Investigators believe that early reduction of engine thrust on a Boeing 767-300ER during a landing at Kabul caused the aircraft to sink, triggering a response which led to a damaging tail-strike.
The Omni Air International twinjet (N768NA) had been conducting a visual approach to the Afghan capital in variable gusting wind conditions, following a charter flight from Bucharest on 20 June 2014.
US National Transportation Safety Board investigators state that the crew had set a landing reference speed of 145kt but added 10kt to account for the winds. The airspeed fluctuated between 162kt and 138kt during the approach, and the engine thrust varied accordingly.
The inquiry states that the aircraft’s vertical navigation system was not indicating a vertical path.
As the aircraft descended through 1,000ft on the approach to runway 29, the tower controller advised that the winds were from 360° at 20kt but gusting to 40kt.
The aircraft was above the glidepath and slightly fast at a height of 200ft, and the captain reduced thrust for correction, but the crew heard an automated ‘sink rate’ alert at about 100ft.
“The captain stated that he increased power and started to flare,” says the NTSB. “As the crew sensed the [aircraft] drop, he increased pitch attitude to arrest the sink rate but landed hard and over-rotated in the flare.”
Analysis of the wind conditions indicated a “significant change” in the final moments of the approach, it adds, with an updraft transitioning to a downdraft, and a 17kt headwind component shifting to a 1kt tailwind component, in the 5s prior to touchdown.
Data from the enhance ground-proximity warning system showed it had not generated a windshear warning during the approach. The NTSB notes that windshear alerts are suppressed below 50ft.
The aircraft touched down at about 140kt. While guidance in the operator’s manuals pointed to a 4-6° nose-up attitude during a normal landing, flight-data recorder information showed the pitch increased from 1° just before the flare to a maximum pitch of 9.5°. The touchdown impact was around 2.5g.
“Charts and guidance included in the manufacturer’s and operator’s flight manuals indicated that, at touchdown, the pitch attitude exceeded the contact limit of the [aircraft],” says the inquiry.
Four of the 16 crew members – but none of the 65 passengers – received minor injuries during the event. But the aircraft suffered substantial damage to its aft lower fuselage, with “multiple” holes and cracks over a 25ft length. Five bays were damaged at the lower end of the aft pressure bulkhead.
The captain of the aircraft had accumulated 865h on 767s out of a total flight time of over 16,300h.
Omni Air International drew attention to tail-strike risk after two occurrences involving 767s two years before the Kabul landing. The communication reviewed flight-training manual information and identified a slower-than-normal airspeed as a cause of tail-strike incidents.