A mid-air miscommunication between pilot and co-pilot in response to a recurring technical issue caused AirAsia flight QZ8501 to plunge into the Java Sea last December, Indonesian investigators have said.
The Airbus A320-200 was en route to Singapore from the Indonesian city of Surabaya on the morning of 28 December when it lost contact with air traffic control 42 minutes after takeoff.
The plane wreckage and bodies of the passengers were later pulled from the Java Sea in waters off Kalimantan, the Indonesian part of Borneo island. Seven crew members and 155 passengers were on board.
Using a toy airplane to demonstrate the erratic flight path, investigators said the crew lost control of the plane.
Their findings indicate the pilot was dealing with a repeated technical problem with the Rudder Travel Limiter (RTL), leaving the co-pilot to take control of the plane.
Electrical interruption to the RTL happened three times in the space of thirteen minutes, eventually causing the autopilot to disengage.
With the loss of the autopilot function, the co-pilot was left to fly the plane manually, and it was at that point that there was a miscommunication between the captain and co-pilot, investigators said.
According to information gleaned from two black boxes and a cockpit recording, the pilot instructed the co-pilot to “pull down”, an order that was taken literally, sending the plane soaring up to 38,000 feet, at a rate of 20,000 feet per second.
“[The pilot] said, ‘Pull down, pull down.’ But when you pull down [the gear controls] the plane goes up. To make the plane go down you need to push, so this order was confusing,” said accident investigator Nurcahyo Utomo.
The report said the plane went into a “prolonged stall condition that was beyond the capability of the crew to recover”. At one point the two men appeared to be pushing their controls in opposite directions, it added.
The findings from an almost year-long investigation by Indonesia’s National Transportation Safety Commission are the first official explanation of what caused the Airbus to crash.
One recommendation made on Tuesday was for AirAsia to have a more standardised terminology to avoid such accidents from happening again.
“To AirAsia we recommend that pilots have a standard callout or terminology to communicate so there is no misinterpretation and all pilots are trained in procedures of how to take control,” said Utomo.
The reason the co-pilot had initially taken control of the plane was because the captain was trying to fix a problem with the RTL, an issue that investigators revealed had occurred 23 times on that specific plane since January 2014.
The problem was related to a cracked solder joint caused by exposure to extreme temperatures. In the months leading up to crash the problem had been occurring more frequently on the plane, including on a flight three days earlier between Surabaya and Kuala Lumpur.
Before the crash, Malaysian airliner AirAsia had a sound safety record.
Indonesia’s aviation safety record has been undermined by a series of fatal crashes. In June, an Indonesian military plane careered into the side of a building in Medan, Sumatra, killing more than 140 people. Since August three separate fatal plane crashes in Sulawesi and West Papua have killed 64 people.
AirAsia’s chief executive, Tony Fernandes, said on Tuesday that “there is much to be learned here for AirAsia, the manufacturer and the aviation industry”.
He added: “We will not leave any stone unturned to make sure the industry learns from this tragic incident.”