Indonesian investigators have released the first report into the fatal crash of Lion Air’s Boeing 737 MAX 8. The preliminary findings, based on the data retrieved from the flight data recorder, confirm that the jetliner experienced speed and altitude issues on its previous flights and states it was in an “un-airworthy” condition on the day it crashed. The report also seems to focus onto Lion Air’s safety culture and the pilots’ actions, including the decision to continue flying the malfunctioning aircraft.

The preliminary report into the loss of Lion Air Flight JT610, published by Indonesia’s National Transport Safety Committee (KNKT) on November 28, 2018, revolves around data recovered from the first and so far, the only found ‘black box’, which indicated a malfunction in the Maneuvering Characteristics Augmentation System (MCAS).

“The Digital Flight Data Recorder (DFDR) recorded a difference between left and right Angle of Attack (AoA) of about 20° and continued until the end of recording. During rotation the left control column stick shaker activated and continued for most of the flight,” the report states. This is despite the fact that prior to its last two flights, the AoA sensor had been replaced and tested.

The MCAS is Boeing’s new anti-stall system, installed on MAX planes. It is activated when angle of attack (AOA) sensors indicate that the airframe is in a dangerously high angle and attempts to correct it by pushing the plane nose down. But if the AOA signal is erroneous, the MCAS can cause an airplane to stall.

The flight data reveals that the aircraft’s systems had detected an imminent stall due to the faulty indicator, causing the “stick-shaker” (which vibrates the aircraft’s steering-wheel and warns the captain) to activate and remain so throughout most of the flight. To prevent a stall, the automated anti-stall system directed the aircraft’s nose down.

The document reveals pilots had repeatedly reported to the Jakarta Air Traffic Control (ATC) the aircraft was experiencing “flight control problem” asking for the aircraft’s altitude as they struggled to steady the plane. Before losing contact, the flight crew had taken manual control to keep the aircraft’s nose up and were successful in counteracting the anti-stall system, but only for a brief time.

Before the final dive, the flight crew asked the controller to “block altitude 3,000 feet above and below” to avoid traffic. They could not determine their altitude due to all instruments showing different readings. The flight crew’s last words in response to controller’s request for an appropriate altitude, were “five thou”.

Faulty system or faulty maintenance?

What investigators really draw attention to in the report is the airspeed and altitude issues the very same aircraft (PK-LQP) experienced during its four previous flights, particularly the one that preceded the day of the crash. During pre-flight check, the Captain of the October 28, 2018, Denpasar-Jakarta flight had discussed with Lion Air’s maintenance on what had been performed on the plane, including the replacement of an AoA sensor.

Similarly to what occurred a day later, the Boeing 737 MAX 8 also ended up being automatically pitched nose down, but the flight crew shut down the MCAS (the anti-stall system) and continued the flight manually, landing “without incident”. That flight recorded “unreliable” airspeed and altitude disagreement warning, and the Aircraft Flight Maintenance Log later revealed a malfunction in the left primary flight display.

“The flight from Denpasar to Jakarta experienced stick shaker activation during the takeoff rotation and remained active throughout the flight. This condition is considered as un-airworthy condition and the flight shall not be continued,” the report states.

In their safety recommendations, investigators also highlight (referring to the CASR Part 91.7 Civil Aircraft Airworthiness and the Operation Manual part A subchapter 1.4.2) that “the Pilot in Command is directly and specifically responsible for, and is the final authority as to, the operation of the aircraft. Therefore, he is responsible for ensuring the aircraft is in condition for safe flight and must discontinue the flight when un-airworthy mechanical, electrical, or structural conditions occur.”

After landing safely the night before the crash, the Captain informed Lion Air’s maintenance about the flight condition and the aircraft problem. The report does not state what measures were taken by the airline to fix the recurring problem or why the 737 was cleared to fly.

"The plane was no longer airworthy and it should not have kept flying," Nurcahyo Utomo, an investigator at KNKT, told the media, as Channel News Asia reports. Question remains on whether it is a “Boeing” or an “airline issue”, as he puts it. According to the official, the agency had not yet determined if the MCAS was a contributing factor, Reuters writes.

Following the publication of the initial report, Boeing released a statement in which the company assures that the 737 MAX “is as safe as any airplane that has ever flown the skies”. The manufacturer also outlines series of questions that are yet to be answered. For instance, it is known that an AOA sensor was replaced before the flight preceding JT610. Boeing calls for the installation records, calibration of the new sensor, or clarity on its condition – “whether the sensor was new or refurbished.”

Another question surrounds the runaway stabilizer, as it is called. After the previous flight was affected by a malfunction similar to what seems to have doomed JT610, the pilot ran a “non-normal checklist” on the runaway stabilizer. However, the report "does not state that he communicated that fact in the maintenance documentation following that flight”.

The last one is aimed at JT610 itself, as Boeing points out that the initial report does not say whether the pilots “performed the runaway stabilizer procedure or cut out the stabilizer trim switches”, a procedure that would have allowed them to override the MCAS. Those three questions might be answered later in the investigation and are key to understand the causes of this accident.

The Boeing 737 MAX was headed for the town of Pangkal Pinang on October 29, 2018, when it crashed just 13 minutes after take-off from Jakarta. The tragedy was the first involving Boeing’s latest model of the 737 series, 737 MAX, which entered service a year ago.

Questions have surrounded the safety of the aircraft and other models comprising the best-selling 737 MAX family (the -7 and the -9). The KNKT states that the apparently faulty AoA sensor will undergo further testing and analysis. The investigation team is also planning to conduct aircraft simulator exercises in the Boeing engineering simulator configured for 737 MAX 8.