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FAA Details Boeing 737 MAX MCAS Oversight Handover

MCAS, which provides automatic stabilizer inputs that put the aircraft's nose down, is the focus of the October 2018 crash of Lion Air flight JT610, a 737 MAX 8.

FAA retained oversight of the Boeing MAX’s new maneuvering characteristics augmentation system (MCAS) early on in the aircraft's certification process, but later delegated it to Boeing once the agency was confident the company had the expertise to manage it, FAA acting administrator Dan Elwell told a Senate hearing March 27.

MCAS, which provides automatic stabilizer inputs that put the aircraft's nose down, is the focus of the October 2018 crash of Lion Air flight JT610, a 737 MAX 8. The system is also being eyed in the March 10 crash of Ethiopian Airlines flight ET302, also a MAX 8. In both cases, the new aircraft struggled to maintain altitude and dove to impact. 

"As a new device on an amended type certificate, we retained the oversight [of MCAS],” Elwell said. As the organization designation authorization process for the MAX was refined “under very strict review,” MCAS was among the items shifted to the manufacturer.

The revelation, made before the US Senate aviation and space subcommittee chaired by Sen. Ted Cruz (R-Texas), sheds additional light on how MCAS was vetted and approved, and suggests FAA had more input on the system than previously understood.

In written testimony presented to the subcommittee, Elwell said the agency “was directly involved in the System Safety Review” of MCAS.

“FAA engineers and flight test pilots were involved in the MCAS operational evaluation flight test” and 133 of the 297 MAX certification flight tests, he said. Elwell could not immediately provide a timeline on when MCAS was delegated to Boeing, as part of the certification process.

The acting administrator said MCAS—added to the MAX to make it handle like its 737NG predecessor in certain fight profiles—was not flagged by pilots as a relevant change from the 737NG during certification.

The 737 MAX flight standardization board (FSB), made up of 737NG pilots from carriers around the world, flew 737 MAX aircraft and simulators to compare the two models—a routine process when manufacturers develop new models under amended type certificates.

“After many scenarios and flights in all regimes, there was a consensus that there was no marked difference in the handling characteristics of these two aircraft,” Elwell said. This, he explained, was the primary reason that more information on MCAS was not provided to pilots.

The Lion Air probe is focusing on how pilots reacted when MCAS, which was relying on erroneous angle-of-attack (AOA) data, pushed the nose down when the pilots did not want the flight-control inputs. Boeing, relying on the FSB feedback and the concept that MCAS was an expansion of the 737NG speed-trim system (STS) and not a standalone addition to the design, did not include MCAS-related training in the 737 MAX documentation. MCAS is not referenced in 737 MAX flight manuals. 

Boeing first alerted the pilot community to it shortly after the Lion Air 737 MAX crashed, when initial information from the probe suggested that MCAS may have activated during the accident sequence.

Data pulled from the Lion Air aircraft’s flight data recorder shows the pilots used manual inputs to counteract MCAS, but the system—still being fed incorrect AOA data—kept responding with nose-down inputs. They apparently did not diagnose the problem as a runaway stabilizer, which is what Boeing and FAA expected would happen if MCAS malfunctioned. Both 737NG and MAX pilots are given the same checklists for runaway stabilizer, including a step to activate cut-out switches that cut power to the stabilizer. It is supposed to be memorized.

“Pilots are trained that if they get an input that they did not ask for, they go through the appropriate procedure,” Elwell said. 

Runaway stabilizer on the 737NG and MAX “presents itself the same way, and it's dealt with the same way,” he added.

Cruz pressed Elwell, noting that pulling back a 737NG yoke, which pulls the nose up, activates column cut-out switches that interrupt STS-related automatic stabilizer movements. But because MCAS is designed to push the aircraft's nose down, it bypasses the same switches on the MAX.

Elwell responded by noting—correctly—that the 737NG and MAX “stabilizer runaway” checklist does not include pulling back on the yoke.

TAGS: Airframe
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