Boeing 737 Crashes Caused By A Few Lines Of Missing Code

Authored by Mike Shedlock via MishTalk,

The FAA certified the Boeing 737 Max on Wednesday. Here's the inside scoop on exactly what caused the crashes.

FAA Certifies the Boeing 737 Max

The Boeing 737 Max has been grounded since March of 2019 after two aircraft crashes killing everyone on board.

The first crash killed 189 people, the second crash killed 157.

On Wednesday, November 18, 2020, the FAA Cleared Boeing's 737 Max To Resume Passenger Service

After 20 months on the tarmac following two fatal crashes, Boeing's troubled 737 Max airliner has been given the green light to resume passenger flights, the Federal Aviation Administration announced Wednesday.

The plane's return to the skies will not be immediate, however. The FAA is requiring a series of design changes laid out in a 115-page directive. It also put forward training requirements for pilots and maintenance requirements for airlines.

"This airplane has undergone an unprecedented level of scrutiny by the FAA," Dickson said. "We have not left anything to chance here."

After the FAA announcement, the Air Line Pilots Association released a statement saying it "believes that the engineering fixes to the flight-critical aircraft systems are sound and will be an effective component that leads to the safe return to service of the 737 MAX."

Culture of Concealment

Boeing was aware of issues but did not disclose them to pilots or the FAA.  

Investigators found a "culture of concealment" as well as "grossly insufficient oversight by the FAA."

A Few Lines of Code

Leeham News author Bjorn Fehrm has interesting details in his take  FAA recertifies Boeing 737 MAX

Fehrm says "The 737 is a Safe Aircraft" and this "chain of events will not happen again on an updated 737 MAX".

Much of the discussion in from a pilot's perspective that is hard to follow but the key details are easy to understand even if you do not understand the terminology.

  1. The MCAS (Maneuver Characteristics Augmentation System) software was inaccurately classified as non “hazardous.”

  2. The inaccurate classification allowed a single sensor to control the MCAS.

  3. The MCAS was inaccurately coded.

  4. The original MCAS listened to the Speed Trim reset, “the Pilot trims,” instead of the correct “AoA is below the threshold again.” The result was MCAS trims, the Pilot trims, MCAS trims, the Pilot trims…. After 24 rounds in the Lion Air jet, the Pilots lost the race with MCAS.

  5. "MCAS went from a Pilot assist to a highly hazardous function by this single mistake in the MCAS software code. The whole drama came from the omission of a few code lines in the MAX Flight Control Computers software."

Boeing Changes

I describe the above in sufficient detail so we can understand how little in MCAS needed change to take it from a hazardous function to one that would have caused no trouble if wrongly triggered.

In addition to this change, Boeing has made additional changes to increase safety further. 

A single sensor no longer triggers MCAS. Both AoA sensors on the 737 MAX have to agree on the aircraft AoA, or Speed Trim including MCAS is deactivated (neither is needed to fly the plane. They are augmentation functions, i.e., good to have but not necessary).

On top of the dual-sensor activation of MCAS, its global authority, no matter what, is limited. The Pilot always has enough pitch control to fly the aircraft.

MCAS is Now Safe

To make MCAS safe, we only needed the correct reset criteria. But as the investigations dug deeper into how Boeing and FAA could miss how dangerous the original MCAS was, the requirements for changes grew. All eventualities, even remote ones, should be covered.

About Bjorn Fehrm

My Boeing contact who sent me the Leeham article notes Bjorn Fehrm is a former fighter test pilot and an aero engineer based in France.

Bjorn has said he would pilot the MAX as well as fly in it as a passenger.  

Most Expensive Lines of Code in History

  • Boeing is out $20 billion, not counting pending lawsuits.

  • 346 people are dead. 

  • 450 aircraft are grounded worth about $45 billion.

  • Mistrust of Boeing and the 737 Max will last for years.

Had a few lines of code been properly placed, there would not have been two crashes or 20 months of grounding even though other safety features were needed. 

In retrospect, it is not really the lines of code that were the problem. 

It was the "culture of concealment" coupled with "grossly insufficient oversight by the FAA."