Brazilian investigators of the Amazon mid-air collision between a Gol Boeing 737 and an Embraer Legacy business jet have interviewed the Legacy's pilots for the first time, but still have to determine why the jet's transponder deactivated, with the loss of the collision-warning system.

Both Legacy pilots have denied deliberately carrying out any action that would have switched off the transponder and do not recall doing anything that might have accidentally caused it to stop operating.

All 154 occupants of the Gol 737-800 were killed in the 29 September 2006 collision. The Legacy escaped with less damage and all those on board survived.

Both Legacy pilots involved in the accident listened to the jet's cockpit-voice recorder and were interviewed separately on 29-31 January, in the presence of lawyers, at the headquarters of the US National Transportation Safety Board. Until then, the two pilots had responded to questionnaires via the NTSB, says the Brazilian accident investigation agency CENIPA.

"The pilots affirmed that they did not conduct any intentional action to interrupt the functioning of the transponder, and consequently the aircraft's anti-collision system, and do not perceive or remember having done anything that could have caused the accidental interruption of this equipment," says CENIPA.

CVR data shows the transponder deactivation occurred during a period of silence in the cockpit lasting 1min 43s. During this time, the pilots were performing planning calculations for the next stage of the flight.

The pilot in the right-hand seat was using a laptop computer, but CENIPA says studies to re-enact the flight show there is "no way" the laptop could have struck the radio management unit's control button in such a way as to put the transponder into standby mode - the button would need to have been touched twice in 20s.

Last year the FAA distributed an advisory to Embraer Legacy operators warning that pilots could accidentally deactivate the transponder by putting their feet on the footrests below the instrument panel, and inadvertently nudging the radio management unit tuning control. But CENIPA has not publicly linked this possible action to the collision.

CENIPA says the Legacy footrest has a protective shield that prevents the pilots' shoes from encroaching directly onto the instrument panel. Even if the pilot on the left side flexed his right foot, he could only reach the buttons that change radio frequencies, and not the button to change the transponder mode, the agency adds.

The investigation is in its final phase, says CENIPA, but the air traffic controllers involved at the time of the accident are, on the advice of defence lawyers, "persisting in the position of not participating in any interview" because of concerns that the information will be used in a criminal trial.

Source: Flight International