Why Investigators Fear the Two Boeing 737s Crashed for Similar Reasons

By James Glanz, K.K. Rebecca Lai and Jin Wu

American and Canadian regulators cited newly available satellite-tracking data in grounding all Boeing 737 Max jets on Wednesday, saying there were similarities between the deadly crash involving the plane in Ethiopia on Sunday and a Indonesian flight in October.

The causes of both crashes are still under investigation. But the data cited by regulators points to preliminary indications that the two planes could have been brought down by the same cause, a malfunctioning automated system intended to keep the jet from stalling.

Ethiopian flight path

Chart below is from

this portion of the flight

8:41 a.m.

Last signal

received

Ethiopian flight path

Chart below is from

this portion of the flight

8:41 a.m.

Last signal

received

Ethiopian flight path

Chart below is from

this portion of the flight

By Scott Reinhard | Souce: Path data from Flightradar24

Marc Garneau, Canada’s transport minister, said on Wednesday that the satellite-tracking data revealed “vertical variations” in the Ethiopian Airlines flight reminiscent of those seen before the crash of the Lion Air Boeing 737 Max in Indonesia in October.

The investigation of the Ethiopian crash is still in its early stages, and the full data cited by the authorities has not been publicly released. Many experts cautioned that any conclusions are far from certain and don’t rule out other possibilities, such as pilot error or another malfunctioning system.

But aviation experts who reviewed the patterns in publicly available data said it suggests that the newly installed automated system known as M.C.A.S. may have been activated on the Ethiopian flight and could have contributed to the crash.

Many experts believe that during the first crash, the Indonesian pilots lost their battle with M.C.A.S.

Possibly because of faulty sensor readings, the automated system pushed the nose of the Lion Air plane down. The pilots repeatedly counteracted it and pulled the nose back up, only to be overridden by the system again. Each interval took about 15 to 20 seconds, leaving a repetitive signature on the data for the plane’s vertical speed.

Vertical speed of Lion Air flight (three-minute portion)

Approximate periods of increasing and decreasing vertical speed

Vertical speed of Lion Air flight

(three-minute portion)

Approximate periods of increasing and

decreasing vertical speed

That led to the series of variations of altitude until the pilots crashed.

The public data for the Ethiopian flight is less clear and complete, but it appears to show a similar signature — an interval where the plane was gaining altitude and then leveling out.

Vertical speed of Ethiopian Airlines flight

Approximate periods of increasing and decreasing vertical speed

Vertical speed of Ethiopian Airlines flight

Approximate periods of increasing and

decreasing vertical speed

The Canadian transport minister said that based on satellite data he had reviewed, M.C.A.S. may have played a role.

The oscillation of roughly 15 to 20 seconds is a telltale sign that suggests the M.C.A.S. system may have been involved, said R. John Hansman Jr., a professor of aeronautics and astronautics at the Massachusetts Institute of Technology.

Planes like the Boeing 737 oscillate naturally, he said, because of turbulence and and other effects. But those swings have different time spans: either between five and eight seconds, or a minute or longer. The variations in the intermediate range of 15 or so seconds have no other obvious explanation, he said.

“Even from the available data, there are similarities between the Lion Air case and this case in terms of this 15 second periodicity. That would point toward a similar phenomenon. We’ll know more when we get the flight data recorder,” Professor Hansman said.

Don Thoma, the chief executive of Aireon, the company that provided the new data, said that it “certainly showed something was wrong with the aircraft, and something they should take a hard look at.”