The US Federal Aviation Administration says it doesn't have sufficient data to order the grounding of Boeing's 737 MAX 8 aircraft that has been involved in two crashes and which has been grounded by 41 other countries this week, but US media reports say pilots of at least two of the Boeing planes reported problems last year.

The Associated Press said on Tuesday that reports by two pilots said that soon after engaging the autopilot on Boeing 737 Max 8 planes, the nose tilted down sharply. In both cases, they recovered quickly after disconnecting the autopilot. The tilting problems are similar to the suspected causes of the crash on Sunday in Ethiopia of a MAX 8 that killed 157 aboard and the crash last October in Indonesia that killed 189.

The New York Times reported on Wednesday that a third pilot complained that Boeing hadn't explained changes to the plane's automation to pilots. "I am left to wonder: what else don't I know?" the pilot wrote. "The Flight Manual is inadequate and almost criminally insufficient.'

Other US media, including Politico and the Dallas Morning News, reported that pilots of the MAX 8 anonymously filed complaints about MAX 8 incidents.

The Dallas Morning News reported that pilots cited issues with the aircraft's autopilot system and described the plane abruptly nosing down. The flights referenced took place in October and November, the newspaper reported.

Politico reported that at least one complaint cited the anti-stall system. "The fact that this airplane requires such jury rigging to fly is a red flag," one pilot wrote.t the Dallas Morning News, also

The FAA and two US airlines -- American Airlines and Southwest Airlines -- continue to permit the planes to fly. Both carriers vouched for the safety of the aircraft on Wednesday.

The FAA has said, "This investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions." The agency said it will take appropriate action if it determines otherwise.

Boeing, the world's largest airplane manufacturer, on Tuesday expressed confidence in its aircraft.

Citing safety concerns, Egypt, Kazakhstan and Vietnam closed their airspace to Boeing 737 Max 8 airplanes on Wednesday. The European Union, China and India had previously banned all 737 Max 8 flights.

The pilots' reports were filed last year in a data base compiled by NASA, the National Aeronautics and Space Administration, according to the AP. They are voluntary safety reports and don't publicly reveal the names of pilots, the airlines or the location of the incidents.

The FAA declined to comment on the incidents that pilots reported, but said it wasn't aware of any verified reports of issues related to the anti-stall system, called MCAS for its acronym, according to the media reports. That anti-stall system only activates if the autopilot is turned off, according to documents Boeing has shared with airlines and the FAA.

Boeing said on Monday that will make a major software change to its 737 MAX 8 aircraft flight-control system. The new software for the MAX 8 will enable the plane's stall-prevention system to use multiple data feeds rather than relying on the single sensor employed when the plane was delivered in 2017, Boeing said.