Boeing to warn 737 MAX operators of a potential instrument failure that could cause the jet to nose-dive

Following the fatal crash of a Lion Air jet in Indonesia last week, Boeing was preparing Tuesday evening to warn all airlines operating its new 737 MAX of the potential for an instrument failure that could result in the plane entering a dangerous dive, according to a person briefed on the bulletin’s details.

The safety warning comes in what’s called a service bulletin that goes out to all operators of the airplane and includes instructions on exactly what pilots should do if the condition arises.

It’s normal for the Federal Aviation Administration to follow such a warning with an “airworthiness directive” that makes it mandatory, and this is anticipated in the coming days.

Investigators looking into 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 airplane’s “angle of attack,” which is the angle between the wing of the plane and the flow of air it is moving through.

A plane will have a high angle of attack when climbing. Too high an angle would cause a stall.

The concern caused by the flight pattern and initial investigation of the Indonesian crash is that the sensor may potentially feed false information about this angle to the flight computer, which in turn triggers other errors.

In particular, with the sensor falsely indicating that the nose is too high, when it isn’t, it causes an automatic system response that “trims” the horizontal tail of the plane to begin putting the plane’s nose down.

At the same time, it causes an indicator of the minimum speed to tell the pilot that the plane is near a stall, which also causes the pilot’s control column to shake as a warning. And the air-speed indicators on both sides of the flight deck disagree.

The pilots can use extra force to correct the nose down trim, but the failure condition repeats itself, so that the nose-down push begins again 10 seconds after correcting.

“If the nose is trimmed down on an aircraft, it becomes difficult for the crew to hold it,” said the person briefed on Boeing’s bulletin. “The nose is turning itself down and they are having to fight it. It takes a lot of effort to keep it from diving. Especially if you have a crew that’s confused and doesn’t know what’s going on.”

This description fits exactly the flight pattern of the Lion Air jet that crashed.

For 12 minutes before it crashed, the altitude swung up and down as if the pilots were fighting to maintain height, bringing the plane up, then having it swing down again repeatedly.

Pilots are typically trained on how to handle a “runaway trim” situation, said the person briefed on the Boeing bulletin, but that’s with everything else working as it should. In this case, the control-column shaking, the stall warning and the air-speed-indicator disagreement all combine to create confusion and keep the pilots very busy.

Boeing instructs pilots in the bulletin that if this failure arises, “initially, higher control forces may be needed to overcome any nose-down stabilizer trim.” The instructions go on to say that after the initial stabilization, the automatic trim system on the horizontal tail should be switched off and any trim performed manually.

Pilots can turn off the automatic trim system with a cut-off switch operated by the thumb on the control column.

“This is all coming from the Indonesian crash,” said the person briefed on the Boeing bulletin. “I’m not aware of any other operator having this problem.”

More than 200 MAXs are in service around the world. Boeing builds the 737 in Renton, and expects 40 to 45 percent of those built this year to be MAXs, the rest being the previous model.

News of the service bulletin was first reported Tuesday evening by Bloomberg News. A Boeing spokesman declined to comment.