Pilots of the Lion Air Boeing 737 Max which crashed shortly after departing Jakarta last October had been unaware of the control problems experienced by the crew of the same aircraft on the inbound service, investigators have disclosed.
This inbound service – from Denpasar – had taken place after the aircraft underwent replacement of an angle-of-attack sensor. Its crew had known about the rectification, and Indonesian investigation authority KNKT says this awareness of the aircraft's condition "may have helped" when the pilots encountered control problems after take-off from Denpasar.
As it lifted off, the stick-shaker activated owing to misalignment of the replacement angle-of-attack sensor and the aircraft automatically started repeatedly trimming nose-down.
KNKT says the crew carried out non-normal checklists, including those for unreliable airspeed and runaway stabiliser, and activated the stabiliser trim cut-out to regain control of the jet.
Despite a continuing stick-shaker activation, and the indications of a runaway stabiliser, the captain chose to continue the flight to Jakarta – a decision which the inquiry says was "highly unusual".
After the aircraft arrived and parked in Jakarta, it adds, the captain made entries into the maintenance log referring to three particular problems experienced during the flight, but "did not mention" the activation of the stick-shaker, as he believed this was a symptom of the other issues.
Nor did the captain report the runaway stabiliser or the activation of the stabiliser trim cut-out. The crew had returned the cut-out switches to their normal position after landing.
Finding the cut-out switches engaged, says the inquiry, would have provided "additional information" to the maintenance engineers.
It says the captain's lack of understanding of the relationship between the system failures and their effects meant his maintenance log report was "incomplete", and points out that full reporting is "critical" for engineers to maintain aircraft airworthiness.
Failure to record information may have been crucial, because one of the next crew's responsibilities is to examine the maintenance log and inquire about the technical status of the aircraft before flight.
KNTK says an "absence" of discussion by the next crew regarding the problems that had affected the inbound flight suggests the pilots might not have been aware of the issues, and the possibility of recurrence.
This lack of awareness – particularly of the stick-shaker activation and the uncommanded nose-down trim – would have lessened the ability of the crew to predict, and prepare to mitigate, similar problems.
No information about the Manoeuvring Characteristics Augmentation System – the source of the automatic nose-down trimming – was contained in the flight crew manuals. Crew training did not include MCAS, says the inquiry, and there were no procedures for responding to erroneous angle-of-attack data.
When the aircraft took off again from Jakarta, the stick-shaker and MCAS activated repeatedly, similarly to the way they had behaved on the inbound flight.
But the pilots, caught by surprise, were far less effective than the previous crew at diagnosing and dealing with the problem.
Non-normal checklists were not completed, says the inquiry, and the crew was distracted by numerous air traffic control communications, multiple alerts, and repetitive MCAS activation, resulting in poor crew resource management and loss of control of the aircraft.