At least four pilots made reports following the October crash of a Lion Air flight in Indonesia shortly after takeoff, all complaining that the aircraft suddenly pitched downward, according to documents reviewed by AFP on a flight safety database.
The incidents seem to involve the flight stabilization system designed to prevent the aircraft from stalling, the "MCAS," which was implicated in the fatal accident in the Lion Air crash that killed 189 people shortly after takeoff.
After the latest accident Sunday of another 737 MAX 8 from Ethiopian Airlines, shortly after takeoff, killing 157, numerous airlines and governments around the world grounded the aircraft or banned it from their skies, including Canada which just took the step on Wednesday.
However, the Federal Aviation Administration on Tuesday said there was no reason as yet to ground the planes, even though it has mandated Boeing update its flight software and training on the aircraft.
The cause of the tragedy in Ethiopia has not been determined, although the black boxes with critical data and recordings of the pilot were retrieved Monday.
'DONT SINK DONT SINK'
One pilot logged an incident in November 2018, just weeks after the Lion Air crash, saying the plane "pitched nose down" two to three seconds after engaging the autopilot following takeoff, according to the report on the Aviation Safety Reporting System, maintained by NASA.
"The captain immediately disconnected the autopilot and pitched into a climb," the report said. "The rest of the flight was uneventful."
The report said the flight crew reviewed the incident "at length... but can't think of any reason the aircraft would pitch nose-down so aggressively."
Another pilot on a flight in November said the crew discussed the concerns about the aircraft and "I mentioned I would engage the autopilot sooner than usual."
But again once engaged, there was a quick automated warning of "DONT SINK DONT SINK!"
"I immediately disconnected the AP (autopilot) ... and resumed climb," the officer said. But after review, "frankly neither of us could find an inappropriate setup error."
"With the concerns with the MAX 8 nose-down stuff, we both thought it appropriate to bring it to your attention."
The Lion Air accident had focused attention on Angle of Attack (AOA) sensors connected to the Aircraft Stabilization System (MCAS).
A malfunction of these tools may erroneously correct the path by pitching the aircraft down due to a mistaken assessment that the aircraft is in stall.
The Ethiopian Airlines disaster took place shortly after takeoff and the aircraft experienced irregular climbs and descents just after taking off.
"We're going to decline to comment on specific ASRS reports," an FAA spokeswoman told AFP. "We are not aware of any verified reports of MCAS issues in the US."
The ASRS is a voluntary system of reports that allows research to "lessen the likelihood of aviation accidents."