Federal aviation authorities have identified a new problem in the Boeing 737 Max that they want remedied before the troubled jetliner can fly again.
The discovery of the problem, which is going to require, at very least, additional software fixes and testing to remedy, could further push back the return to service for the jet already grounded during airlines’ busy summer travel season after two deadly international crashes.
The Federal Aviation Administration said that it found a “potential risk that Boeing must mitigate” during simulator testing of the software changes that Boeing has made and wants to have certified. Like the original flaw that led to the plane’s grounding, the new issue also involves the uncommanded movement of the horizontal stabilizer, the little wing near the tail that moves the plane up or down.
In the case revealed Wednesday, the problem involved a stabilizer that moves on its own without being prompted by pilots, an issue known as “runaway stabilizer trim.” If the stabilizers were to move in such a way that the jet’s nose ends up pointed at the ground and pilots are unable to correct it, the plane could crash.
That’s the scenario that is believed to have played out in the two fatal accidents. One involved a Lion Air flight over the Java Sea last October and the other an Ethiopian Airlines flight after takeoff from Addis Ababa in March. Together, they claimed 346 lives and prompted grounding of the plane worldwide.
In the two crashes, the fatal flaw centered on a computer program, the Maneuvering Characteristics Augmentation System, or MCAS, that had been installed to work in the background to automatically keep the plane’s nose up, compensating for a design change in the twin engines from past 737 generations. The Max’s engines are larger and mounted farther forward on the wings.
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This time, the stabilizer problem centers on the way in which data is processed by the plane’s flight computer.
If a microprocessor were to fail, CNN reported, it could push the plane’s nose down. The microprocessor will have to be reprogrammed or replaced. But given the hundreds of hours of testing that Boeing has already put into changes in the Max, it was unclear whether the new fix will have to go through rigorous, time-consuming testing in a way that will further push back the plane’s return to passenger service. Until the new problem surfaced, best estimates were that the Max would not fly again until the fall.
The FAA, under scrutiny for having approved MCAS in the first place, said it is following “a thorough process, not a prescribed timeline, for returning the Boeing 737 Max to passenger service.”
Boeing said in a statement that it agrees with the need to make the new repair as identified by FAA.
“Addressing this condition,” which it did not specify, “will reduce pilot workload by accounting for the potential source of uncommanded stabilizer motion,” the aircraft maker said in the statement.