A rudder control system problem that had occurred 23 times in the previous 12 months coupled with the pilots' response led to last year's crash of an AirAsia plane that killed all 162 people on board, Indonesian investigators said Tuesday.
The National Transportation Safety Committee said an analysis of Flight 8501's data recorder showed the rudder control system had sent repeated warnings to the pilots during the Dec. 28, 2014 flight between the Indonesian city of Surabaya and Singapore.
Aircraft maintenance records for the Airbus A320 showed that problems with the rudder system had been reported 23 times during the year prior to the crash, with nine occurring in December. The investigators said the fault was caused by cracked soldering on an electronic circuit board.
Investigator Nurcahyo Utomo said the problem by itself should not have been dangerous. But after the fourth time an alarm went off during the flight, a crew member apparently went against recommendations and removed a circuit breaker to try to reset the system, he said.
The autopilot became disengaged, and the aircraft began to roll, but no movement was detected on the plane's manual control stick for nine seconds, he said. It then began climbing rapidly before stalling and plummeting into the Java Sea.
Utomo said the voice recorder showed the pilot said "pull down," but in fact the plane was ascending.
"It seemed that there was a miscommunication between the pilot and co-pilot after the fourth fault," he said.
The same warnings had occurred three days before the crash with the same pilot, who witnessed a technician on the ground addressing the problem by removing the circuit breaker and then replacing it, according to Utomo and the investigation report.
The last contact the pilots had with air traffic control indicated they were entering stormy weather. They asked to climb from 32,000 feet (9,753 meters) to 38,000 feet (11,582 meters) to avoid threatening clouds, but were denied permission because of heavy air traffic. Four minutes later, the plane dropped off the radar. No distress signal was issued, and investigators said weather conditions did not play a role in the crash.
Earlier reports indicated that the co-pilot was at the controls and the captain had left his seat. “The co-pilot pulled the plane up, and by the time the captain regained the controls it was too late,” one of the people familiar with the investigation said, reported the International Business Times.
"There is much to be learned here for AirAsia, the manufacturer and the aviation industry," tweeted Tony Fernandes, chief executive of the Malaysia-based budget carrier. "We will not leave any stone unturned to make sure the industry learns from this tragic incident."
The disappearance of Flight 8501 on Dec. 28, 2014, prompted an intense 17-day international search for the aircraft. The black boxes were located after three Indonesian ships detected two strong pings being emitted from their beacons, about 20 meters (22 yards) apart. The voice recorder captures all conversations between the pilots and with air traffic controllers, as well as any noises heard in the cockpit, including possible alarms or explosions. The flight data recorder saves information on the position and condition of almost every major part in the plane, including altitude, airspeed, direction, engine thrust, the rate of ascent or descent and what up-or-down angle the plane was pointed.