One of the aspects of the Ethiopian Airlines Boeing 737 Max crash which had remained unclear during the investigation was the cause of the original angle-of-attack sensor malfunction that initiated the accident sequence.
The sensor suddenly failed during the take-off run from Addis Ababa on 10 March 2019, transmitting flawed information to various aircraft systems including the MCAS horizontal stabiliser trim function – which led the aircraft to enter a nose-down attitude and rapid descent from which its crew was unable to recover.
Ethiopia’s air accident investigation bureau believes the sensor was defective and that the sensor malfunctioned “most likely” as a result of a “power quality problem”.
At the time of the malfunction the flight-data recorder showed heater power to the left-hand sensor was lost.
“Evidence indicates the loss of power was likely due to a production-related intermittent electrical [or] electronic failure involving the airplane’s electrical wiring interconnection system,” the inquiry claims.
It points out that the aircraft had been experiencing various electronic glitches shortly after its delivery to Ethiopian Airlines in November 2018.
But the US National Transportation Safety Board argues that a bird-strike, which damaged the angle-of-attack vane, is a more plausible explanation for the sudden change in sensor data.
Readings from the left- and right-hand angle-of-attack sensors began to deviate sharply just as the aircraft lifted off. The left-hand figure fluctuated from 11.1° to 35.7° and then 74.5°, while the right-hand instrument remained largely constant at about 15°.
Ethiopian investigators, states the NTSB, omitted “key findings” about the sensor failure in their draft report into the accident.
Collins Aerospace manufactured the sensor and was appointed to the investigation team as a technical adviser to look into possible reasons for the malfunction, including manufacturing defects, component and heater faults, and structural failures.
The company conducted vibration, acceleration and flight-simulation testing, and modelled the sensor’s performance.
While the Ethiopian inquiry refers to Collins’ analysis, the NTSB says it “does not acknowledge” that the analysis of flight-data recorder information was “not consistent” with any internal failure of the sensor.
“Instead, those data were fully consistent with previous instances of partial [angle-of-attack] vane separation due to a bird strike,” it adds.
The vane appears to have broken at the hub and separated from the sensor, causing the performance deviation evident 44s after the Ethiopian aircraft commenced its take-off roll.
Separation of the vane resulted in an open circuit for the vane heater and resulted in a fail annunciation about 6s later.
According to the analysis the aircraft was travelling at about 170kt at the time and a 0.23kg (0.5lb) bird impact would have been sufficient to break the vane away from the jet. Large birds, such as steppe and tawny eagles, are common around Addis Ababa airport.
“Windtunnel test data and the [angle-of-attack] dynamic performance model show that the failure mode involving separation of the vane at the hub is consistent with the large and near-instantaneous initial change in the left [angle-of-attack] value and the resulting [angle-of-attack] dynamics observed,” says the NTSB.
The Ethiopian inquiry states that an inspection of departure runway 07R, covering about a quarter of its 12,400ft length, did not locate any bird remains or debris from the vane.
But the NTSB states that the search took place eight days after the accident and did not include the area around taxiway D, where the erroneous left-hand angle-of-attack sensor readings began.
It adds that the lack of debris located during the inspection is not sufficient to rule out a foreign-object impact on the vane.
The NTSB says the inquiry “misses the opportunity” to address improvement of wildlife management at Addis Ababa.
But the Ethiopian investigators, while conceding that a bird-strike “may be” a cause, has leaned towards technical failure, insisting that a “wide range of power quality problems” can lead to erroneous angle-of-attack sensor output.