"We at Boeing are sorry for the lives lost in the recent 737 Max accidents", Muilenburg said on Thursday via Twitter after a new report by the Ethiopian government revealed the pilots on Ethiopian Airlines fight 304 followed Boeing's recommended procedures when the plane began to nose dive.
Responding to the preliminary report, Boeing Commercial Airplanes president, Kevin McAllister, said: "Understanding the circumstances that contributed to this accident is critical to ensuring safe flight".
He said investigators are going to be focussed far more on the interaction between software and pilots rather than mechanical issues in future.
At the same time, the autopilot disconnected as it couldn't handle the mismatch in information it was receiving, and the crew reported to Air Traffic Control that they were experiencing flight control problems.
"We're taking a comprehensive, disciplined approach, and taking the time to get the software update right", Muilenburg said. Boeing has reportedly stated that the issues are unrelated to the anti-stall system, and called the problem "relatively minor".
Data from the Ethiopian Airlines flight indicates the aircraft was flying nose-heavy and not in a "neutral" attitude when pilots hit the stabilizer cutout switches to disable the MCAS system, the preliminary report showed.
"If we don't end the cozy relationship between the patsy FAA...and the Boeing company, 5,000 of these fatally flawed planes will be in the air all over the world with millions of passengers", Nader said.
A preliminary report on the Ethiopian Airlines crash released on Thursday did not directly blame the MCAS, but did speak of the plane's constant and uncontrollable nose-diving, which could have been caused by the software if it were fed false data from a damaged sensor.
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"Aviation authorities shall verify that the review of the aircraft flight control system has been adequately addressed by the manufacturer before the release of the aircraft for operations", she added.
Separately, National Transportation Safety Board (NTSB) chairman Robert Sumwalt told reporters that US investigators were given the raw data from Ethiopian Airlines Flight 302 as soon as it was read in France last month.
Boeing is cutting its 737 production rate as the company works to return the MAX airplanes to flight.
In the final struggle, the report describes the aircraft pitching downward, eventually reaching a 40-degree nose dive as it crashed to ground at 575 miles per hour (925 kmh).
Boeing said it is still working with the FAA and regulatory agencies to develop and certify a software update created to keep the system from being activated unintentionally, along with additional training for pilots. The agency was also reluctant to ground the planes after the Ethiopian Airlines crash and was among the last agencies to do so.
Pilots in both airplanes seem to have had trouble regaining control of the aircraft after the MCAS system pushed the nose of the jets down to keep them from stalling.
"With an MCAS failure such as they suffered, the nose pitching down radically multiple times would create literally the most hard situation I would imagine in an aircraft", pilot Anthony Roman told CBS correspondent Kris Van Cleave.
Officials said a full report would be completed within a year of the crash.