The preliminary report into the Oct. 29 Lion Air flight 610 crash confirms that the pilots struggled with the automatic trim as it responded to faulty inputs, although they appear to have not activated the cut-out switch for this system.
The report by the Indonesian National Transportation Safety Committee details the maintenance timeline for the Boeing 737-8, which experienced similar issues on previous flights. It also outlines the sequence of events on the prior flight on Oct. 28, and flight data recorder [FDR] information and air-traffic control communications from the flight that crashed. All 189 passengers and crew were killed as the plane plunged into the sea shortly after takeoff from Jakarta.
While the report lists recommendations and actions that have already been taken, it also includes an additional recommendation related to pilot procedures. Boeing has issued a statement highlighting certain aspects of the report.
Before the prior flight on Oct. 28, the pilot discussed with the engineer “the maintenance actions that had been performed including replacement of the AoA [angle of attack] sensor and had been tested accordingly,” the report said. During this flight, “the stick shaker activated during the rotation and remained active throughout the flight.” The pilot noticed the ‘IAS [indicated airspeed] disagree’ warning, and that the aircraft was automatically trimming aircraft nose down (AND).
“After three automatic AND trim occurrences, the second-in-command (SIC) commented that the control column was too heavy to hold back,” the report said. The pilot “moved the STAB TRIM switches to CUT OUT and the SIC continued the flight with manual trim without auto-pilot until the end of the flight.” Boeing introduced an automatic trim system known as the Maneuvering Characteristics Automation System on the 737 MAX.
After landing, the pilot informed the engineer about the problem and entered IAS and ALT disagree, and ‘feel differential pressure,’ on the flight maintenance log. The engineer flushed the left pitot air data module (ADM) and static ADM “to rectify the IAS and ALT disagree” followed by an operations test on the ground, and was “satisfied,” the report said. The feel differential pressure was rectified by cleaning the electrical connector plug of the elevator feel computer. Tests on the ground “found that the problem had been solved,” the report said.
During the Oct. 29 flight, the FDR information showed “a difference between left and right [AoA] of about 20 degrees and continued until the end of recording … during rotation the left control column stick shaker activated and continued for most of the flight.” The FDR data also showed multiple attempts to manually trim up to counteract automatic trim down. The report does not indicate that the pilots cut out the automatic trim as the pilots on the previous flight had done.
While the cockpit voice recorder has not yet been located, records of communications with the ground show that the crew advised controllers of flight control problems and that they were flying the aircraft manually. The controller gave the crew a heading to return to the airport. The pilots also told the controller that “the altitude of the aircraft could not be determined due to all aircraft instruments indicating different altitudes.”
The last communications noted in the report started with the pilot asking the controller “to block altitude 3,000 feet above and below for traffic avoidance.” The controller then “asked what altitude the pilot wanted,” and the pilot responded with “five thou.” About 20 seconds later the FDR stopped recording, according to the report.
The report lists several actions, advisories and recommendations that have already been taken or issued by Lion Air, Indonesian authorities, the FAA, Boeing, and MRO provider Batam Aero Technic.
The committee “acknowledges the safety actions taken by Lion Air” since the crash “and consider[s] that the safety actions were relevant to improve safety, however there still safety issues [that] remain to be considered.”
One of these additional recommendations calls for Lion Air to ensure the implementation of a section of the operations manual that advises pilots to “discontinue the flight when unairworthy mechanical, electrical, or structural conditions occur.” The report notes the stick-shaker event on the prior Oct. 28 flight, and that this is “considered as [an] un-airworthy condition and the flight [should] not be continued.”
Another recommendation calls for better flight documentation since there was one more flight attendant on board than was listed on the weight and balance sheet.
In its statement released following the preliminary report, Boeing said it is “taking every measure to fully understand all aspects of this accident, working closely with” authorities.
Boeing noted that maintenance logs “recorded problems related to airspeed and altitude on each of the four flights that occurred over the three days prior to Flight 610.” The logs “indicate that various maintenance procedures were performed, but issues related to airspeed and altitude continued on each successive flight.”
The preliminary report “does not include records as to the installation or calibration of the new [AoA] sensor, nor does the report indicate whether the sensor was new or refurbished,” Boeing said.
Although the report states that the pilot on the Oct. 28 flight “was satisfied by the information relayed by the engineer that the AOA sensor had been replaced and tested,” during the flight “the pilots again experienced problems with erroneous airspeed data, and also experienced automatic nose down trim.”
Boeing highlights the fact that the pilots on the Oct. 28 flight “turned off the stabilizer trim switches within minutes of experiencing the automatic nose down trim.” They followed checklists as “the appropriate procedure to address unintended horizontal stabilizer movement, regardless of source.”
After landing, the pilot reported some of the issues both on the aircraft maintenance log and to engineering, Boeing said. The manufacturer also noted that “the report states that the pilot ran the runaway stabilizer non-normal checklist, but it does not state that he communicated that fact in the maintenance documentation following that flight.”
The report shows that shortly after taking off, the pilots on the Oct. 29 flight “experienced issues with altitude and airspeed data that the pilots had previously experienced on the earlier flights, due to erroneous AoA data,” Boeing said. However, “unlike as is stated with respect to the prior flight, the report does not state whether the pilots performed the runaway stabilizer procedure or cut out the stabilizer trim switches.”
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