Airbus A320 operators are being advised to modify the type's warning logic in a bid to provide additional safeguards against accidental failure to retard thrust on landing.
The 'enhanced retard' or 'F5' modification is intended to alert pilots if they have not correctly reduced thrust during touchdown.
Investigation of the fatal TAM A320 overrun at Sao Paulo Congonhas three years ago determined that, for unknown reasons, the crew only retarded one of the two throttle levers on landing. There had been previous instances of similar omissions, notably the overrun of a TransAsia Airways A320 at Taipei in 2004.
Standard landing procedure on the A320 requires pilots to retard both throttle levers during landing. This is necessary for flight controls such as the spoilers to deploy.
European Aviation Safety Agency regulators are drawing up an airworthiness directive aimed at ensuring that the A320 logic continues to warn pilots if they do not follow this procedure correctly.
While the 'retard, retard' call-out normally stops if reverse-thrust is engaged on one of the two throttles, Airbus says the 'F5' change ensures that the warning will continue as long as one engine remains above idle thrust.
It will only stop if the pilot sets both throttle levers to idle, or sets one lever to idle and the other to reverse.
"In addition, the warning itself changes in both loudness and also in the frequency of the call-outs," says Airbus. The rate at which the 'retard, retard' alert is spoken becomes faster and more intense.
EASA is proposing the new airworthiness directive after analysing the consequences of pilots' failing to follow correct procedures or mismanaging the throttle levers during landing.
"The investigation results identified the need for improvements in the identification of throttle mispositioning and so providing further opportunity for the flight crew to identify an incorrect thrust-lever configuration and to correct this," it states.
While aircraft can safely be dispatched with one thrust-reverser inoperative, EASA points out that this scenario has led to cases of one lever being left in forward thrust, while the other is set to idle or reverse. The TAM A320 had been dispatched with a single inoperative reverser.
EASA adds that a second asymmetric thrust scenario can occur when one throttle lever is moved forward after landing, usually when returning it from reverse to idle.
Its proposed directive will require operators to ensure that flight warning computers on A320-family aircraft meet the required standard within four years, although update of the A319 Corporate Jet must wait for a further modification to the auxiliary fuel-management computer due early next year.