Boeing left a safety feature off the software on its 737 Max planes that was present in an earlier version of the system, The Wall Street Journal reported.

The anti-stall software system, called MCAS, misfired in the two fatal crashes that killed 346 people, pointing the noses of both planes down into dives from which they never recovered.

The system relied on a single external sensor — a design widely criticised by experts.

But a military jet designed before the Max used MCAS with multiple sensors, sources told the Journal.

Boeing said in response that the two systems "are not directly comparable."

However, part of the company's proposed fix for the 737 Max includes data from a second sensor.

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Boeing left a safety feature off the software system on its 737 Max planes that was linked its two fatal crashes, despite including it on an earlier version of that system used elsewhere, a new report claims.

The version of the automated anti-stall system, called MCAS, on the 737 Max relied on data from only one of the plane's two angle-of-attack sensors, which measure the plane's angle in the sky.

But Boeing engineers first created the MCAS system more than a decade ago for a military jet used for re-fuelling, The Wall Street Journal reported, citing people familiar with the matter.

In this plane, the system worked with multiple sensors — giving the pilots more control over the plane, according to the report.

Undelivered Boeing 737 Max planes sit idle at a Boeing property in Seattle, Washington, in August 2019. The planes cannot be flown by their airlines as the plane has been grounded around the world since March. David Ryder/Getty Images

Boeing confirmed in April that an erroneous sensor readings triggered the plane's MCAS software in the two fatal crashes: a Lion Air flight in Indonesia in October 2018 and an Ethiopian Airlines flight in March 2019 in Ethiopia. The crashes killed a total of 346 people.

Read more: The complete history of the 737 Max, Boeing's promising yet problematic workhorse jet

Preliminary reports from investigations into the two crashes suggested that there were problems with the sensor readings.

In both flights, the planes nosedived and pilots were unable to regain control.

Objects recovered from the sea at Tanjung Priok Port, Indonesia, after the Lion Air Boeing 737 Max crash in October 2018. Eddy Purwanto/NurPhoto via Getty Images

The MCAS system is designed to prevent the aircraft from stalling — which can occur when a plane is angled too sharply up — by automatically pointing the nose down.

Experts have criticised Boeing's decision to have the plane rely on just one sensor input.

Peter Lemme, a former Boeing flight-controls engineer, told CNN in May that the plane should have had "a fail-safe design" that "relied on two inputs to make sure that you weren't sensitive to one failure."

Read more: As the FAA head defended the agency's handling of the Boeing 737 Max, he suggested that the agency might change its process for certifying new planes

Another flaw is that the sensors are fixed to the outside of the plane, where they can be easily damaged in flight.

US Rep. Peter DeFazio, the chair of the House Transportation and Infrastructure Committee, also said that his committee's investigation into the 737 Max would look at how its software relied on a single sensor.

The cockpit of a Boeing 737 Max plane. Ted S. Warren/Associated Press

Boeing has since defended its design. In response to the CNN report, a spokesperson said that "single sources of data are considered acceptable in such cases by our industry."

But Boeing in its actions appears to see a benefit in a second sensor. The company is updating the 737 Max design, and said that its update will incorporate data from two sensors instead of just one.

The plane will not fly again until regulators in the US and around the world approve Boeing's fixes — a process that most in the industry do not expect to be completed until the end of 2019 at the earliest.

Sources for The Wall Street Journal said this fix will make the MCAS system in the Max plane more similar to the one in the military jet.

Read more: 400 pilots have joined a growing number of airlines in demanding payback from Boeing for its 737 Max disasters — here's the full list

A Boeing spokesman declined to explain to The Wall Street Journal why the systems are different on the two planes. The spokesman said said: "The systems are not directly comparable."

Kenyans mourn family and friends, who were victims of the Boeing 737 Max Ethiopian Airlines plane crash, at the crash scene in March. REUTERS/Baz Ratner

An Air Force official and other unnamed sources told the Journal that the military refuelling jet with the MCAS system also has another safeguard, which lets pilots override the system by pulling on the controls.

Will Roper, an assistant Air Force secretary who serves as the branch's procurement chief, said: "We have better sensor data."

"But most importantly, when the pilot grabs the stick, the pilot is completely in control."

Max pilots have criticized Boeing for what they described as a plane design that lessens their control over the plane, in ways they say they were told of in advance.

Read more: The Boeing 737 Max could be grounded through the holidays thanks to a series of new delays, and airlines are cancelling thousands of flights because of it

American Airlines pilots confronted Boeing executives about the MCAS system in November 2018, after the first crash but before the second.

One pilot said: "These guys didn't even know the damn system was on the airplane, nor did anybody else."

Boeing is currently facing lawsuits from the families of those killed on the planes, as well as federal investigations and demands for compensation from airlines around the world who have had their planes grounded and unable to fly since March.

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