In Ethiopia Crash, Faulty Sensor on Boeing 737 Max Is Suspected

New York Times –

Black box data from a doomed Ethiopian Airlines flight suggests the crash was caused by a faulty sensor that erroneously activated an automated system on the Boeing 737 Max, a series of events suspected in an Indonesian disaster involving the same jet last year.

Data
from a vane-like device, called the angle-of-attack sensor, incorrectly
activated the computer-controlled system, according to several people
who have been briefed on the contents of the black box in Ethiopia. The
system, known as MCAS, is believed to have pushed the front of the plane
down, leading to an irrecoverable nose-dive that killed all 157 people
aboard.

The black box, also called
the flight data recorder, contains information on dozens of systems
aboard the plane. The black boxes on the jets, Boeing’s latest
generation of the 737, survived the crashes, allowing investigators to
begin piecing together what caused the disasters. Both investigations
are continuing, and no final determinations have been made.

The
new connections between the two crashes point to a potential systemic
problem with the aircraft, adding to the pressure on Boeing. The company
already faces scrutiny for its role in the design and certification of
the plane. The Federal Aviation Administration delegated significant
responsibility and oversight to Boeing.

The
company is now on the defensive, as investigators, lawmakers and
prosecutors try to determine what went wrong. The Justice Department is
investigating the jet’s development, while the Transportation
Department’s inspector general is looking into the certification
process. The inspector general has issued a subpoena to at least one
former Boeing engineer for documents related to the 737 Max, according
to a person familiar with the investigation.

MCAS was originally designed to activate based on data from a single angle-of-attack sensor, which measures the level of the jet’s nose relative to oncoming air. Air-safety experts, as well as former employees at Boeing and the supplier that made the sensor, have expressed concern that the system had this single point of failure, a rarity in aviation.

“That’s not a good engineering system,” said Bjorn Fehrm, an aeronautical engineer and a former fighter pilot for the Swedish Air Force. “That’s where they screwed up royally.”

In
a tacit acknowledgment that the initial design was flawed, Boeing this
week unveiled a software update that specifically addresses the concerns
about MCAS and the sensors. American authorities will have to approve
the fix before the planes start flying again.

Regulators
around the world grounded the Max this month, and airlines are not
expected to use them soon. On Friday, Southwest Airlines said it planned
its flight schedule through May without its 34 Max jets.

Boeing
said it could not comment on the black box findings until investigators
released their official report, per international aviation agreements.
Rosemount Aerospace, a subsidiary of the industrial giant United
Technologies based in Burnsville, Minn., made the sensor. A United
Technologies spokeswoman did not immediately respond to requests for
comment.

Angle-of-attack sensors are
highly reliable and have been used on passenger jets for years, but like
any aircraft component, they can fail. Given that, former Boeing and
Rosemount engineers said it was surprising that Boeing would allow a
single sensor to activate a crucial system that pushes the aircraft
toward the ground.

The sensors, which
are effectively wind vanes on the jet’s nose, have malfunctioned in the
past, for a variety of reasons, including bird strikes, according to
the former engineers. They have also been broken by jetways that attach
to the plane for passengers to board and exit the plane.

The
sensors can also malfunction if water pools around them and then
freezes when the plane reaches a certain altitude, the engineers said.
The sensors have built-in heaters to prevent freezing at such high
altitudes, but they sometimes do not work quickly enough or can fail
outright.

“Generally speaking,
aviation components are highly reliable,” said Mel McIntyre, a retired
Boeing engineer who worked with such sensors for years. “But everything
can fail. Nothing is invincible.” Mr. McIntyre declined to comment
further on any specifics of the 737 Max.

Investigators of the Indonesia crash in October, who have produced a preliminary report and released some of the information from the black box, found one sensor produced a reading that was at least 20 degrees different from the other as the plane began its ascent. Based on the bad data, MCAS was activated, erroneously pushing the nose of the plane down.

The pilots on the Indonesian flight tried repeatedly to override the system. But after about 12 minutes, they lost their battle.

None
of the people briefed on the contents of the Ethiopian Airlines black
box said whether its data indicated that the pilots tried to counteract
the system. But the plane in Ethiopia had the same bouncing, bobbing
trajectory seen in the Indonesian flight as the pilots tried to save the
plane, according to publicly available flight data.

Air
traffic controllers in Ethiopia also said they saw the oscillating
trajectory before the plane crashed on March 10. The pilot radioed to
them that he was having trouble controlling the aircraft, but did not
give details on what systems were causing problems.

Boeing
has defended the 737 Max, its best-selling jet, which is expected to
bring in hundreds of billions of dollars in the coming years. And most
airlines, which have ordered thousands of planes, have stood by the
manufacturer.

“The 737 is a safe
airplane,” Mike Sinnett, Boeing’s vice president for product strategy,
said at a briefing on Wednesday. “And the 737 Max builds on that
tremendous history of safety that we’ve seen through the last almost 50
years.”

But Boeing’s coming software fix specifically addresses the suspected problems that may have led to the two deadly crashes.

The
F.A.A. classifies potential problems with aircraft at different levels
of risk. The agency did not classify a failure of MCAS as the highest
risk level, meaning Boeing was allowed to design the system to rely on
data from a single sensor, according a company official with knowledge
of the matter. After the crashes, the company wanted to make the system
more robust and rely less on pilots to intervene if it failed, the
official said.

The software fix will
make the system rely on two sensors, rather than just one. The update
will also limit MCAS, in most cases, from engaging more than once, a
concern in the Lion Air crash in Indonesia. And it would prevent the
system from pushing the plane’s nose down more than a pilot could
counteract by pulling up on the controls.

“The
rigor and thoroughness of the design and testing that went into Max
gives us complete confidence that the changes that we’re making will
address any of these accidents,” Mr. Sinnett, the Boeing vice president,
said on Wednesday.

David Gelles contributed reporting. Kitty Bennett contributed research. A version of this article appears in print on March 30, 2019, on Page A1 of the New York edition with the headline: Black Box Hints At Similar Cause Of Air Disasters. Order Reprints | Today’s Paper | Subscribe

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