Investigators have been unable to conclude whether a replacement angle-of-attack sensor was properly tested after being fitted to the ill-fated Lion Air Boeing 737 Max 8 which crashed shortly after departure from Jakarta last year.
Indonesian investigation authority KNKT says a 21° misalignment in the left-hand sensor activated the stick-shaker, generated airspeed and altitude disagree warnings, and triggered the controversial Manoeuvring Characteristics Augmentation System before the aircraft came down on 29 October last year.
The sensor had been replaced in Denpasar the day before, 28 October, after the aircraft had experienced repetitive air data problems on previous flights, including speed and altitude flags on the captain's primary instrument display.
No spare sensor had been available in Denpasar, so the engineer ordered one from Batam Aero Technic, located in Batam, and the aircraft was grounded in the meantime.
Once the replacement sensor was fitted, the maintenance manual required an installation test using one of two methods.
The recommended method involves using a specific piece of test equipment which was unavailable in Denpasar, says the inquiry.
It states that the engineer resorted to the alternative test method which involves deflecting the angle-of-attack vane to various positions – fully up, centre, and fully down – while verifying indications on the built-in test equipment module of the stall management yaw damper computer.
But the inquiry says the engineer "did not record" the angle-of-attack values shown on the computer during the installation test – despite this being required by Batam Aero Technic procedures.
The engineer in Denpasar claimed the test result was "satisfactory", says the inquiry, and released the aircraft for flight, believing the problems had been resolved.
But KNKT says investigators could not conclude whether the installation test had been successful, pointing out that the sensor was subsequently found to have a 21° bias.
Boeing and the US National Transportation Safety Board carried out a sensor installation test, using the same alternative method employed in Denpasar, with a sensor which had been deliberately misaligned by 33° before fitting.
"The test result indicated that a misaligned [sensor] would not pass the installation test as the [angle-of-attack] values shown on the [stall management yaw damper] computer were out of tolerance," says the inquiry.
This out-of-tolerance situation resulted in a 'sensor invalid' message on the test equipment module, it adds, verifying that this alternative testing method "should have" identified a 21° misalignment on the Lion Air jet's sensor.
Although the engineer in Denpasar provided photos of the stall management yaw damper unit during an installation test, as evidence of a satisfactory result, the inquiry states that the photos were "not valid evidence" because they were not related to the aircraft involved in the accident.
The result of the sensor installation test "could not be determined with any certainty", it adds.
Analysis of flight-data recorder information shows that, after the jet was released for flight from Denpasar to Jakarta, the values recorded by the replaced left-hand angle-of-attack sensor were 21° higher than those of the right-hand sensor.
None of the 181 passengers and eight crew members survived when the aircraft crashed after subsequently departing Jakarta for Pangkal Pinang.