Boeing kept airlines and pilots in the dark about an automated background trim system on the 737 MAX that may be implicated in the first crash of the new model in Indonesia last month. The trim system, which is meant to improve pitch characteristics and stall protection, wasn’t even described in any of the documentation provided to pilots transitioning to the new aircraft.
Lion Air JT610, a new MAX8 with only about 800 hours on the airframe, plunged into the Java Sea off Jakarta on Oct. 29, killing all 189 people aboard. Prior to the crash, the aircraft flew through repeated pitch, airspeed and vertical speed excursions before diving almost directly into the water about 12 minutes after takeoff.
According to sources at two airlines operating the MAX series, the system in question is called Maneuvering Characteristic Augmentation (MCAS) and is intended to improve pitch response at high angles of attack. It was added to the MAX models partly because the aircraft has heavier engines than the previous 737 NG models and the airplane’s center of gravity is biased more forward.
MCAS is activated without pilot input and would typically come alive in steep turns with high load factors, but only when the airplane is being flown manually. According to a minimal description provided to AVweb, MCAS operates only in flaps-up flight and is inhibited in any other configuration. MCAS intervenes at a threshold angle of attack and automatically trims nose down at a rate of 0.27 degrees per second to a maximum of 2.5 degrees. Stabilizer input is lower at high Mach numbers, but more aggressive at low Mach. The Wall Street Journal said Boeing didn’t disclose MCAS details to cockpit crews because it was worried about overwhelming them with more technical detail than needed or could digest. Boeing also said pilots were unlikely to encounter MCAS intervention during their normal flying.
Pilots trained on the MAX weren’t given even minimal briefings on MCAS, according to an interview with Jon Weaks, president of the Southwest Airlines Pilots Association published in the Seattle Times early Tuesday. “We do not like the fact that a new system was put on the aircraft and wasn’t disclosed to anyone or put in the manuals,” Weaks told the Times. And both Boeing and the FAA have warned that the system may not be performing as it’s supposed to.
American Airlines, which also operates the MAX8, also provided its pilots with new documentation on MCAS hurriedly provided by Boeing after the Lion Air crash. Because MCAS relies on angle of attack data from the aircraft’s vane-type sensor, one focus of the Lion Air investigation is on the AoA sensor itself, which appears to have been replaced as faulty prior to the accident. The aircraft also reportedly had a history of unreliable airspeed indications. It’s unclear if the two are related to the accident or how they affect MCAS operation.
On the Pilots of America online forum, an American Airlines pilot posted an informational bulletin from a pilot’s association and added this: “We had NO idea that this MCAS even existed. It was not mentioned in our manuals anywhere (until today). Everyone on the 737 had to go through differences training for the MAX and it was never mentioned there either.”
Boeing said Monday that it’s working closely with investigators and taking every measure “to fully understand all aspects of this incident.” Meanwhile, Indonesian investigators say they expect to release an initial report by the end of November.