Vulnerable To Failure - Boeing 737 Sensors Linked To Over 140 Incidents

In their preliminary report, the team investigating the crash of ET302 determined that while there were actions that the pilots could have taken to avert the crash, the confusing alarms triggered by a faulty sensor ultimately proved too confusing, even as the pilots followed the steps laid out by Boeing in its safety manual.

As the focus of the investigation shifts to sensors that have been installed on all planes (not just those made by Boeing) - though the company has said it will include an additional sensor on 737 MAXs as part of its revamp of the plane's anti-stall software - Bloomberg has published  chilling report that raises serious questions about how these sensors managed to avoid scrutiny, given a history of malfunctions that stretches back to the 1990s.

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Some 140 incidents involving faulty or damaged sensors have been reported in that time, according to a review of publicly available data by Bloomberg. Of those, 25 cases involved the same type of faulty alert that led to the crash of ET302, and all 157 people on board.

Though the investigation into the Lion Air crash is ongoing, it's believed a faulty sensor was also to blame in that crash.

Pilots have for decades relied on the weather-vane-like “angle of attack” sensors to warn them when they near a dangerous aerodynamic stall. But investigators are probing Boeing’s decision to enable the sensors on the Max model to go beyond warning pilots and automatically force the plane’s nose down.

A review of public databases by Bloomberg News reveals the potential hazards of relying on the devices, which are mounted on the fuselage near the plane’s nose and are vulnerable to damage. There are at least 140 instances since the early 1990s of sensors on U.S. planes being damaged by jetways and other equipment on the ground or hitting birds in flight.

In a chilling revelation that raises questions about why these sensors weren't more heavily scrutinized, an Airbus SE crashed in 2008 while on a demonstration flight. All seven people onboard died as the plane plunged into the ocean. Moisture inside the sensors was eventually blamed for the accident, but strangely, nothing was done.

Before the 737 Max crashes, the most recent accident involving the sensors occurred when an Airbus SE A320 on a 2008 demonstration flight went down off the coast of France killing all seven people aboard. Moisture inside two of the plane’s three angle-of-attack vanes froze, confusing the aircraft’s automation system, according to France’s Office of Investigations and Analysis. The report also faulted the pilots for multiple errors.

Examining such previous episodes is all the more important because of Boeing’s decision to use a single sensor as the trigger for the anti-stall mechanism on the Max known as MCAS, or Maneuvering Characteristics Augmentation System.

"With that many pilot reports and with the unknowns that we’re dealing with in these two accidents, that’s an important area to be investigated," said James Hall, the former chairman of the U.S. National Transportation Safety Board.

In an amusing twist, many of these malfunctions were caused by incidents where birds collided with the sensors, damaging them (and killing the birds).

On April 1, 2012, a United Airlines 767-300 was taking off from San Francisco when it struck a flock of western sandpipers, according to a Federal Aviation Administrationdatabase. The birds damaged the left sensor, scrambling the speed readings and auto throttle. The plane returned to the airport.

A Republic Airlines Inc. flight was struck by a tundra swan as it neared arrival into Newark, New Jersey, on Dec. 5, 2016, according to the FAA. It damaged the angle-of-attack sensor and other equipment on the Embraer SA EMB-170, rendering unreliable airspeed and altitude readings on the regional jet. It landed safely.

Though these incidents represent a minuscule fraction of the hundreds of millions of successful flights that have happened in that time, the fact that they sensors weren't investigated before now comes as a shock, according to some experts.

Examining such previous episodes is all the more important because of Boeing’s decision to use a single sensor as the trigger for the anti-stall mechanism on the Max known as MCAS, or Maneuvering Characteristics Augmentation System.

"With that many pilot reports and with the unknowns that we’re dealing with in these two accidents, that’s an important area to be investigated," said James Hall, the former chairman of the U.S. National Transportation Safety Board.

The FAA was aware of previous angle-of-attack failures and considers such incidents when it evaluates aircraft designs for certification, the agency said in a statement.

"As part of the FAA’s oversight of the continuous operational safety of our nation’s aviation safety system, the agency continues to monitor, gather and evaluate all available information and data regarding the performance of aircraft and related components," the agency said.

Even more chilling: BBG's investigation turned up one flight with circumstances eerily similar to those that led to the deaths of everyone aboard both the Lion Air and Ethiopian Airlines flights. However, since the plane involved was an older model 737, it wasn't equipped with MCAS, and thus the nose of the plane wasn't automatically forced lower.

One March 2016 incident closely resembled the recent crashes, except that the plane, an earlier 737-800 model, wasn’t equipped with MCAS and the pilots maintained control.

As soon as the plane got airborne, the captain, seated on the left side, got the loud thumping noise and vibrating control column warning that the plane was about to stall, according to the NASA report. The captain’s airspeed and altitude displays disagreed with the copilot’s, indicating an error and setting off additional alerts. All of those symptoms occurred on the two recent Max crashes.

The pilots opted to continue onto their destination in spite of the multiple failures. Both the captain and the copilot said that they regretted continuing the flight and didn’t realize that they had violated their airline’s procedures by disabling the stall warning.

"A return, while considered, should have been accomplished," said the captain.

Despite this report, Boeing shares rebounded on Thursday as worries about more order cancellations failed. But the report raises an interesting question about the Airbus crash: Since the deaths of seven people likely didn't make a splash in the headlines, did that make it easier for regulators at the time to ignore?