It's the report that could prove Boeing's biggest oversight — the known problems surrounding the angle-of-attack sensor.

In an exclusive report by CNN, the AoA device — which is believed to have played a role in the Lion Air and Ethiopian Airline crashes — was flagged in more than 200 incident reports submitted to the Federal Aviation Administration (FAA).

However, according to the report, despite the recorded incidents, Boeing did not flight test a scenario — such as an accidental nosedive — in which the sensor malfunctioned.

The sensors' job is to accurately determine how much the aircraft is pointing up or down. Since the horror Ethiopian Airlines crash in March, air safety experts have focused their attention on the tiny fin that extends from the airliner's nose.

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If the nose is angled too high, and the aircraft moves too slow, the amount of air rushing over and under the wings may not be enough to generate the lift necessary to keep it flying. It's called a stall.

Boeing is desperately trying to fix the software implicated in the crashes to end a global grounding of its signature aircraft. Photo / Stephen Brashear, Getty Images
Boeing is desperately trying to fix the software implicated in the crashes to end a global grounding of its signature aircraft. Photo / Stephen Brashear, Getty Images

The artificial intelligence that comprises modern airline autopilots and safety systems takes input from a multitude of sensors spaced around the aircraft. This tells them everything from air speed, engine throttle settings, air temperature and height.

If an aircraft detected an angle and speed at risk of producing a stall, this data should have triggered Boeing's Manoeuvring Characteristics Augmentation System (MCAS) artificial intelligence.

The MCAS system on a MAX-8 aircraft is constantly fed data from the two synchronised sensors, located on the plane's nose. In the event of pitch, the MCAS automatically moves the aircraft's nose down, without pilot input. Although pilots can cut off the system manually, its sudden activation can confuse pilots.

In a MAX 8, which was the aircraft involved in both Lion Air and Ethiopian Airlines incidents, the fully automated system of the AoA sensor bypasses the pilot. It takes over the controls and forces the aircraft's nose down to what it judges to be a "safe" angle.

Investigators say the flight characteristics of both the Ethiopian Air and Lion Air MAX8 aircraft before they crashed was consistent with pilots "fighting" the aircraft's computers.

Discussing the sensors and the multitude of complaints made about the device, former Boeing engineers and aviation analysts say the aviation company's original software design is vulnerable to defects.

A former unnamed Boeing pilot, who tested the 737 MAX, said Boeing didn't appreciate "the ramifications of a … failure of an AoA probe" in an interview with CNN.

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Another source claims the failure of an AoA sensor was not flight tested and instead was "analysed in the design and certification" of the aircraft. According to CNN, it was determined trained pilots would have been able to handle the failure if it were to happen.

"I would be very curious to know what their logic was on that … and what drove them to think that was a suitable solution," a second former Boeing test pilot said in an interview.

In support of the concern around the sensor system, CNN says the FAA's Service Difficulty Reporting website received at least 216 reports of AoA failures or the sensor having to be repaired, replaced or adjusted since 2004.

Of that figure, one-fifth involved Boeing planes where sensors "were frozen, improperly installed, struck by lightning or even hit by flying birds".

"In some cases, faulty sensors led to stall warnings, forcing pilots to abort takeoffs or perform emergency landings," CNN said.

In one of the incident reports received by the FAA, the flight crew on a Boeing 737-800 reported the "angle of attack and airspeed failed" and declared an emergency. The AoA sensor was replaced. The report suggests the FAA issued two directives for various Boeing aircraft models before the 737 MAX was released, indicating Boeing was aware of the potential for the sensors to cause problems in its planes.

Boeing chief executive officer Dennis Muilenburg speaks during a news conference after the company's annual shareholders' meeting at the Field Museum in Chicago. Photo / Jim Young-Pool, Getty Images
Boeing chief executive officer Dennis Muilenburg speaks during a news conference after the company's annual shareholders' meeting at the Field Museum in Chicago. Photo / Jim Young-Pool, Getty Images

One of these directives published in 2016 warned the sensors on Boeing MD-90-30 aircraft needed to be modified and tested to address "the unsafe condition on these products".

Earlier this week, the Wall Street Journal reported neither the FAA or Southwest Airlines, Boeing's largest 737 MAX customer, knew the safety alert (which measures the pitch of the plane's nose) was deactivated.

It is understood the MAX fleet was supposed to have a standard cockpit alert that would warn pilots when sensors outside the plane were feeding in incongruous data.

In response to FAA and Southwest Airlines, Boeing said it had a safety alert that only worked in aircraft with an optional feature.

"Boeing did not intentionally or otherwise deactivate the disagree alert on its MAX aeroplanes," the company said in a statement.

"The disagree alert was intended to be a standard, stand-alone feature on MAX aeroplanes. However, the disagree alert was not operable on all aeroplanes because the feature was not activated as intended."

The revelation is likely to add more regulatory scrutiny to Boeing as it tries to fix the software implicated in the crashes and gain the approval it needs from the FAA to end a global grounding of its signature passenger aircraft.

Report suggests the angle-of-attack sensor was flagged to FAA 216 times. Photo / Stephen Brashear, Getty Images
Report suggests the angle-of-attack sensor was flagged to FAA 216 times. Photo / Stephen Brashear, Getty Images

On Monday, Boeing CEO Dennis Muilenburg defended the company's safety record and declined to take any more than partial blame for two deadly crashes of its best-selling plane even while the company finishes an update that "will make the aeroplane even safer."

Mr Muilenburg said Boeing followed the same design and certification process it had always used to build safe planes, and he denied the MAX was rushed to market.

Mr Muilenburg pointed to actions by the pilots on the two flights who he said did not "completely" follow the standard procedure when uncommanded tail movements begin to push the jet's nose down.

Boeing has conceded in both accidents, the MCAS was triggered by faulty readings from a single sensor that pushed the planes' noses down.

Boeing 'withheld information' on 737 Max

Boeing failed to tell airline pilots about features of a new flight-control system suspected of playing a role in the fatal crash of an Indonesian 737 MAX, according to US pilots

How MCAS (The Manoeuvering Characteristics Augmentation System) works.
How MCAS (The Manoeuvering Characteristics Augmentation System) works.

HOW MCMAS WORKS

The Manoeuvring Characteristics Augmentation System

Level flight:

Normal angle of attack (AOA) at which airflow hits aircraft

Nose-up flight:
High AOA puts aircraft at risk of stalling. MCAS automatically triggered, moving horizontal stabiliser to trim anticlockwise - pushing jet's nose down

October 29 Lion air crash killed 189 people in Indonesia, investigators determined that AOA sensor was feeding faulty data to the onboard flight computer, repeatedly activating MCAS in spite of no risk of stall.