Key 737 MAX upgrade to ease pilot workload

When Boeing certifies and delivers the new 737-10, the company will debut an updated avionics system that reduces the possibility of erroneous air data and eases pilot workload in the case of a failure.

“Right now, if a pilot experiences a problem with the angle-of-attack system, they have to run through an extensive checklist. We’re significantly simplifying that. It’s going to be much easier for pilots,” said Capt. Bill Quashnock, Boeing’s 737 deputy chief pilot.

Quashnock has demonstrated the enhanced angle-of-attack (AOA) system for multiple 737 operators and “they all love it.”

The new system – which will be retrofitted on other 737 MAX models – reflects a robust engineering solution in response to learnings from two tragic accidents.

Critical sensors

The Boeing 737 is equipped with two “angle of attack” vanes on either side of the airplane’s nose. The small metal devices serve as critical sensors that measure the angle of the airplane relative to the airflow. AOA readings stream into the avionics system, which uses the data to help calculate other flight information.

“If there’s an issue with that vane, it can lead to bad airspeed and altitude data presented to the pilots as well as to the downstream systems like flight controls that use that data,” said Nate Bement, an avionics engineering manager.

In both 737 MAX accidents in 2018 and 2019, one of the two AOA sensors malfunctioned and fed erroneous data into the system, triggering a cascading set of events that contributed to two fatal accidents.

All options on the table

In the aftermath of the accidents, Boeing implemented several technical and operational updates to improve safety, reduce pilot workload and strengthen the airplane’s handling of sensor failures.

A Boeing engineering team also set out to redesign the AOA system, pulling together experts from Avionics, Flight Controls, Crew Operations, Flight Sciences and the Flight Test pilot community.

“All options were on the table,” said Jonathan Morrow, a Boeing Technical Fellow and avionics expert. “We locked ourselves in a room for about four hours a day every day, for six months straight, coming up with what became the enhanced angle-of-attack package.”

Monitoring for errors

The engineering spark came when the team focused on the fact that AOA vanes behave in a certain way during specific phases of flight. The engineers developed five ‘monitors’ in the air data system to check whether the AOA sensors produce the expected reactions, including:

• At takeoff: As the airplane starts to pitch up and lift into the air, the AOA vanes are supposed to rotate momentarily to a high value.
• After takeoff: Once an airplane builds up sufficient air speed, the AOA sensors are supposed to measure near zero as aerodynamic forces rotate the AOA vane back down.
• After a bird strike: When an AOA vane collides with an object, such as a bird, it can rotate to a very high value and lose its anti-ice electric current.

In any of the five situations, if an AOA sensor does not produce an expected result, the monitoring software will flag the data as faulty and stop it from passing on to other systems. Alternatively, the monitors will replace the erroneous data with a default value, so that it doesn’t corrupt other calculations.

Additionally, the system will flash an “AOA FAIL” alert – identifying either the left or right vane – on the pilots’ primary flight displays so they can easily see the issue without having to go through an extended checklist.

Stopping the shaking

The development team also devised a hardware addition to the 737 MAX flight deck: A physical switch that pilots can pull to turn off ‘stick shaker’.

• The stick shaker initiates a mechanical response that causes the control yoke to vibrate strongly and loudly to warn pilots the airplane is nearing a stall condition. In the two 737 MAX accidents, the flight control system activated the stick shaker because the erroneous AOA sensor readings led the system to believe an aerodynamic stall was imminent.

Canada’s aviation regulator said the stick shaker’s constant noise and vibration affect pilots’ ability to continue safely flying the airplane. As part of supporting the 737 MAX’s return to commercial service in 2020, the Canadians mandated that Boeing add a switch to disable the stick shaker in the event of a nuisance alert.

Engineers say the new switch serves as a manual backup for the flight crew.

“If something did get past the new monitors, they can use the switch to at least turn off persistent stick shaker to be able to more easily go through their checklist and determine the condition of the airplane,” said Bement.

Testing the solution

The development team put the enhanced AOA system through extensive lab testing, analysis and peer reviews, while Boeing test pilots spent months in a flight simulator evaluating the solution.

The Flight Test team then flew a large matrix of test conditions and scenarios to validate the system’s response, including inserting simulated failures into the system in the middle of a flight.  

“Whenever we inserted it, we would see that the monitor would capture that and give us the correct indications,” said test pilot Quashnock.

Moreover, engineers scoured several million 737 NG and 737 MAX flight records and found every case of AOA sensor failure.

“We analyzed these events through the lens of today's eAOA improvements,” said Morrow. “And we can confidently say that every single one of those events would have been caught by one of these new monitors.”

Sense of success

Engineers Morrow and Bement say the multi-year effort has been well worth it as the enhanced system has passed all certification tests and garnered positive pilot and customer feedback.

“That was the goal from the beginning,” said Morrow. “We wanted the design improvements to be intuitive to the pilots. Sensors can fail but what we can do is try to bound the failures and try to make things as clear and unambiguous as possible for the pilots.”

Boeing expects the 737-10 to be certified this year with deliveries beginning in 2027.

Following certification of the 737-10, the enhanced AOA system will become the standard for all 737 MAX models coming off the production line and a retrofit will be made available for in-service airplanes. 

By Bernard Choi

“We analyzed these events through the lens of today's eAOA improvements and we can confidently say that every single one of those events would have been caught by one of these new monitors.”