JAKARTA, Indonesia — The pilots of AirAsia Flight 8501 cut power to a critical computer system that normally prevents planes from going out of control shortly before it plunged into the Java Sea, two people with knowledge of the investigation said.
The action appears to have helped trigger the events of Dec. 28, when the Airbus Group NV A320 plane climbed so abruptly that it lost lift and it began falling with warnings blaring in the cockpit, the people said. All 162 aboard were killed.
The pilots had been attempting to deal with alerts about the flight augmentation computers, which control the A320's rudder and also automatically prevent it from going too slow. After the initial attempts to address the alerts, the flight crew cut power to the entire system, which is comprised of two separate computers that serve as backups to each other, the people said.
While the information helps show how a normally functioning A320's flight-protection system could have been bypassed, it doesn't explain why the pilots pulled the plane into a steep climb, the people said. Even with the computers shut off, the pilots should have been able to fly the plane manually, they said.
Flight 8501 climbed more than 5,000 feet (1,524 meters) in less than 30 seconds, rising above the altitude where it was authorized to fly, Ertata Lananggalih, an investigator with Indonesia's National Transportation Safety Committee, said in Jakarta on Thursday.
The co-pilot, with 2,247 hours of flying experience, was at the controls and communicating with the ground while the captain, who had 20,537 hours, was monitoring, said Mardjono Siswosuwarno, the lead investigator of the crash. The account was the first description of the last moments of the plane.
The investigators didn't address whether pilots had cut power to the flight augmentation computer system and said they wouldn't release more information on the case.
From a cruising altitude of 32,000 feet, the single-aisle A320 climbed to 37,400 feet as pilots probably tried to avoid bad weather, Lananggalih said. The aircraft then descended slowly for three minutes before it disappeared, he said.
"The pilots were conscious when the maneuver happened," he said. "They were trying to control the airplane."
The aircraft, operated by the Indonesian affiliate of Malaysia-based AirAsia, disappeared from radar en route to Singapore from Surabaya.
Indonesia won't release a preliminary report on its investigation into Flight 8501 because fact-findings could change rapidly, Tatang Kurniadi, head of the commission, said Thursday. Indonesia sent the preliminary findings to all countries involved in the investigation on Jan. 28, Kurniadi said.
The pilots had sought permission from air traffic control to turn left and then to climb to 38,000 feet because of storm clouds. Four minutes after the request, a controller cleared the pilots to climb to 34,000 feet, he said.
Satellite imagines showed storm clouds that reached as high as 44,000 feet, according to investigators.
The aircraft was in "good condition," said Siswosuwarno.
All Airbus models produced since the 1980s are designed to prevent pilot errors from causing crashes. The planes are controlled by multiple flight computers, which limit pilots from overly steep turns or getting too slow.
In the event of a malfunction or loss of power, the flight protections will shut down and leave the pilots to fly the plane manually. That appears to be what happened before Flight 8501 entered the steep climb and stalled, the two people said.
Investigators are still trying to determine why the pilots would cut power to the FACs by pulling a circuit breaker in the cockpit.
Indonesian authorities have so far recovered at least 70 bodies from the search. Investigators still haven't managed to lift the fuselage of the jet. The tail section of the plane has been retrieved. Indonesia's military pulled out of the search this week.
The cockpit-voice recorder captured the pilots' voices and no explosion was heard, Nurcahyo Utomo, an investigator with the committee, said last week. The flight-data recorder captured 1,200 parameters and the voice-recorder captured the last two hours and four minutes of the flight, the investigators said. The two devices are called the black boxes. After studying data from the black boxes, authorities ruled out terrorism as a factor that brought down the plane.
Flight 8501 appeared to have stalled after climbing steeply, Minister of Transportation Ignasius Jonan said earlier this month. A stall occurs when airflow over the wings is disrupted or becomes too slow to provide lift and keep a plane aloft.
Indonesia has said it intends to shut the agency responsible for coordinating aircraft flight slots in three months. That's after the AirAsia flight took off on a Sunday, without a Ministry of Transportation permit to fly that day.
The government has since suspended the license of AirAsia for that route, found other airlines in breach of permits and removed officials involved from the ministry, AirNav Indonesia and state airport company PT Angkasa Pura 1.
The airline made an administrative error in flying QZ8501 on Sunday, AirAsia Indonesia Chief Executive Officer Sunu Widyatmoko said Jan. 13. The carrier didn't inform the Directorate of Air Aviation on the schedule revision, he told parliament in a hearing.
Park reported from Singapore. Andrea Rothman contributed from Toulouse, France.
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