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False information from a sensor located on the outside of the aircraft was identified as having contributed to the crash of the Lion Air JT610 flight last week. This sensor is capable of triggering uncontrolled movements of the jet's horizontal tail only on the MAX, said one expert.
The security bulletin that Boeing sent Tuesday night to all airlines operating its 737 MAX jet aircraft indicates at least one possible cause of the deadly crash of Lion Air that killed 189 passengers and members of the air force. # 39; crew.
The Boeing Alert, which concerns only the single-aisle Model 737 MAX model built by Renton, highlighted an automated system designed to prevent the aircraft from stalling, but which can potentially run wild if false data are fed by a small sensor type "bladelike". the outside of the plane.
Although there is not enough information to know exactly what caused the accident, the investigators pay particular attention to this system, which can move the horizontal tail of the jet to tip the head forward or backward.
John Cox, a former pilot and director of aviation consulting company Safety Operating Systems, said that although a version of this automated system is installed on the 737 since the construction of the first, in 1967, this sensor can only trigger that on the MAX uncontrolled movements of the horizontal tail of the jet.
The resolution of the problem, if it occurs in flight, should only require a standard pilot procedure – a procedure recently emphasized by Boeing in his security bulletin on Tuesday.
Because of this, Cox stated that he considered the MAX safe and would fly tomorrow without hesitation.
But he suggested to accident investigators and Boeing's own engineers to look closely at the chain of events on the Lion Air cockpit and determine how the pilots were managing the chaos, which should be clearly indicated by the cockpit voice recorder, to determine if Boeing should address the problem. system design.
Another expert, Bjorn Ferhm, an analyst at Leeham.net, a former fighter jet pilot and aeronautical engineer, said that it was possible to change the internal logic of the MAX's flight computer system, but according to him A more likely suspect is a hardware failure of the computer from theft of the plane that crashed.
It recalls that when an AirAsia Airbus A320 in Indonesia crashed in 2014, killing 162 people, the pilots also had to deal with a series of repeated failures of the instruments, including the investigation determined that they had been triggered by a cracked weld seam.
So, Ferhm also thinks that the MAX is as safe as any 737. If something happens to the pitch control system, pilots simply have to follow the procedures outlined in the warning. from Boeing, he said.
After Boeing's security warning, the FAA released Wednesday an emergency airworthiness directive, effective immediately, requiring airlines to update flight procedures in accordance with the alert's instructions. The FAA said the directive is based on an analysis by Boeing.
An FAA engineer, speaking on condition of anonymity, said Boeing was trying to understand how they react to the various instrument failures described in the Lion Air crash. .
Should the accident investigation eventually endanger the design of Boeing's nose pitch control system, questions would arise regarding the safety certification process, much of which is delegated by the FAA to Boeing. -even.
Boeing refused Wednesday to comment.
FAA calls for action
The FAA stated that its directive dealt with the potential effects of false information from the sensor located on the outside of the aircraft and indicating its "angle of attack" (AOA), that is, the l 'angle of attack. angle between the wing and the flow of air through the jet. .
This key data point is introduced into the flight computer with air temperature and speed. These three metrics are affected and are used by various systems controlling the flight of the aircraft.
The FAA has stated that false AOA readings "can potentially bring the horizontal (the tail) to repeatedly tilt the nose of the plane down, making the aircraft difficult to control. "
The danger, warns the FAA, is that it could "lead to excessive dive attitude, significant altitude loss and possible impact on the ground".
Boeing's service bulletin to the airlines indicates that, although pilots can pull the yoke and adjust the horizontal tail to lift the nose, the nose-up condition can then resume five seconds later.
The flight data of the Lion Air jet in the 12 minutes before its crash show that the aircraft has regularly lost altitude and found it before diving fully into the air. ocean.
Automated system acting on false data
The problem comes from the "automatic trim" system, which adjusts the position of the horizontal tail so as to tilt the nose up or down.
When a pilot pulls or pushes the control column, he exerts a force to adjust the moving horizontal tail and to raise or lower the nose of the aircraft. Cropping means the system must keep the tail in this position without the driver doing anything else. The force exerted on the control column is released to zero and the system maintains the position.
A driver can manually adjust each time that he adjusts the speed or direction. He can also configure the system in automatic mode so that he himself makes the adjustments to maintain the position.
But if the automatic compensation system is powered with fake angle of attack (AOA) measurements, everything will go wrong.
If the sensor tells the system that the AOA is too high, the automatic system starts moving the tail to point the nose down. Even when the pilot corrects this by pulling on the column, if the sensor continues to say that the AOA is too high, the system starts to poke down again.
"On no other 737, there is a system based on the angle of attack that will move the compensation (horizontal tail). This is unique to MAX, "said Cox, CEO of the aviation consulting firm. "I was surprised that only one angle of attack indicator could cause this system to activate."
The FAA directive directs operators to revise the aircraft flight manual to indicate to the flight crew the procedures to follow in order to know exactly what to do when this situation arises in order to Avoid that the plane is swaying repeatedly.
Boeing's instructions tell the pilots that they have to move away with difficulty to control the dive, then turn off the system with a cutoff switch to automatically cut the horizontal tail to keep the speed and flight level .
Instead of using this automatic compensation system, which goes wrong every 10 seconds, the driver must then set the compensation positions manually at each change of speed or direction.
However, in the stress of a real situation, this may not be so simple.
The FAA directive tells operators that misreading the AOA can create a cascade of false indications on the cockpit that can create confusion for the flight crew.
These include continuous or intermittent control column shaking and reduced speed indication, both of which suggest that the aircraft is approaching a stall; increase nose control forces, which the pilot must forcefully remove from the control column to counteract; and warning lights indicating false measurements of speed and altitude.
Mr. Ferhm said that an "uncontrolled assembly" scenario, in which an automated system was moving the flight control rudders in the wrong direction, "is something you train in every plane."
"This is one of the most common situations you have," he said. "You train for that and learn to turn off the system."
Still, he said, he would not blame the pilots. He said the Lion Air flight model suggests "that they had hell up there" as they struggled to control the plane.
"The plane was throwing curved balls at them all the time. Why they did not remove the automatic cropping system, we do not know. In a way, they did not press the switches. We need more information. We have to listen to the voice recorders to understand why these pilots were so stressed, "said Ferhm.
A total of 246 MAXs fly around the world, including 45 in the United States, the Southwest, the United States and the United States.
In a statement, the FAA stated that she "continued to work closely with Boeing and that as a member of the Lion accident investigation team. Air in Indonesia, she could take other appropriate measures based on the results of the investigation ".
