New Flaw in 737 MAX 8 Computer Found that Could Send Plane into Nosedive

Sources close to the ongoing effort to recertify the 737 MAX 8 told CNN that flight simulator testing uncovered a completely different way that the plane could suddenly pitch down, and that this one could have been even harder for a pilot to handle in an emergency. According to the report, a microprocessor failure in the 737 MAX 8's computer system could result in something known in the industry as runaway stabilizer trim.

The stabilizer is the smaller, horizontal wings on the tail of the aircraft that normally stabilizes the aircraft in flight, and the degree to which they are facing relative to the wind passing over or under them--known as the angle of attack, or AOA--can lift the tail upward or push it downward relative to the rest of the plane.

Too far in either direction and it is said to "runaway", at which point a pilot is supposed to cut power to the stabilizer using a wheeled lever in the cockpit. This should correct the stabilizer and return it to its normal, mostly forward-facing angle.

This mechanism is at the heart of the MCAS anti-stall system whose malfunction investigators suspect was a major factor in the crashes of Lion Air flight 610 and Ethiopian Airlines flight 302. Those crashes, both 737 MAX 8 aircraft, occurred less than six months apart, prompting the worldwide grounding of the 737 MAX 8 back in March which is still in effect. In the case of the MCAS, if the aircrafts AOA sensors detect an angle that indicates the plane is at risk of stalling, the MCAS is supposed to increase the stabilizer's AOA to compensate, leveling the aircraft out.

If the AOA sensor on the aircraft feeds the MCAS system bad readings, which appears to be the case in both the Lion Air and Ethiopian Airlines crashes, rather than stabilize the aircraft to an appropriate level, the additional lift of the tail pushes the nose of the aircraft down and can send it into an unrecoverable nosedive in as little as 40 seconds.

Since the investigations into those crashes is still ongoing, it isn't known whether this latest flaw had any role in either of those crashes, since both failures would have had essentially the same effect on the aircraft.

The FAA Does Its Job

This latest microprocessor flaw was detected when government pilots working with the Federal Aviation Administration (FAA) tested the effects of a microprocessor failure in the 737 MAX 8's computer. During these tests, pilots found that once the microprocessor failed, the plane would start pitching forward and if they did not intervene quickly, the plane would go into an unrecoverable nosedive.

"It was difficult for the test pilots to recover in a matter of seconds," one source told CNN. "And if you can't recover in a matter of seconds, that's an unreasonable risk."

The FAA would not confirm the specific issue when CNN contacted them for comment, but the FAA did tell them that "the FAA's process is designed to discover and highlight potential risks. The FAA recently found a potential risk that Boeing is required to mitigate."

Boeing didn't specifically cite a microprocessor failure in a statement the company released yesterday but did confirm that the FAA had identified an "additional requirement" during flight simulator tests that involved runaway stabilizer trim, and that Boeing agreed that it needed to be fixed.

"The safety of our airplanes is Boeing’s highest priority. During the FAA’s review of the 737 MAX software update and recent simulator sessions, the Federal Aviation Administration (FAA) identified an additional requirement that it has asked the company to address through the software changes that the company has been developing for the past eight months," the company's statement said.

"The FAA review and process for returning the 737 MAX to passenger service are designed to result in a thorough and comprehensive assessment," they added. "Boeing agrees with the FAA's decision and request, and is working on the required software. Addressing this condition will reduce pilot workload by accounting for a potential source of uncommanded stabilizer motion. Boeing will not offer the 737 MAX for certification by the FAA until we have satisfied all requirements for certification of the MAX and its safe return to service."

Whether a software fix alone will be sufficient or whether Boeing will have to physically replace all of the microprocessors in question on all of the existing airplanes stuck in aircraft hangers around the world is unclear. Whatever fix will ultimately be required, given the circumstances, it appears that the FAA is putting Boeing's 737 MAX 8 through the kinds of rigorous testing that it didn't do the first time around.