The Boeing 737 MAX-8 that dropped from the sky in October, killing 189 people, could have crashed a day earlier — had it not been for an off-duty pilot who happened to be in the cockpit, according to a report Tuesday.

An extra pilot, hitching a ride in the Lion Air plane Oct. 28, 2018, showed his colleagues how to disable a malfunctioning automated flight-control system, saving the plane, Bloomberg reported.

The next day, a different crew aboard the doomed aircraft faced what investigators said was the same malfunction and plunged into the Java Sea.

The detail could explain how some pilots of the now-controversial 737 MAX planes were able to avoid disaster and others lost control and crashed.

Authorities are scrutinizing the automated safety system, dubbed MCAS, in the recent Ethiopian Airlines crash, in which 157 people died.

Boeing’s 737 MAX was grounded by US regulators and across the world in the wake of the accident.