Pilots repeatedly voiced safety concerns about the Boeing 737 Max 8 to federal authorities, with one captain calling the flight manual "inadequate and almost criminally insufficient" several months before Sunday's Ethiopian Air crash that killed 149 people, an investigation by The Dallas Morning News found.
The News found at least five complaints about the Boeing model in a federal database where pilots can voluntarily report concerning aviation incidents without fear of repercussions.
The complaints are about the safety mechanism cited in preliminary reports for an October plane crash in Indonesia that killed 189.
The disclosures found by The News reference problems with an autopilot system during takeoff and nose-down situations while trying to gain altitude during flights of Boeing 737 Max 8s. While records show these flights occurred during October and November, information regarding which airlines the pilots were flying for at the time is redacted from the database.
Records show a captain who flies the Max 8 complained in November that it was "unconscionable" that the company and federal authorities allowed pilots to fly the planes without adequate training or fully disclosing information about how its systems differed.
The captain's complaint was logged after the FAA released an emergency airworthiness directive regarding the Boeing 737 Max 8 in response to the crash of Lion Air Flight 610 in Indonesia.
An FAA spokesman said the reports found by The News were filed directly to NASA, which serves as a neutral third party for reporting purposes.
"The FAA analyzes these reports along with other safety data gathered through programs the FAA administers directly, including the Aviation Safety Action Program, which includes all of the major airlines including Southwest and American," said Lynn Lunsford, southwest regional spokesman for the FAA.
A federal audit in 2014 said that the FAA does not collect and analyze its voluntary disclosure reporting in a way to effectively identify national safety risks.
U.S. regulators are mandating that Boeing upgrade the plane's software by April, but have so far declined to ground the planes. China, Australia and the European Union have grounded the 737 Max 8, leaving the U.S. and Canada as the only two countries flying a substantial number of the aircraft.
Sen. Ted Cruz (R-Texas), who leads a Senate subcommittee overseeing aviation, said in a statement Tuesday that U.S. authorities should ground the planes.
"Further investigation may reveal that mechanical issues were not the cause, but until that time, our first priority must be the safety of the flying public," Cruz said.
At least 18 carriers -- including American Airlines and Southwest Airlines, the two largest U.S. carriers flying the 737 Max 8 -- have also declined to ground planes, saying they are confident in the safety and "airworthiness" of their fleets. American and Southwest have 24 and 34 of the aircraft in their fleets, respectively.
"The United States should be leading the world in aviation safety," said John Samuelsen, the president of a union representing transport workers that called Tuesday for the planes to be grounded. "And yet, because of the lust for profit in the American aviation, we're still flying planes that dozens of other countries and airlines have now said need to grounded."
The fifth complaint from the captain who called into question the 737 Max 8's flight manual ended: "The fact that this airplane requires such jury-rigging to fly is a red flag. Now we know the systems employed are error-prone -- even if the pilots aren't sure what those systems are, what redundancies are in place and failure modes. I am left to wonder: what else don't I know?"
The Maneuvering Characteristics Augmentation System (MCAS) was included on the Max 8 model aircraft as a safety mechanism that would automatically correct a plane entering a stall pattern. If the plane loses lift under its wings during takeoff, and the nose begins to point far upward, the system kicks in and automatically pushes the nose of the plane down.
After the Lion Air crash, the FAA's issued an airworthiness directive, stating: "This condition, if not addressed, could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain."
Officials have not yet determined what caused Ethiopian Airlines 302 to nosedive into the ground on Sunday, but many experts have noted similarities between this week's crash and the one in Indonesia.
A spokesperson for Southwest Airlines told The News that it hasn't received any reports of issues with MCAS from its pilots, "nor do any of our thousands of data points from the aircraft indicate any issues with MCAS."
American Airlines did not respond to questions from The News.
The FAA issued a statement to The News on Tuesday afternoon that said that it is "collecting data and keeping in contact with international civil aviation authorities as information becomes available."
"The FAA continuously assesses and oversees the safety performance of U.S. commercial aircraft. If we identify an issue that affects safety, the FAA will take immediate and appropriate action."
Jon Weaks, president of the Southwest Airlines Pilots Association, said in a press release Monday night: "We fully support Southwest Airlines' decision to continue flying the MAX and the FAA's findings to date."
Boeing, which posted a record $101 billion in revenue last year, issued a new statement Tuesday saying that no grounding of planes was necessary. "Based on the information currently available, we do not have any basis to issue new guidance to operators," the company said.
Samuelsen of the transport workers union said it's "unconscionable" that the FAA has not yet grounded the planes in the U.S., given the number of deaths that have occurred.
"This pressure should not be on these pilots to overcome an engineering flaw that Boeing themselves acknowledges," said Samuelsen.