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Boeing employees surround the 10,000th 737 jet — a 737 MAX 8 built for Southwest Airlines — during a ceremony in Renton, Wash., in March 2018. (Boeing Photo)

Two optional safety features that might help pilots head off a scenario that’s at the heart of investigations into two catastrophic crashes of Boeing 737 MAX jets will become available free of charge, The New York Times reported.

The features are an indicator that shows pilots the readings from two sensors that monitor an aerodynamic characteristic known as the angle of attack, and a “disagree light” that flashes when those sensor readings are at odds with each other.

Spurious readings from the angle-of-attack sensors are thought to have played a role in last October’s crash of a Lion Air 737 MAX 8 plane in Indonesia, which killed 189 people on board; and this month’s crash of an Ethiopian Airlines 737-8 in Ethiopia, which killed 157.

The current leading theory is that in both cases, bad data from a single sensor caused an automatic flight control system — known as the Maneuvering Characteristics Augmentation System, or MCAS — to kick in repeatedly.

The MCAS was added to the 737 MAX’s control system to compensate for the aerodynamic effect of the model’s bigger engines and guard against an excessive upward lift and stall. But investigators suspect that, in the fatal crashes, the spurious data caused the system to force the plane into a nose dive.

In both cases, pilots complained about control problems minutes into their doomed flights. Today The Wall Street Journal quoted Ethiopian Airlines CEO Tewolde Gebremariam as saying he believed the MCAS system was activated on the Ethiopian jetliner, based on what he has learned about the investigation.

Boeing says pilots can use a procedure to disengage the MCAS system in the event of a problem, but the procedure apparently did not come into play in the Lion Air or Ethiopian Airlines scenarios.

Over the weekend, Boeing brought pilots and trainers to its 737 MAX facility in Renton, Wash., to discuss potential safety modifications to the plane and test out simulations of an MCAS problem scenario. The New York Times quoted sources as saying that pilots using the simulators were able to land their virtual planes safely.

About 200 pilots, technical leaders and regulators are due to attend another session on Wednesday.

“This is part of our ongoing effort to share more details about our plan for supporting the safe return of the 737 MAX to commercial service,” Boeing said in a statement. “We had a productive session this past Saturday and plan to reach all current and many future MAX operators and their home regulators.”

Boeing is preparing to release a software update and revised training guidelines that are aimed at addressing the MCAS issue. Reportedly, one of the changes will involve having the MCAS system take in data from both angle-of-attack sensors instead of just one, and refrain from kicking in if the readings disagree. Another change would reportedly limit the system’s ability to kick in repeatedly. The Journal said the changes have won tentative approval from the Federal Aviation Administration.

The debate over the 737 MAX safety features has touched on cockpit instrumentation as well: Last week, The New York Times reported that the angle-of-attack indicator and the disagree light were optional features that operators had to pay extra for. But on Sunday, the Times said the disagree alert would become standard on all new 737 MAX planes, and the angle-of-attack indicator would be provided free of charge for customers who want it.

A statement from John Hamilton, chief engineer for Boeing Commercial Airplanes on 737 MAX flight deck displays, added to the instrumentation debate:

“All primary flight information required to safely and efficiently operate the 737 MAX is included on the baseline primary flight display. Crew procedures and training for safe and efficient operation of the airplane are focused around airplane roll and pitch attitude, altitude, heading and vertical speed, all of which are integrated on the primary flight display. All 737 MAX airplanes display this data in a way that is consistent with pilot training and the fundamental instrument scan pattern that pilots are trained to use.

“The AOA (angle of attack) indicator provides supplementary information to the flight crew. The AOA disagree alert provides additional context for understanding the possible cause of air speed and altitude differences between the pilot’s and first officer’s displays. Information for these features is provided by the AOA sensors.

“There are no pilot actions or procedures during flight which require knowledge of angle of attack.”

Boeing also provided these definitions for the terms used by Hamilton, which are helpful for the wider debate. The definitions make clear that the disagree alert and angle-of-attack indicators are considered software-based features rather than hardware:

  • Angle of Attack – the difference between the pitch angle (nose direction) of the airplane and the angle of the oncoming wind.
  • Angle of Attack Disagree Alert – a software-based information feature that alerts flight crews when data from left and right angle of attack sensors disagree.  This can provide pilots insight into air data disagreements and prompts a maintenance logbook entry.
  • Angle of Attack Indicator – a software-based information feature that provides angle of attack data to the flight crew through the primary flight displays.
  • Angle of Attack Sensor – hardware on the outside of the airline that measures and provides angle-of-attack information to onboard computers; also referred to as an AOA.
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