A new airworthiness directive issued by the US Federal Aviation Administration requires Boeing 777 operators to inspect and potentially replace the aircraft's dual pitch rate sensors (PRS).

The directive, which applies to 777-200LRs, -300s, -300ERs and 777 freighters, comes in response to reports received by the agency of dual failures of the sensors.

When both sensors fail, the aircraft's primary flight computer transitions from primary to secondary mode, which causes the autopilot to disengage, says the directive, made public by the US Office of the Federal Register on 19 November.

"We are issuing this AD to prevent a dual PRS failure that could cause an automatic disengagement of the autopilot and autoland," says the DOT's directive.

Losing both autopilot and autoland can be dangerous, particularly if it "occurs at low altitude and the flight crew is unable to safely assume control and execute a go-around or manual landing", adds the document.

The directive will be published in the Federal Register tomorrow and becomes effective 35 days later. It will require US operators to inspect, within 60 months, all four PRS on their 777s to determine the sensors' part number. Then, prior to further flight, operators must replace any PRS with part number 402875-05-01, says the directive.

DOT estimates airlines will spend $170 to inspect each aircraft and $340 to swap parts.

The new requirements come roughly 16 months after a Boeing 777 operated by Asiana crash in San Francisco after the pilots failed to understand complexities of the autopilot systems.

The National Transportation Safety Board pegged crew failures as the ultimate cause, but added that the pilot falsely assumed that the autothrottle was maintaining speed.

The aircraft's speed deteriorated when the autothrottle transitioned to "hold" mode while the flight computer was in "flight-level change mode", a configuration under which the autothrottle does not maintain speed.

The crash led to the death of three passengers.

Source: Cirium Dashboard