German investigators believe Airbus should assess aspects of A320 flight logic after determining that partial loss of lateral control authority contributed to the Lufthansa wing-strike in Hamburg two years ago.
While the investigation agency BFU states that a combination of circumstances led to the wing-strike during an abortive landing attempt in strong crosswinds, it determined that the aircraft switched from 'flight' mode to 'ground' mode at a critical moment, even though it was still technically airborne.
The mode switch was triggered by the brief touchdown of the left main landing-gear during the 1 March 2008 landing. Even though the right main gear did not make contact, the control logic transitioned to 'ground' mode, halving the aileron deflection available through the pilot's sidestick.
Just after the touchdown the aircraft lost contact with the runway, and in the gusting wind conditions banked 23° left. Both pilots reacted with full right sidestick, and up to 14° right rudder, but the limited control authority meant they were unable to counter the bank enough to avoid the wing-tip strike.
"The aircraft's system behaviour contributed to a flight attitude which was unintended and undesired by the pilots, and ground contact with the wing-tip could not be prevented," says BFU's final report into the incident, in which the A320's wing fence suffered damage.
The crew executed a go-around and landed safely on a different runway at the airport.
BFU says the logic transition to 'ground' mode, even though the aircraft was "still in the air", resulted in a situation in which system functionality "was not unambiguous". The investigation agency adds that it "regards this as safety relevant", particularly because the reduced effect of the controls was "unknown" to the pilots and the airline's training department.
"It is impossible to clarify whether the pilots would have taken a decision for a go-around procedure earlier, had there been a risk evaluation that included aircraft system behaviour, then unknown to the crew," says BFU, but says it believes this knowledge "would probably have influenced the decision".
While the Airbus flight control operations manual says there is "no discontinuity" in the A320's control laws, BFU says this is "inaccurate" because the transition from 'flight' to 'ground' mode can take place with a single main-gear touchdown.
Airbus has responded with a justification of the system design. It states that the limitation of the flight controls during touchdown is designed to avoid pilot-induced oscillation during landing.
BFU says this is "understandable" but has recommended that Airbus assess the transition logic with the aim of ensuring that 'ground' mode engages "only if the aircraft is indeed on the ground".