In the months before an Ethiopian Airlines crash killed 157 people Sunday, the second recent deadly crash of a Boeing 737 MAX 8 jetliner, American pilots complained to authorities about perceived safety problems with the same aircraft.

Two pilots reported their aircraft unexpectedly pitched nose down after they engaged autopilot following departure. Another pilot reported a “temporary level off” triggered by the aircraft automation. The captain of a flight in November 2018 called part of the aircraft’s flight manual “inadequate and almost criminally insufficient.”

“The fact that this airplane requires such jury rigging to fly is a red flag,” that captain – who was not identified by name – wrote in a report to the federal Aviation Safety Reporting System. The captain said part of the plane’s flight system was “not described in our Flight Manual.”

Records show that federal aviation authorities received at least 11 reports concerning the Boeing 737 MAX 8 from professional aviators logged from April 2018 to December 2018.

Sunday's crash in Ethiopia followed the crash Oct. 29, 2018, of Lion Air Flight 610, in which 189 passengers and crew died when it plunged into the Java Sea outside Indonesia. Both flights crashed after experiencing drastic speed fluctuations during ascent, and their pilots tried to return to the ground after takeoff.

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Regulators and industry experts suspect that MAX 8’s Maneuvering Characteristics Augmentation System, or MCAS, may have caused the jets to make unwanted dives.

Flight data recovered from the Indonesia crash indicated pilots repeatedly tried to get the plane’s nose up before impact. After the crash, Boeing issued a service bulletin warning pilots that erroneous flight data fed into the MCAS could force the aircraft into a dive for up to 10 seconds.

After the crash in Ethiopia, the company said it had no new guidance.

The pilot complaints, first reported Tuesday by The Dallas Morning News, emerged as aviation regulators around the world were hustling to respond to the two crashes in five months.

In one documented complaint, a pilot said the plane's downturn triggered the ground proximity warning system, which is designed to alert pilots when their planes are in immediate danger. The complaint states an alarm sounded “don’t sink, don’t sink” before the captain disconnected the autopilot and manually adjusted the plane to climb.

This week, the European Union, United Kingdom, China, Australia, Ethiopia, Indonesia and Malaysia grounded the MAX 8 over safety concerns.

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Federal Aviation Administration spokeswoman Lynn Lunsford declined to comment on the specific pilot complaint reports, which are logged by the Aviation Safety Reporting System. Lunsford noted the reports do not discuss MCAS, the feature suspected to have played a role in both crashes.

Kristy Kiernan, an assistant professor at Embry-Riddle Aeronautical University’s college of aeronautics in Daytona Beach, Florida, cautioned against drawing conclusions from the MAX 8 aircraft complaint reports, which she reviewed for USA TODAY.

“There’s nothing that really struck me as a pattern,” Kiernan said. “I just don’t think there’s anything you can draw from it at all.”

Although the reports involve the MAX 8, the issues are different from those flagged about the MCAS system in the Lion Air crash, Kiernan said. For example, two pilots raised concerns about issues after engaging the plane’s autopilot, but Kiernan noted that the MCAS system is disengaged when the autopilot is turned on. Another of the reports focused on a different system, the aircraft’s auto throttles.

The anonymous reports are submitted by pilots on a voluntary basis to capture safety concerns and are used by regulators to support policies aimed at decreasing the likelihood of accidents.

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As aviation regulators around the world this week suspended the MAX 8, the FAA stated that its "investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions."

Boeing has sold nearly 400 of the airliners, including 74 to domestic carriers, and has taken orders for thousands more. A company spokesperson stressed “full confidence in the safety" of the planes Tuesday.

“The Federal Aviation Administration is not mandating any further action at this time, and based on the information currently available, we do not have any basis to issue new guidance to operators,” Boeing said in a statement.

Contributing: John Bacon, Bart Jansen and Chris Woodyard