The US FAA is proposing that operators of 686 US-registered Boeing 757 perform corrective actions on the twin-jets within five years to prevent possible unintended roll axis motions just before landing in windy conditions.
"We have received numerous reports of Boeing 757 events where flight crews experienced unintended lateral oscillations during final approach, just before landing," says the FAA in a notice of proposed rulemaking to be published on 24 May. "One event resulted in a nose gear collapse after a hard landing and another event resulted in a tail strike during a landing that was aborted because of the oscillations."
The FAA notes that the oscillations "are characterized by large [control] wheel inputs at high rates that are out of phase with the airplane response and typically occur under certain gusty and turbulent wind conditions during landing."
Modifications for some aircraft will include the installation of new relays that will prevent the deployment of certain wing spoilers (used in conjunction with ailerons for large roll manoeuvres) when landing flaps are selected.
The proposed directive is related to a 2006 airworthiness directive (AD) that called on 757 operators to install a control wheel damper and flap vortex generators to reduce unintended roll control oscillations due to spoiler-induced flow separation on the flaps near touchdown.