The response of pilots to a computer failure caused a loss of control and led to the crash of AirAsia Flight QZ8501 last year, killing all 162 aboard, investigators said Tuesday.
The main flight control computer on the Airbus A320 had a cracked joint that caused it to malfunction repeatedly — including four times during the flight, and 23 times the previous year — Indonesia's National Transportation Safety Committee concluded in a report [PDF link here.]
While the pilots struggled to handle an influx of electronic warning messages, the plane rolled sharply several times, climbed too high and ultimately stalled before crashing into the Java Sea.
The midair emergency unfolded over two-and-a-half minutes, the report said, suggesting passengers were aware of a serious problem. In its final moments, the plane rolled 104 degrees to the left before plummeting at a rate of up to 20,000 feet per minute.
The flight had been normal until warning messages prompted the pilots to pull a circuit breaker in an effort to reset the computer, against the advice of the plane's operating manual.
"Subsequent flight crew action resulted in inability to control the aircraft ... causing the aircraft to depart from the normal flight envelope and enter a prolonged stall condition that was beyond the capability of the flight crew to recover," the report said.
Despite the malfunction, the report described the plane as "airworthy" and said the crew should have been able to recover from the roll or stall if they had correctly followed their training.
It was less than halfway into a two-hour flight from the Indonesian city of Surabaya to Singapore on Dec. 28 last year.
The co-pilot was at the controls, under the supervision of the captain. Despite the unfolding crisis on the flight deck, the captain did not retake control as required by the airline's procedures, according to analysis of the 'black box' cockpit voice recorder.
At one stage the plane rolled 54 degrees to the left over the course of 9 seconds before the co-pilot intervened to stabilize it. "The investigation could not determine where the [pilot's] attention was directed at that time. The [co-pilot] might have been startled when he realized the unusual attitude of the aircraft, as indicated by the [black box] record of self-expression."
As the plane reached a nose-up angle of 24 degrees, the captain ordered "pull down…pull down" but the co-pilot pulled back further on the controls, sending the nose even higher.
While the captain pushed forward on his controls to bring the nose down, the co-pilot was still pulling back, causing the plane's automated fly-by-wire system to cancel out the opposing instructions.
"The degraded performance and ambiguous commands might have decreased the [co-pilot's] situational awareness and he did not react appropriately in this complex emergency," the report said.
The report has many echoes of Air France Flight 447, which crashed on en route from Brazil to Paris in 2009. Investigators concluded the crew mistakenly responded to a speed sensor failure by pulling the nose up, causing a dangerously high pitch and ultimately a deadly stall over the southern Atlantic Ocean.
In a statement, AirAsia Indonesia CEO Sunu Widyatmoko said the carrier had made changes to its procedures, including recurrent training for pilots on how to recover from an unstable situation.
"There are many lessons to be learned for the entire aviation industry and we will continue to be dedicated on ensuring that AirAsia Indonesia's safety standards remain at the highest level in the industry," the statement said.