An alarming new report says that the two fatal Boeing 737 Max 8 crashes which killed almost 350 people were missing safety features which were sold as optional extras by the manufacturer, and not included as standard.

As entire fleets of the Boeing 737 Max 8 aircraft remain grounded across the globe amid investigations into safety practices at the airline giant, details have emerged about missing safety features, including additional sensors that would have operated as fail safes for the existing ones on board the aircraft and alerted the pilots to any potential issues.

The “angle of attack (AOA) disagree” light warns the pilot when the plane is about to stall based on factors such as the airflow and nose direction, but this does not come as standard when airlines purchase the aircraft.

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Another missing feature was the AOA indicator which gives pilots a visual representation of the airflow relative to the aircraft’s nose.

The Maneuvering Characteristics Augmentation System (MCAS) is designed to detect an imminent stall and adjust the plane’s stabilizers to pitch the nose forward to increase airspeed, thus preventing a stall. However, when this is improperly timed, it can be fatal.

To make matters worse, another anti-stall system that does come as standard on the Boeing 737 Max planes reportedly only used one sensor at a time despite having two. Had the additional sensors been fitted and in use, the pilots could potentially have overridden the MCAS and prevented the tragic loss of life.

In response to the tragedies involving Ethiopian Airlines Flight 302 on March 10 and Lion Air Flight 610 five months earlier, Boeing will add the AOA disagree light as an update by the end of April, a source told the New York Times, and both sensors will operate by default. The AOA indicator will still be considered an optional extra, however.

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Boeing charges extra for things ranging from superficial things like leather seats to back-up fire extinguishers. Brazilian carrier Gol Airlines reportedly paid $6,700 extra for crew oxygen masks, and $11,900 for a weather radar system control panel.

Former Boeing engineers have reported that the company may have misled the Federal Aviation Authority about the efficacy of and risk associated with the MCAS, adding that Boeing didn’t have any external oversight for the aircraft’s safety certification.

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