Pilots fought with jet in death dive
Lion Air crew were unable to correct for faulty sensor
Amalfunctioning sensor and an automated response from the aircraft’s software stymied pilots’ efforts to control a doomed Indonesian flight that went careening into the sea last month, according to preliminary investigative findings.
The report, which stops short of determining the cause of the crash or analysing findings, chronicles the chaotic moments on the Lion Air flight before it crashed into the waters off Java a month ago, killing all 189.
It details how sensors and other equipment were checked and fixed before the aircraft’s final flight, but not the “angle of attack” sensor. That measures where the nose is pointing and was showing erroneous readings.
With the sensor insisting the nose was too high, an automatic feature kicked in, sending the plane plummeting as the pilots wrestled to regain control. Unable to trust their readings, the pilots asked air traffic control about speed and altitude.
Lion Air Flight 610 plunged into the Java Sea on October 29 just after taking off from Jakarta, killing the eight crew members and 181 passengers. The crash appears to have been caused by a mix of new technology and cockpit confusion as the pilots fought to gain altitude. The crew — at an altitude of just 1525m — had very little time to resolve the issue before the plane crashed at 725km/h.
The pilots asked to return to Jakarta just two minutes after takeoff, reporting a “flight-control problem” but not specifying what it was.
Black-box data showed that the pilots were pulling back on the control column, attempting to raise the plane’s nose, with almost 45kg of pressure before they crashed.
The Indonesian National Transportation Safety Committee, which produced the report, also said that Lion Air, a Jakarta-based low-cost airline, should improve its “safety culture”. No engineer briefed the pilots on the multiple problems the aircraft experienced on previous flights, and it was up to him to review the maintenance logs.
“Our job isn’t to find faults,” safety committee investigator Nurcahyo Utomo said.
The aircraft was the most recent incarnation of the Boeing 737. The 737 Max was equipped with morepowerful engines that are mounted farther forward on the wing, requiring that additional software be added to the autopilot to provide more control.
That software, which has been described as several lines of coding, was identified in the Boeing manual as the manoeuvering characteristics augmentation system, or MCAS.
When the sensors transmitted faulty data to Flight 610, the new MCAS system sensed a stall — that point at which planes do not have enough airspeed to create lift — and sought to correct for it by repeatedly pointing the nose of the aircraft down.
A feature in previous 737 models that allowed pilots to manually override an “electric trimming” process — which automatically budges the nose downward to prevent a stall, does not work in Max 8 planes, Boeing explained in a November 7 bulletin.
That same week, the Federal Aviation Administration issued an emergency notice to all airlines that fly the 737 Max, warning them that erroneous sensor inputs “could cause the flight crew to have difficulty controlling the airplane,” leading to “possible impact with terrain”. The deviation probably was caused by what is called a “runaway stabiliser”. Stabilisers are essentially those small wings on either side at the tail end of the plane. They each have flaps — called elevators — that help control the elevation of the plane.
In case of a runaway stabiliser, pilots are instructed in the cockpit checklist to hold the control column firmly, disengaging the autopilot that, in this case, contained the MCAS program. Next, they are told, disengage the auto throttle and manually fly the plane. “This corner of the performance charts is called the ‘coffin corner’,” said Mary Schiavo, an aviation lawyer and former inspector general of the US Transportation Department, “and good pilot training teaches you how to get out of coffin corner, but did these pilots realise the plane itself was putting them in coffin corner? Apparently not.”
It is not clear whether the pilots attempted the runaway stabiliser procedure.
Unions representing pilots at Southwest and American airlines said they were not properly informed about the new system during training. “We did not know this was on the Max models,” Southwest Airlines Pilots Association president Jon Weaks said on November 13, referring to a new automated flight-control feature.
Soerjanto Tjahjono, who heads the safety committee, said last week that the plane’s black box showed that “the technical problem was the airspeed, or the speed of the plane. There were four flights that experienced problems with the airspeed indicator.” The angle-of-attack sensor contributes to the airspeed readings.
Utomo said last week that the antistall system had activated on the plane as it flew into Jakarta the night before the crash but that pilots managed to shut it off. Utomo said the plane, on both the doomed flight and the previous flight, had experienced a stick shaker — “a warning that showed that the plane was going to stall”.
The report said differing data between the sensors appeared rectified by cleaning an electrical plug and a “test on the ground found the problem solved.” But it was not, because when the plane took off the two flight-speed sensors did not agree on the aircraft’s speed.—