Brazilian investigators of the Amazon mid-air collision between a Gol Boeing 737 and an Embraer Legacy business jet have directly interviewed the Legacy’s pilots for the first time, but have still 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 which would have switched off the transponder and do not recall doing anything which might have accidentally caused it to stop operating.
Last year the US FAA warned that Legacy pilots could accidentally nudge the radio management unit – which also controls the transponder – while using the cockpit footrest.
But Brazilian accident investigation agency CENIPA has not publicly linked this possible mechanism to the collision, and has even appeared to detract from the possibility by highlighting contrary evidence.
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 recording and were interviewed separately over 29-31 January this year, in the presence of lawyers, at the headquarters of the US National Transportation Safety Board. Until then, says CENIPA, the two pilots had responded to questionnaires via the NTSB.
“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, adding that the pilots were interviewed voluntarily and that three NTSB members participated.
Cockpit-voice recorder data shows the transponder deactivation occurred during a period of silence in the cockpit lasting 1min 43s. During this period 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 that studies to re-enact the flight show there is “no way” that the laptop could have struck the radio management unit’s control button in such a way as to put the transponder into ‘standby’ mode. It says 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 placing their feet on the footrests below the instrument panel, and inadvertently nudging the radio management unit tuning control.
CENIPA, citing ergonomic considerations, subsequently stated that the footrest could not be used during flight if the pilot’s seat was in its normal position, and that the CVR would have picked up the movement of the seat. It said there was “no record of [seat] movement” on the CVR during the period in which the transponder stopped operating.
But it added that the CVR record did contain a comment from the left-hand pilot recommending that feet should not be put up in the cockpit during flight.
CENIPA said the Legacy footrest has a protective shield which prevents the pilots’ shoes from encroaching directly on the instrument panel. Even if the pilot on the left side flexed his right foot, it said, he could only reach the buttons that change radio frequencies, and not the button to change the transponder mode. But CENIPA did not, however, discuss the potential for interference from the pilot in the right-hand seat.
The investigators state that the information supplied by the pilots will be cross-referenced with other information to construct possible scenarios which could have led the transponder to stop functioning.
CENIPA says the investigation is in its final phase. But it adds that the air traffic controllers involved at the time of the accident, on the advice of defence lawyers, are “persisting in the position of not participating in any interview”, because of concerns that the information will be used in a criminal trial.
Source: flightglobal.com's sister premium news site Air Transport Intelligence news