Feds: Boeing 737 MAX Will Need Additional Redundancies To Fly Again

The proposed changes look to remove from allowable inoperative equipment systems that might come into play in the case of problems with the MCAS system, linked to two air tragedies.

The 737 MAX fleet was grounded in March 2019 in the wake of those two accidents, Lion Air Flight 610 and Ethiopian Airlines Flight 302, totaling 246 fatalities. The accidents drew attention to the MAX's MCAS system, which Boeing admitted contributed to each accident. The Maneuvering Characteristics Augmentation System was designed to make the 737 MAX handle like other 737s, introducing a nose-heavy feel as the aircraft reaches a high angle of attack in clean configuration.

A single failure point in an angle-of-attack probe, however, made the aircraft susceptible to an unconventional pitch trim runaway. This system wasn't covered in differences training for pilots who trained on other 737 models. The fleet's grounding, intended to allow the manufacturer to develop training and flight control system changes, has stretched beyond a year as multiple issues with the aircraft's design and certification have surfaced and delayed its return to service. At the time of the grounding, airlines were operating 387 aircraft of this type. Boeing continued production until January 2020, parking an additional 400 new aircraft without delivering them.

Although the timeline is still far from concrete, details are emerging that Boeing's 737 MAX will return to the skies with additional protections and systems that previously weren't required to be operational. The 737 MAX Master Minimum Equipment List, in a revision published April 10, outlines which of the aircraft's systems can be inoperative when it's dispatched for flight. While this manual doesn't outline any new modifications to the aircraft, it does require more of the aircraft's auto-flight and flight control systems to be operable prior to flight.

Transport category aircraft are manufactured with layers of redundancy to ensure safe operation, and an isolated system often allows the aircraft to operate safely until repairs can be made. Deferring redundant parts of aircraft systems is an industry practice that allows airlines to dispatch their aircraft reliably and ensure that customers get to their destination in a timely manner.

Previously, the 737 MAX could depart on a flight with both autopilot computers inoperative, so long as the flight conditions allowed. Many instrument approaches require the aircraft to be flown by autopilot, for example, which would limit destinations for weather. Other factors included the pilots' agreeing that the flight wasn't so long as to make hand-flying the entire leg a fatiguing act. The 737 Max's latest MEL published indicates that it must have one operating autopilot to depart.

Additional redundancies and indications, previously deferrable, are now mandatory. These include various warnings to indicate trim and autopilot failures, which were previously allowed to be deferred under certain circumstances. Both yoke switches for disconnecting the autopilot and controlling the pitch trim are now required to be operable. Many of these systems were previously allowed to be deferred so long as the pilot flying had working controls.