Boeing has once again been shaken by its 737 Max saga, this time after it was revealed that safety features for the controversial airliner were inactive – which was not what the airlines flying the craft had been led to believe.
The American planemaker's chief exec, Dennis Muilenberg, gave a press conference yesterday in which he insisted the controversial Maneuvering Characteristic Augmentation System (MCAS) was not to blame for the two fatal crashes of Boeing 737 Max airlines within the last few months. Those crashes killed everyone aboard both airliners.
Muilenburg told the world's media the MCAS met its "design and certification criteria" and, judging by various reports of his remarks, appeared to suggest that the pilots of the doomed aircraft may have been partly to blame, saying:
"As in most accidents, there are a chain of events that occur. It is not correct to attribute that to any single item."
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An accident report eventually issued by the Ethiopian authorities found the pilots of flight ET302 followed Boeing's recommended procedures and checklists, though it appeared that while the crew had cut out the electric automatic trim, the resulting manual backup situation left them physically unable to keep their 737 Max 8 from nosediving into the ground.
Separately, various American news outlets reported that Boeing had failed to tell airlines flying the 737 Max series that safety features those airlines had ordered on their new aircraft were not operational. The manufacturer said it did not "intentionally or otherwise deactivate" those warning systems, which were a warning caption on the pilot's display screen showing the message AOA DISAGREE (angle-of-attack; two probes on either side of the 737 Max’s nose gather angle-of-attack data, which the autopilot uses to determine nose-up/nose-down trim settings) and a raw readout of computed AoA data, as a gauge on the pilot's display.
With raw data being displayed to the pilots, or an automated "sensors disagree" warning, the extra information lets flight crews take immediate action if the MCAS system (explained here) malfunctions. Southwest Airlines, one of Boeing's larger customers, had reportedly thought their aircraft had the disagree option enabled by default.
"The disagree alert was intended to be a standard, standalone feature on Max airplanes. However, the disagree alert was not operable on all airplanes because the feature was not activated as intended," Boeing said in a statement.
A software fix is still in the works between Boeing and American regulator the Federal Aviation Authority.
Why was MCAS installed?
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MCAS was a software-based fix for physical handling characteristics. The 737 Max is essentially a stretched version of the basic 737-8 model, intended to compete with Airbus's A321 – itself a longer model of the basic A320 shorthaul airliner.
When Boeing stretched the Max, it also fitted larger, more powerful and more fuel-efficient engines. Those engines' size meant they had to be mounted further forward than previous 737s, bringing them out from under the wings where previous models' engines were slung. This changed the aircraft's flight characteristics and introduced the potential for a dangerous nose-up stall condition. MCAS was supposed to add nose-down trim if the 737 Max got itself into a high-AoA situation with certain combinations of landing gear and flaps deployment.
The idea of automatically adding trim was to help the pilot overcome potentially high control column forces and avoid a possibly fatal stall. Normal drills to escape a stall are to lower the nose and increase power – but on modern airliners with underslung engines, increasing power actually pushes the nose up through an effect known as the pitch-power couple. In effect this is vertical torque; the aircraft moves around its centre of gravity (CofG), meaning its engines slung below the CofG produce an upwards rotation as they produce more power. To counteract this, pilots must push forward on their control columns.
What nobody appeared to know at the time all of this was certified as safe was that if one of the 737 Max's AoA sensors malfunctioned for whatever reason, MCAS could decide the jet was in that dangerous nose-high condition and begin inappropriately applying nose-down trim, forcing the airliner into a nosedive at an unexpected moment.
Both fatal crashes happened in the minutes after takeoff: a high workload situation as the pilots safely retract wheels and flaps, maintain the planned flight path, change air traffic control frequencies as they leave the airport's control zone and run through standard post-takeoff checklists. It is possible that MCAS systems being deployed simply overloaded flight crews who hadn't been adequately trained on what to do if the system malfunctioned. ®