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Fox Business Network’s Maria Bartiromo asks Boeing CEO Dennis Muilenburg about the Lion Air 737 MAX crash. (Fox Business Network)

What did 737 MAX pilots know, and when did they know it? That’s become a subject of debate in the wake of last month’s fatal Lion Air crash in Indonesia and the potential role played by an automatic control system that Boeing added to new-model 737s.

The control feature — known as the maneuvering characteristics augmentation system, or MCAS — is designed to guard against a stall by automatically pushing down the plane’s nose under potentially hazardous aerodynamic conditions.

The MCAS system relies on readings from sensors on the plane, including devices that monitor how air is flowing over the wings. The problem is that erroneous data from those angle-of-attack sensors, or AOA sensors, could cause an unwarranted nose-down condition, putting the plane into an aggressive dive.

Preliminary findings from investigators looking into the Oct. 29 crash of the Lion Air 737 MAX 8 suggest that faulty angle-of-attack data contributed to the drastic ups and downs that preceded the loss of the plane and all 189 people who were aboard.

Because of those findings, Boeing issued a service bulletin on Nov. 6, pointing airplane operators to “existing flight crew procedures to address circumstances where there is erroneous input from an AOA sensor.” One of the procedures involves disengaging the MCAS automatic control system. The FAA followed up the next day with an emergency airworthiness directive covering the same ground.

As word of the warnings spread, some pilots said this was the first they heard about the MCAS system and the potential control issue.

The Seattle Times quoted Jon Weaks, president of the Southwest Airlines Pilots Association,as saying that the airline and the pilots “were kept in the dark.”

“Is there anything else on the MAX Boeing has not told the operators?” Weaks was quoted as saying. “If there is, we need to be informed.”

The Allied Pilots Association, which represents American Airlines pilots, suggested something similar in a message to its members. “This is the first description you, as 737 pilots, have seen,” the association said. “It is not in the AA 737 Flight Manual Part 2, nor is there a description in the Boeing FCOM [flight crew operations manual]. It will be soon.”

Aviation Week said it verified that the MCAS system wasn’t covered in the operations manual or in the supplemental training materials provided to pilots coming over to the 737 MAX from the previous generation of 737s.

Boeing CEO Dennis Muilenburg took a different perspective today during an interview on Fox Business Network. He said the procedures for handling the control issue were “part of the training manual.”

“It’s an existing procedure, so the bulletin that we put out again last week and over the weekend pointed to that existing flight procedure,” Muilenburg told Fox’s Maria Bartiromo.

Muilenburg acknowledged that the MCAS was a new feature for the 737 MAX, and that its potential failure modes merited further investigation:

“There are new systems on the airplanes that are designed to take advantage of the capabilities of the airplane and provide control capability and high angle-of-attack conditions and those systems operate properly. And again, in certain failure modes if there’s an inaccurate angle-of-attack sensor feeding information to the airplane, there’s a procedure to handle that. And so, again, as part of the investigation process we’re going to make sure we fully understand that.  We’re going to make sure that we’re providing all the information necessary and appropriate training, and go back to the core value here … that the airplane is safe, we know how to fly it safely, and we’re very confident.”

Muilenburg assured Bartiromo that Boeing would “continue to fully cooperate in the investigation” of last month’s crash.

Reuters quoted an unnamed U.S. government official as saying Boeing was expected to unveil a software update to reduce the risks posed by the MCAS. Meanwhile, Soerjanto Tjahjono, the head of Indonesla’s National Transportation Safety Committee, said his country’s regulators would tighten requirements for pilot training.

“Because this incident happened, we know we need additional training,” Reuters quoted him as saying.

Update for 11:59 p.m. PT Nov. 13: The Wall Street Journal reports that the FAA is reviewing details surrounding the safety analyses that Boeing has provided over the years as part of the certification process for the 737 MAX 8 and MAX 9 models. FAA officials are also looking into training requirements for pilots, the Journal says.

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