Seattle, USA, Nov 06: 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 airspeed 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.