Ethiopian Air crash, Lion Air crash and the MCAS system: a simple explanation

Boeing 737 MAX is the latest generation of the 52-year-old Boeing 737 that has had impressive safety records, until these last five months.

Boeing’s better design

In an effort to be competitive, Boeing hoped to make its new planes more cost effective to its buyers. One way to do this was to keep a similar design to prior models. This would preclude the need for lengthy training on a new aircraft; pilots could begin flying the new models almost immediately.

Another way to be competitive was to create larger engines.

These larger engines had to be mounted higher and farther forward on the wings to provide adequate ground clearance.

This new placement–under some conditions, like low air speed at takeoff–could cause the nose to lift and the plane to stall. The MCAS was created to counteract that.


The MCAS (Maneuvering Characteristics Augmentation System) lowers the nose automatically to prevent a stall if it detects that the angle of the plane’s nose is too high relative to the ground.

Lion Air crash

The preliminary investigation of the Lion Air flight shows that prior to the crash, the MCAS had engaged, probably without the pilots’ knowledge.

why the MCAS engaged

Investigators preliminarily believe that faulty sensors on the plane’s fuselage indicated to the MCAS that the nose of the plane was too high relative to the ground.

The MCAS lowered the nose.

In response, Lion Air pilots manually raised the nose.

Sensors again triggered the MCAS to lower the nose. Pilots again manually raised the nose.

This pattern continued until the fateful plunge into the sea. This is a similar flight pattern that which was seen in the Ethiopian air flight.

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