Boeing has assured passengers and operators that its 737 Max aircraft remain safe following the release of a preliminary report into the Lion Air 737 Max 8 crash, while also highlighting some points that the report did not address.
In an unusual move, the manufacturer's detailed statement largely re-iterated key points from the interim report released by Indonesia's National Transportation Safety Committee (NTSC). These include how the aircraft had encountered repeated problems with airspeed and altitude data in the days before the 29 October crash.
Boeing points out that prior to a Denpasar-Jakarta flight on 28 October, the flight directly before the doomed JT610, an engineer had informed the flight crew that the jet's angle-of-attack sensor had been replaced and tested.
“The report does not include records as to the installation or calibration of the new sensor, nor does the report indicate whether the sensor was new or refurbished,” states the manufacturer.
“Although the report states that the pilot was satisfied by the information relayed by the engineer that the AOA sensor had been replaced and tested, on the subsequent flight the pilots again experienced problems with erroneous airspeed data, and also experienced automatic nose down trim.”
Boeing also notes that while the pilot ran the runaway stabiliser non-normal checklist after encountering issues on the flight from Denpasar, the report "does not state that he communicated that fact in the maintenance documentation following that flight”.
The manufacturer has previously indicated that following the runaway stabilizer check list would disengage the aircraft’s Maneuvering Characteristics Augmentation System (MCAS), which commands stabiliser movements to pitch the aircraft down, without pilot input, when it is being flown manually at high angles of attack with the flaps up.
In the 28 November statement, Boeing also points out that the report does not state whether the pilots operating flight JT610 “performed the runaway stabilizer procedure or cut out the stabilizer trim switches.”
Those procedures allow for pilots to override the MCAS and trim the aircraft manually. The system is unique to the Max series, and was introduced to counter centre-of-gravity issues associated with the mounting of its CFM International Leap-1B engines.
Following the Max 8 crash, Boeing issued a flight crew operations manual bulletin emphasizing the runway stabiliser procedure must be followed if uncommanded horizontal trim stabiliser movement and other conditions, including stick shaker activation and speed and altitude disagreement alerts occur during manual flight.
The NTSC report notes that the US Federal Aviation Administration soon issued an airworthiness directive to 737 Max operators to include text from the bulletin in their flight manuals.