Investigators believe the prime suspect behind the loss of transponder transmissions from the Embraer Legacy involved in the Amazonian mid-air collision to be accidental deactivation while the commander was accessing fuel data.
Interruption of the transponder crucially rendered the TCAS collision-avoidance system inactive for nearly an hour before the Legacy struck wings with a Gol Boeing 737-800 on 29 September 2006.
While the reason for the sudden loss of the transponder remains undetermined, Brazilian investigation agency Centro de Investigacao e Prevencao de Acidentes Aeronauticos (CENIPA) has drawn up a “most likely” scenario after analysing five possible sets of circumstances.
Transponder status is governed through the two radio management units (RMU), located in the central instrument panel area. Switching the transponder to "standby" requires the pilot to press a specific button – the fourth down, on the left side of the unit's screen - twice in 20 seconds.
In the minutes leading to the transponder interruption the pilots had been examining take-off calculations from Manaus, the Legacy’s destination, because they had received NOTAM information about runway length restrictions. While the co-pilot was using a notebook computer on his lap, neither crew member was able to recall what the commander was doing.
But CENIPA says that the cockpit-voice recorder indicates the commander was looking at fuel data on the RMU display, to assist the co-pilot in the Manaus calculations. The RMU shows the status of the TCAS but the display can be switched to show other information, including fuel data.
"While the [commander] was apparently trying to clarify the issue of the airplane fuel status...his radio management unit was used in a way that changed the...transponder...to 'standby'," says CENIPA’s final report into the accident.
It says the change to ‘standby’ happened just as the co-pilot suddenly informed the commander, after a period of silence, that he had finally obtained some data about departing Manaus.
"The most likely explanation...is that the [commander], while inattentively trying to return his radio management unit to the page of communications after consulting the fuel page, did not notice that he had pushed a button that would chance the status of the transponder to 'standby'."
The inquiry says the commander would have then focused his attention on his work with the co-pilot over the Manaus situation.
CENIPA has considered accidental contact between the notebook computer and the RMU switch but discarded the possibility. It says this would have required the notebook to be placed between the pilots, in a position that made its use “impossible”.
Intentional deactivation has been discarded as highly unlikely – especially in reduced vertical separation minima airspace – and extensive testing by manufacturer Honeywell has turned up no evidence of intermittent or continuous failure of the avionics.
While a fifth scenario – that the commander used the footrest situated next to the RMU, and accidentally nudged it – has been explored, CENIPA has also dismissed this hypothesis.
It says cockpit-voice recordings give “no sounds” that are compatible with seats being moved back into a position for using the footrests. The stops on the footrests themselves would make the movement to touch the RMU buttons unnatural and uncomfortable.
CENIPA calculates the probability of transponder deactivation this way at 5.2 x 10-15, equating to once in 192 trillion flight hours: "This kind of condition can be considered virtually impossible to occur during the fleet's lifetime."
During interviews undertaken in the progress of the investigation the Legacy’s commander was unable to recall whether he might have been verifying the fuel system. CENIPA says he was “definite” in stating that he did not remember taking any action that could have caused the transponder to stop operating.