An examination of the flight data recorder (FDR) of a Malaysia Airlines (MAS) Boeing 777-200 that experienced control problems in August on a flight from Perth, Western Australia, has revealed that the aircraft’s air data inertial reference unit (ADIRU) provided unusual and instantaneous acceleration values that resulted in the aircraft pitching.
Following the incident, the US Federal Aviation Administration issued an airworthiness directive to 777 operators to reinstall old software to prevent a possible failure of Honeywell ADIRUs (Flight International, 6-12 September).
The MAS 777 was 1h from Perth on a flight to Kuala Lumpur and on autopilot when the crew observed a low airspeed warning as the aircraft was climbing through flight level 380. At the same time the aircraft’s slip/skid indication deflected to the full right position on the primary flight display (PFD), the Australian Transport Safety Bureau (ATSB) says in its preliminary report.
The PFD then indicated that the aircraft was simultaneously approaching the overspeed limit and the stall speed limit. The aircraft pitched up and climbed to FL410 and the indicated airspeed dropped from 270kt (500km/h) to 158kt, while the stall warning and stick shaker devices also activated.
The pilot disconnected the autopilot, but continued to have control problems, with the aircraft pitching up and climbing another 2,000ft (600m). The crew requested a return to Perth and flew manually. At 3,000ft, on preparation for an instrument landing system approach to Perth, the PFD again indicated a low airspeed condition and the autothrottle system responded by commanding an increase in thrust. The aircraft landed safely.
The aircraft’s FDR, cockpit voice recorder and ADIRU have been examined, says the ATSB, revealing unusual acceleration values provided by the ADIRU. The values were used by the primary flight computer (PFC) during both manual and autopilot flight, although the severity of the pitching was reduced by the PFC comparing the ADIRU information with that from the standby air data and attitude reference unit.
EMMA KELLY/PERTH
Source: Flight International
