FACTBOX-Nine factors that contributed to Lion Air 737 MAX crash

Oct 23 (Reuters) - Mechanical and design issues are among nine sets of factors that contributed to the crash of a Boeing 737 MAX jet operated by Lion Air last October, Indonesian investigators have found.

Following is a transcript of slides presented by officials to relatives at a briefing on Wednesday, obtained by Reuters, together with an unofficial explanation of key technical terms.

The full report will be released on Friday.


1. During the design and certification of the Boeing 737-8 (MAX), assumptions were made about pilot response to malfunctions which, even though consistent with current industry guidelines, turned out to be incorrect.

2. Based on the incorrect assumptions about pilot response and an incomplete review of associated multiple flight deck effects, MCAS's reliance on a single sensor was deemed appropriate and met all certification requirements.

3. MCAS was designed to rely on a single AOA sensor, making it vulnerable to erroneous input from that sensor.

4. The absence of guidance on MCAS or more detailed use of trim in the flight manuals and in pilot training, made it more difficult for flight crews to properly respond to uncommanded MCAS.

5. The AOA DISAGREE alert was not correctly enabled during Boeing 737-8 (MAX) development. As a result, it did not appear during flight with the mis-calibrated AOA sensor, could not be documented by the flight crew and was therefore not available to help maintenance identify the mis-calibrated AOA sensor.

6. The replacement AOA sensor that was installed on the accident aircraft had been mis-calibrated during an earlier repair. The mis-calibration was not detected during the repair.

7. The investigation could not determine that the installation test of AOA sensor was perform(ed) properly; however the mis-calibration was not detected.

8. Lack of documentation in the aircraft flight and maintenance log about the continuous stick shaker and use of the Runaway Stabilizer NNC meant that information was not available to the maintenance crew in Jakarta nor was it available to the accident crew, making it more difficult for each to take the appropriate actions.

9. The multiple alerts, repetitive MCAS activations and distractions related to numerous ATC communications were not able to be effectively managed. This was caused by the difficulty of the situation and deficiencies in manual handling, NNC execution and flight crew communication, leading to ineffective CRM application and workload management. These deficiencies had previously been identified during training and reappeared during the accident flight.

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Following is an unofficial explanation of the abbreviations used, based on previous Reuters reports and industry documents.

AOA: Angle of Attack, a key flight parameter which indicates the angle between the wings and the oncoming flow of air. If the angle becomes too high, the aircraft can stall or lose lift. The data is drawn from two sensors, one on which side of the nose.

AOA DISAGREE: An indicator, optional at the time in the 737 MAX, indicating the two AOA sensors are producing values that differ by an excessive margin. Boeing plans to make this feature standard as part of a redesign following the Lion Air accident and a separate crash five months later in Ethiopia.

ATC: Air Traffic Control

CRM: Crew Resource Management, a method of coordination between pilots designed to improve their responses to errors and reduce stress. It was developed in response to earlier crashes and is now a standard part of pilot training and procedures.

MCAS: Maneuvering Characteristics Augmentation System (MCAS), a flight control software patch designed to lower the nose of the aircraft when there is an elevated Angle of Attack.

It was developed for the latest version of 737, the MAX, to compensate for the tendency of the plane's nose to drift higher due to the size and shape of the its newer and larger engines.

Boeing has redesigned the software in order to soften its impact and prevent repeated misfirings.

NNC: Non-Normal Checklist or problem-solving procedure.

RUNAWAY STABILIZER: Uncommanded movements of the plane's horizontal stabilizers - small wings near the bottom of the tail that are designed to control the pitch or up/down movement.

(Reporting by Fanny Potkin, Jessica Damiana, Bernadette Christina Munthe, Tim Hepher Editing by Nick Zieminski)