After months of relative quiet, Boeing on 27 March took its 737 Max message public.
During a briefing with reporters, a top Boeing executive defended the safety of the 737 Max and described a software update intended to address concerns about the aircraft's manoeuvring characteristics augmentation system (MCAS),
Officials also stressed pilot procedures Boeing says arrests the type of wayward stabiliser movement suspected of causing at least one of two recent Max crashes.
The media briefing came the same day Boeing hosted some 200 pilots to discuss software upgrades.
"All of us at Boeing are deeply affected by the tragic loss of the Lion Air and Ethiopian passengers and crew," Boeing vice-president of product strategy Mike Sinnett tells reporters. "Safety is core to everything we do. The 737 is a safe airplane."
During the briefing Boeing avoided discussion of cause, noting investigations into the 737 Max 8 crashes remain open. Investigators have said the Lion Air pilots lost a battle after MCAS repeatedly pushed the aircraft's nose down. Officials have noted similarities to the Ethiopian flight.
Boeing used the media briefing partly to better explain the new system.
MCAS sits within the 737's "speed trim system," which Boeing added to the 737NG when transitioning from the 737 Classic. The speed trim adjusts the stabiliser to provide a "constant increasing gradient of force" as pilots pull back on the control column, Boeing says.
Engineers added MCAS to the Max's trim system to account for that aircraft's engines, which are heavier, larger in diameter and more forward on the fuselage than the 737NG's engines. The system activates when the autopilot is off and the flaps are up, and, as originally designed, relies on data from one of two AOA vanes.
MCAS gives pilots "a linearly increasing gradient of column force" through elevated angles of attack, says a top Boeing official who declines to be named. "That's what the MCAS function is designed to do."
MCAS also gives the 737NG and 737 Max "consistent handling".
So consistent, indeed, that some pilots say they needed complete less than 1h of iPad-based training to convert to the Max.
Boeing's 737 production line in Renton on 27 March. Production continues despite the ongoing global 737 Max grounding
The MCAS upgrades announced by Boeing make the system rely on two AOA sensors, and the computer monitors sensor variation and deactivates if sensors differ by more than 5.5°.
The changes prevent MCAS from activating repeatedly and ensure that pilots can always override MCAS stabiliser movement by pulling the control column, says Boeing.
While stressing that investigations remain ongoing, Michel Merluzeau, director at aerospace research and consulting company Air, thinks Boeing's proposed changes "will go a long way in preventing any issues with the system and its behavior".
"I'm generally satisfied with what [Boeing] presented, from a standpoint of addressing" MCAS concerns, he says.
Boeing also developed new "differences training" to help pilots better understand MCAS. Pilots will need to complete the roughly 30min course before returning to the Max, Boeing says.
The company declines to speculate when regulators might certify the software and training updates, but observers suspect the grounding will lift incrementally as countries independently conduct reviews.
Broader geopolitical factors – including US-China trade battles and competitive tension between the aerospace industries of the USA, Europe and China – could also influence the pace at which 737 Maxes get airborne, observers say.
Once certified, Boeing can get the software updates to airlines in about one day, and updating aircraft takes 1h, according to the company.
Though Boeing says the changes address MCAS concerns, it continues stressing that pilots can quickly counteract MCAS-induced stabiliser movement.
"The flight crews can override [stabiliser movement] at any time with a switch under their thumb and with the manual trim," the official says. "Pilots can always electronically or manually override the automatic systems."
Boeing has repeated that message since the Lion Air crash.
Merluzeau thinks recent crashes suggest a need to strengthen crew training and crew resource management (CRM), a type of training intended to help pilots work together to solve issues.
"There is a very important training and CRM element to be addressed," he says. "It… needs to be at the forefront of aviation safety."
During the briefing Boeing also addressed pointed questions about whether software upgrades indicate an original faulty design.
"It does not", the Boeing official responded.
Asked why MCAS originally relied on one sensor, that official noted that all aircraft manufacturers design systems based on "hazard categories", which reflect system failure risks. If risks can be adequately mitigated, a one-data-source system might be deemed safe.
"There is a class of failures for which mitigation of that failure can be very quickly performed by a trained pilot using established procedures," the official says. In such cases, "standard practice allows a single input".
Boeing also addressed criticism for making an "AOA disagree" cockpit warning an optional feature.
"All of our procedures and all of training is centered around airplane pitch, airplane airspeed, airplane vertical speed and altitude. And none of our procedures or training reference angle of attack at all," Boeing says.
Boeing has since made that feature, which notifies pilots if AOA indicators differ by more than 10° for more than 10s, standard equipment.
With the software upgrade developed, Boeing now faces the task of winning confidence from regulators, airlines, pilots and flying public.
Boeing insists the aircraft, with the updated MCAS, is good to go.
"We have conducted some thorough audits since the Lion Air accident of all aspects of systems on 737 Max, with a particular focus on things that have change since the 737NG," the official says. "We have uncovered nothing that concerns" us.