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Cause of Indonesia AirAsia plane crash finally revealed

Chief of Indonesian National Transportation Safety Committee Soerjanto Tjahjono holds a model plane during a news conference announcing the result of their investigation on the crash of AirAsia Flight QZ 8501 into the Java Sea last year, in Jakarta, Indonesia, Tuesday, Dec

Liza Yosephine (The Jakarta Post)
Jakarta
Tue, December 1, 2015

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Cause of Indonesia AirAsia plane crash finally revealed Chief of Indonesian National Transportation Safety Committee Soerjanto Tjahjono holds a model plane during a news conference announcing the result of their investigation on the crash of AirAsia Flight QZ 8501 into the Java Sea last year, in Jakarta, Indonesia, Tuesday, Dec. 1, 2015. Indonesian investigators say a faulty rudder control system and the pilots' response led to the crash of an AirAsia plane last year that killed all 162 people on board. (AP Photo/Achmad Ibrahim) (AP Photo/Achmad Ibrahim)

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span class="inline inline-center">Chief of Indonesian National Transportation Safety Committee Soerjanto Tjahjono holds a model plane during a news conference announcing the result of their investigation on the crash of AirAsia Flight QZ 8501 into the Java Sea last year, in Jakarta, Indonesia, Tuesday, Dec. 1, 2015. Indonesian investigators say a faulty rudder control system and the pilots' response led to the crash of an AirAsia plane last year that killed all 162 people on board. (AP Photo/Achmad Ibrahim)

The initial conclusion that large thunderclouds had brought down Air Asia Flight QZ8501 has been contradicted by the results of an investigation by the National Safety Transportation Board (KNKT), which revealed on Tuesday that the trigger of the plane crash was the flight control system.

KNKT Head Soerjanto Tjahjono explained that a series of interferences to the Rudder Travel Limiter Unit (RTLU) was the cause of the QZ8501 flight from Surabaya's Juanda International Airport to Singapore on Dec. 28, 2014 that killed all 162 people on board.

'€œWeather conditions and engine problems were not a factor in the downing of the Airbus 320,'€ Soerjanto said.

According to KNKT investigator Nurcahyo Utomo, four master caution signals were detected in relation to damage in the RTLU system. The first three caution signals were followed by the proper Electronic Centralized Aircraft Monitoring (ECAM) procedure by the flight crew.

'€œHowever, following the fourth call, the flight crew didn'€™t follow procedure,'€ KNKT investigator Nurcahyo told reporters at a press conference sharing the investigation results.

Nurcahyo added that RTLU failure was not a significant or fatal problem if the standard ECAM procedures were followed.

The Flight Data Recorder (FDR) indicated that the pilot had followed ECAM procedures after the first three failures of the RTLU. The FDR also suggested that the Circuit Break (CB) could have been pulled, however, there was no concrete evidence, such as a video recording, to prove this Nurcahyo added.

Investigators said that the CB was reset during each time the Flight Augmentation Computer (FAC) failed.

During the intermittent FAC failures, the FAC 1 went offline followed by FAC 2. The pilot repetitively reset FAC 1 and 2. Electrical interference on FAC 1 and 2 occurred during the fourth caution signal, which in turn deactivated both the auto-pilot and auto-thrust. Kinetic energy turned into potential energy and caused the airplane to roll.

The rudder shifted 2 degrees and the airplane rolled at 54 degrees for 9 seconds following the deactivation of the auto-pilot and auto-thrust. This caused the side stick to lift the plane'€™s nose by 48 degrees, accompanied by wind hitting the plane at a 40 degree angle.

At this point, the conditions were beyond the capabilities of the pilot to return the plane to normal conditions.

The plane rolled to an angle of 104 degrees at the peak height of flight of 38,000 feet, where its slowest speed recorded was 57 knots.

Investigation revealed that damages to the RTLU of the aircraft were recorded 23 times in the last 12 months. In the three months prior to the crash, the recorded reports came at increasingly closer intervals.

Nurcahyono said that AirAsia pilots had not been trained in recovering airplanes during upset conditions because it was indicated in the official AirAsia documents that such training was not recommended.

In conclusion, the KNKT explained that there were five contributing factors to the accident.

First, there was a crack in the solder joint of both channel A and B that resulted in a loss of electrical continuity and led to the RTLU failure.

The second factor was that existing maintenance data analysis led to unresolved repetitive faults occurring at shorter intervals for three months prior to the accident. The same fault that occurred that times during the flight.

The third factor was that following the fourth fault, the flight crew'a action was not in accordance with the ECAM procedures. The FDR recorded different signatured that were similar to FAC CB'€™s being reset, resulting in electrical interruption to the FAC's.

Fourthly, the electrical interruption to the FAC'€™s caused the autopilot to disengage and the flight control logic to change from Normal Law to Alternate Law, the rudder deflecting 2 degrees to the left resulting in the aircraft bank by up to a 54-degree angle.

Finally, subsequent flight crew action resulted in the inability to control the aircraft in the Alternate Law causing the aircraft to depart from the normal flight envelope and entering prolonged stalled condition that was beyond the capability of the flight crew to recover. (dan)

 

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