US investigators have determined that two passengers on board a United Boeing 757-200 suffered bone fractures after the crew responded to a conflict alert with an ”abrupt” pitch input.
Two cabin crew also received minor injuries during the incident, says the National Transportation Safety Board.
The aircraft had been operating from Newark to San Francisco on 19 September last year.
According to the first officer, who was flying, the captain switched on the ‘fasten seatbelt’ sign near the top of descent in preparation for the possibility of having to descend quickly, because the crew was still awaiting clearance to a lower altitude.
After descending and levelling at 32,000ft the jet was then cleared to 31,000ft but, since the flight was above the desired vertical flightpath, the first officer increased the descent rate using ‘flight-level change’ mode.
About 500ft above the assigned altitude, the crew received a traffic advisory from the collision-avoidance system. The advisory related to an aircraft crossing 1,500ft below the 757.
As the first officer adjusted the descent rate by switching to ‘vertical speed’ mode, the traffic advisory upgraded to a resolution advisory – an instruction to take avoiding action.
According to the captain’s testimony to the safety board, “the startle factor in the [resolution advisory] was extremely high because of how fast it occurred [after the initial traffic alert]”.
The first officer responded by disengaging the autopilot and autothrottle, and pitching the aircraft nose-up using guidance on the primary flight display.
Recorded flight data indicates that the aircraft’s pitch increased by 3° in the space of 1s – from 1.76° nose-down to 2.81° nose-up – halting the descent. Vertical forces ranged from 2.3g to 0.6g over 2s.
Although the ‘fasten seatbelt’ sign was on, some passengers had not returned to their seats prior to the manoeuvre.
One passenger in an aft lavatory sustained a spinal vertebra fracture in the lower back after being thrown upwards and forwards. Another passenger, exiting a lavatory, was similarly jolted upwards and fractured an ankle as he landed.
Two cabin crew members, located in the forward galley, fell to the floor during the manoeuvre and suffered minor injuries.
After the aircraft (N12125) landed on San Francisco’s runway 28L, paramedics met the flight at gate F21 and transported the injured passengers to hospital.
