US investigators probing the Atlas Air Boeing 767-300 freighter crash have yet to explain fully the initiating circumstances behind the elevator deflection which pushed the aircraft into a fatal dive on approach to Houston.
Crucially the National Transportation Safety Board has shifted its immediate emphasis, through the unusual decision to amend its phrasing while detailing preliminary findings.
While the NTSB had initially stated that the aircraft had pitched down “in response to column input”, it subsequently revised this, saying the downward pitch was the result of “nose-down elevator deflection” – an amendment designed to avoid premature conclusions being drawn over the relation, if any, between actions in the cockpit and the unusual attitude of the aircraft.
Although the initial use of the term “column input” might suggest there was a nose-down command of some degree, the NTSB has not clarified the extent of any pressure placed on the yoke – or the reason – nor whether the elevator deflection was in line with the command.
Weather radar images indicate that the 767 would have encountered the edges of a band of precipitation as the jet headed west over the north-eastern shore of Trinity Bay.
The inquiry says the aircraft, which was being vectored to avoid the heaviest of the weather, appeared to enter a region of turbulence as it briefly levelled at around 6,200ft.
Investigators then found that the aircraft, for reasons still unclear, then experienced an increase in engine power to maximum thrust, even though the airspeed was steady at 230kt. The jet pitched upwards, to around 4°, although the NTSB has not specified whether this was a natural consequence of the increased power.
There is no evidence of a stall – the stick-shaker was not activated – and the NTSB has not explained whether the subsequent nose-down manoeuvre was a reaction to the pitch-up attitude, an input to continue an expedited descent to 3,000ft previously advised by air traffic control, or attributable to other factors.
But the extraordinary transition to a 49° nose-down pitch, which took place over 18s, is central to the inquiry. The NTSB has not specified whether the aircraft was in cloud at the time of the transition, but it had clearly emerged from the cloud base into good visibility during the last few seconds of its descent.
With investigators yet to establish conclusively whether there is a connection between control column movements and the aircraft’s excessive nose-down attitude, the possibility of a mechanical reason for the elevator deflection is yet to be ruled out.
The 767 has previously been the subject of airworthiness directives including measures to prevent corrosion of ballscrew components in the drive mechanism for the horizontal stabiliser, which could potentially lead to loss of stabiliser control.
Elevator power control actuators have also been a previous focus of 767 directives; a 2014 bulletin ordered checks to ensure aircraft were not operating with failed shear rivets in the actuator mechanism and to prevent jamming and a possible elevator hardover – which could result in a significant pitch upset.
The NTSB has not disclosed any information on the position of the horizontal stabiliser or the condition of the drive mechanism and the elevators’ mechanical linkages.
But it does indicate that the severity of the dive had lessened as the aircraft descended towards Trinity Bay, with the pitch reducing by some 30°, to around 20° nose-down, before impact. The inquiry has not stated, however, whether this was the result of recovery actions – including column input – in the cockpit, movements of the elevator or stabiliser, or other aerodynamic effects.