Last week, Lion Air's plane crash in Indonesia prompted Boeing to issue a security alert to all airlines operating the 737 MAX, informing pilots of what to do if a specific sensor failure occurs.

Following the fatal crash of a Lion Air jet in Indonesia last week, Boeing prepared on Tuesday evening to warn all airlines operating their new 737 MAX of a possible instrumental failure that could lead the plane into a dangerous dive advised about the details of the bulletin.

The safety alert comes in a service bulletin addressed to all operators of the aircraft and provides instructions on what the pilots should do exactly when the condition occurs.

It is normal for the Federal Aviation Administration to follow such a warning with an "airworthiness directive" that mandates it, and this is expected in the coming days.

Investigators investigating the cause of the Lion Air crash, which killed all 189 passengers and crew, have identified a potential failure of a sensor that tells the pilot and the flight control computer the aircraft's "angle of attack" the airflow through which it moves.

An airplane has a high angle of attack during the ascent. Too high an angle would lead to a standstill.

Concern about the flight pattern and the initial investigation into the Indonesian crash is that the sensor may be relaying false information about this angle to the flight computer, which in turn triggers other errors.

If the sensor falsely indicates that the nose is too high, it will result in an automatic system response that "trims" the aircraft's horizontal end and pushes the nose of the aircraft down.

At the same time, the pilot is informed by an indication of the minimum speed that the aircraft is near a standstill, causing the pilot's control column to tremble as a warning. And the airspeed indicators on both sides of the flight deck do not match.

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The pilots can apply extra force to correct the nose trim, but the fault condition repeats so that the nose depressor restarts 10 seconds after the correction.

"When the nose is cut down on an airplane, it becomes difficult for the crew to hold it," said the person who was informed in Boeing's bulletin. "The nose turns off and they have to fight it. It is very tiring to prevent diving. Especially if you have a crew that is confused and does not know what's going on. "

This description fits exactly with the flight pattern of the downed Lion Air-Jet.

The altitude rocketed up and down for 12 minutes before the crash, as if the pilots were struggling to hold the altitude, bring the aircraft up, and then repeatedly rock it down.

The pilots are usually trained on how to handle a "runaway trim" situation, said the person who was briefed on the Boeing bulletin, but for everything else, it works the way it should. In this case, the blurring of the control column, the warning warning and the mismatch of the airspeed indicator cause confusion and keep the pilots very busy.

Boeing instructs the pilots in the Bulletin that when this error occurs, "higher initial control forces are required to overcome a nose-down stabilizer crash." The instructions state that after the initial stabilization, the automatic trim system is horizontal. The tail should be off and the trimming should be done manually.

Pilots can turn off the automatic trim system with a disconnect switch that operates with the thumb on the control column.

"It all comes from the Indonesian crash," said the person who was notified in the Boeing Bulletin. "I am not aware that another operator has this problem."

More than 200 MAX are in use worldwide. Boeing builds the 737 in Renton and expects to be 40 to 45 percent of the MAXs built this year, with the remainder being the predecessor.

News from the Service Bulletin was first reported by Bloomberg News on Tuesday night. A Boeing spokesman declined to comment.


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