Failure by ground engineers to understand data confirming an Airbus A320 had suffered a severe hard landing in the Azores allowed the aircraft to be cleared to continue flying despite sustaining structural damage.
After the landing at Ponta Delgada, the captain of the SATA International flight told a ground engineer that he suspected a hard touchdown. But while they analysed a load report from the data management unit, they were "unable to clarify" the information, says Portuguese investigation agency GPIAA, and "suspected it might be inconsistent".
In its final report into the incident GPIAA says the aircraft - which was barely two months old, with only 533h - had experienced a 2.13g touchdown on Runway 30 after a high descent rate. It then bounced to a height of 12ft (3.6m), before dropping and hitting at 4.86g. An impact above 2.6g is categorised as hard.
Two figures were given on the load report, identifying the extent of the impact and the limiting vertical acceleration threshold.
But because the engineering department was closed, owing to the late hour, the ground staff could not reach anyone to help interpret the report. The pilots and engineer visually inspected the A320, but could not see anything irregular, and the event was not written up in the technical log.
"As they were unable to understand the load report, they concluded that the displayed data might be erroneous," says GPIAA. The aircraft was flown back to Lisbon, but a second attempt by engineers there to decode the load report was also unsuccessful.
Time pressures meant the aircraft was prepared and cleared for its next flight without the report being decoded, and the A320 flew six sectors after the landing before an A-check revealed damage to the wing shroud box on both sides, as well as compression damage to the main-gear tyres.
The aircraft subsequently underwent a dedicated full inspection programme. Both main-gear legs and their tyres were eventually replaced, for tests to be conducted on the originals, and the aircraft was returned to flight on 30 November 2009, nearly four months after the 4 August incident.
While the ground engineers were qualified, they did not carry out the actions required by the aircraft maintenance manual, says GPIAA, which adds that SATA has retrained all ground engineers to ensure they can correctly read load report data.
Flight-control logic in the A320 led the aircraft computer to deploy the spoilers as the thrust levers were retarded in mid-bounce, destroying lift and causing the heavy second impact.
While Airbus recommends executing a go-around during a high bounce, the airframer in 2010 began to introduce a change to its spoiler control logic to aid pilots.
The modification, SEC 120, enables a 10° spoiler extension on initial touchdown without retardation of the thrust levers, partly counteracting lifting forces and dampening any bounce.
Introduction of this partial lift-dumping logic to the A320 line started from airframe 4472 having already been developed for the A330 and A340.
GPIAA says that, had the SATA aircraft been similarly modified, the force of the second impact would have been reduced to 1.7g.