilots' flight display that read "TCAS off" in white lettering.  Similarly, a loss of their transponder functionality would have been indicated by a small message on the radio management unit reading "ATC fail" or "standby."  Today's systems don't require pilots to acknowledge or cancel these warnings.

NTSB Recommendations

Subsequent to this accident, the NTSB has determined that the Legacy's TCAS and transponder annunciations are largely consistent with what is found on all airplanes that are TCAS equipped.  Therefore, to help prevent a reoccurrence, it is recommending that the FAA mandate TCAS installations have a method of warning the pilot if the system isn't functioning properly.

The NTSB's recommendation would have the FAA impose such a mandate for all aircraft that require a TCAS.  The rule would require both presently installed TCAS, and future TCAS installations, to include an aural and visual warning to the pilot of any failure to function properly while airborne, and to get a pilot acknowledgment of the warning.

The Board also wants the FAA to evaluate the feasibility of providing similar aural and visual warnings for future systems that provide ground collision avoidance, and if feasible, require that they include this function.

Finally, the Board wants the FAA to inform all pilots who use transponders or TCAS about the circumstances of this accident and ask all pilots who use transponders and TCAS units to become familiar with their current annunciations.

U.S. Federal Aviation Regulations require TCAS to be installed on all turbine-powered airplanes of more than 33,000 pounds maximum certificated takeoff weight operating under 14 CFR Parts 121, 125, and 129 and on all airplanes with 10 to 30 seats that are operated under Parts 121, 129.

A complete discussion of the NTSB's findings and rationale can be found at http://www.ntsb.gov/recs/letters/2007/A07_35_37.pdf.  05-04-2007.

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