Faulty sensors blamed in Ethiopian Boeing 737 Max crash
David Wagman | March 09, 2020Investigators probing the March 2019 crash of an Ethiopian Airlines Boeing 737-8 Max aircraft pointed to a faulty sensor as leading to the accident, which killed all 157 people on board.
The investigators called on Boeing Corp., which manufactured the aircraft, to include redundant systems to verify angle of attack (AOA) values and also to incorporate simulator training to train pilots to respond effectively to normal and abnormal operation of the Maneuvering Characteristics Augmentation System (MCAS) flight control system, which was intended to enhance the pitch stability of the airplane.
Diagram depicting the concept of angle of attack. Source: BoeingThe report was published by the Ethiopia Civil Aviation Authority and was released the day before the first anniversary of the fatal March 10, 2019 accident. (Read the report here.)
As one of its major findings, the report said that shortly after takeoff, the left and right recorded AOA values deviated. The left AOA values were found to be erroneous and reached 74.5° while the right AOA reached a maximum value of 15.3°.The difference between the left and the right AOA values was 59° and remained at that variance until near the end of the recording, which coincided with the crash.
Similar factors in Indonesia accident
In November, Indonesian air transportation authorities said that a combination of human errors and technical problems that included the MCAS led to the Oct. 29, 2018, crash of Lion Air Flight 610, a Boeing 737 Max 8.
The plane crashed into the Java Sea 13 minutes after takeoff, killing all 189 people on board. In a 322-page report, the Indonesian transportation agency Komite Nasional Keselamatan Transportasi (KNKT) said that the MCAS malfunction led to a "series of aircraft and flight crew interactions which the flight crew did not understand or know how to resolve."
The report also said that problems with the aircraft occurred as early as three days before the fatal flight. Events during those critical three days were marred by human errors and technical problems.
Ethiopian accident timeline
The Ethiopian air transport ministry offered details into the fatal March 10, 2019 flight of Ethiopian Airlines Flight 302, a Boeing 737-8(MAX), ET-AVJ. It said that at 05:38 UTC, the flight departed from Addis Ababa Bole International Airport bound for Jomo Kenyatta International Airport in Nairobi, Kenya.
The Ethiopian Airlines 737 Max that was involved in the fatal March 10, 2019 accident. The February 2019 photo shows the plane as it departed Ben Gurion International Airport in Israel. Source: LLBG Spotter / CC BY-SA 2.0The takeoff roll and lift-off was normal, the accident report said, including normal values of left and right angle-of-attack (AOA) sensors. During the takeoff roll, the engines stabilized at about 94% N1, a reference to the jet van speed.
Shortly after liftoff, the left AOA sensor recorded value became erroneous. The left stick shaker activated and remained active until near the end of the recording. In addition, the airspeed and altitude values from the left air data system began to deviate from the corresponding right-side values.
In particular, the accident report said the left AOA decreased to 11.1° then increased to 35.7° while the right AOA indicated 14.94°. The left AOA value then reached 74.5° in less than one second, while the right AOA reached a maximum value of 15.3°.
As soon as the flaps were retracted the automatic nose-down trim activated and engaged for nine seconds positioning the stabilizer trim to 2.1 units. The pilot flying pulled to pitch up the airplane with a force greater than 90 lbs, the report said.
At 5:40:22, a second automatic nose-down trim activated. Following nose-down trim activation GPWS "Don't Sink" sounded for three seconds and “Pull Up” also displayed for three seconds. This was followed seconds later by a third automatic trim nose-down activation.
At 05:40:50, the Captain told the First Officer to tell air traffic controllers that the crew wanted to maintain its current altitude due to a flight control problem.
Following approval of the controllers, a new target altitude of 14,000 ft was set. However, the Captain was unable to maintain the flight path and requested to return to the airport.
The Lion Air 737 Max that was involved in the fatal 2018 accident. Source: PK-REN / CC BY-SA 2.0At 05:43:21, roughly five seconds after the last manual electric trim up input, a fourth automatic nose-down trim activated for about five seconds. The stabilizer moved from 2.3 to 1 units, and the report said the rate of climb decreased followed by a descent.
One second before the end of the automatic trim activation, the average force applied by the crew fell from 100 lbs to 78 lbs in 3.5 seconds. In these 3.5 seconds, the pitch angle dropped from 0.5° nose up to -7.8° nose down; the descent rate rose from -100 ft/min to more than -5,000 ft/min.
Following the fourth automatic trim activation and despite recorded force of up to 180 lbs, the pitch continued decreasing. The descent rate and the airspeed continued to rise between the fourth automatic trim activation and the last recorded parameter value.
At the end of the flight, the report said the computed airspeed values reached 500 knots (575 mph), pitch values were greater than 40° nose down and descent rate values were greater than 33,000 ft/min. Both recorders stopped recording at around 05:43:44.
The aircraft crash site consisted of a crater roughly 10 m deep with a hole about 28 m wide and 40 m long. Most of the wreckage was found buried in the ground; small aircraft fragments were found scattered around the site in an area by about 200 m width and 300 m long.
NTSB recommendations
In late September, the U.S. National Transportation Safety Board (NTSB) issued seven safety recommendations to the Federal Aviation Administration (FAA), calling upon the agency to address concerns about how multiple alerts and indications are considered when making assumptions as part of design safety assessments.
The recommendations stemmed from from the NTSB’s examination of safety assessments conducted as part of the original design of Boeing’s MCAS. The NTSB said it was concerned that the process "needs improvement" given its ongoing use in certifying current and future aircraft and system designs.
“We saw in these two accidents that the crews did not react in the ways Boeing and the FAA assumed they would,” said NTSB chairman Robert Sumwalt. “Those assumptions were used in the design of the airplane and we have found a gap between the assumptions used to certify the MAX and the real-world experiences of these crews, where pilots were faced with multiple alarms and alerts at the same time.”