SINGAPORE/PARIS - Investigators probing the crash of an AirAsia jetliner are examining maintenance records of a key part of its automated control systems, and how the pilots may have handled the plane if it failed, two people familiar with the matter said.
An outage of the twin Flight Augmentation Computers (FAC) could not have directly caused the Dec 28 crash, experts say, but without them the pilots would have had to rely on manual flying skills that are often stretched during a sudden airborne emergency.
"There appears to be some issue with the FAC," a person familiar with the investigation said, adding that more information was being sought from the manufacturer and airline.
Indonesia has said the Airbus A-320 jet climbed abruptly from its cruising height and then stalled, or lost lift, before plunging out of control into the Java Sea, killing all 162 people on board.
A second person familiar with the probe said investigators were looking at how the pilots dealt with the chain of events leading up to the crash. Neither person agreed to be identified, because details of the investigation remain confidential.
The pair of computers comprising the A-320's FAC system is mainly responsible for controlling rudder movements and helping to keep the airplane stable, as well detecting windshear, or sudden changes in wind speed or direction.
Indonesian magazine Tempo reported a series of maintenance problems with the computerised rudder system of that particular aircraft in the days and months before the loss of flight QZ8501.
Pictures of wreckage retrieved from the Java Sea provide little evidence that the crash was caused by problems with the rudder.
But, after partially analysing data from the "black box" voice and flight data recorders, investigators have extended their interest to the FAC computers, the two people familiar with the probe said.
A problem with the system may help explain another key element of the crash - why the jet did not automatically correct itself before entering into a stall, even if accidentally encouraged to do so by crew.
Airbus jets are designed to provide "flight envelope protection", making it virtually impossible to push them outside safe design limits when operating in normal flying mode.
But when the computers are unable to perform their tasks, control is automatically handed to the pilots who must fall back on training and fly manually, in so-called "alternate" mode.
A failure of both FAC computers - one primary, the other back-up - is one of those rare circumstances that can cause the usual stall protection to trip.
That alone would not explain why such a jet might crash and it is unlikely to be the only scenario being considered by investigators.