He said aviation manufacturers needed to develop better safety-analysis processes to holistically examine aircraft designs for risks.
Daniel Carey, the president of the Allied Pilots Association, which represents American Airlines pilots, also said Wednesday that Boeing had mistakenly created MCAS as a single-point-of-failure system, meaning that if one part of it failed, pilots would have to identify and correct that error.
He said Boeing had failed to disclose the existence of MCAS to pilots and to provide adequate 737 Max training materials for pilots trained on older 737 models.
While both Carey and Sullenberger recommended additional training requirements for 737 Max pilots, Sullenberger said simulator training should be mandatory before the aircraft returns to the skies.
"Reading about it on an iPad is not even close to sufficient," Sullenberger said.
Carey said pilots could get computer- and video-based training, then receive simulator training in the months after the aircraft returns to service, though he expressed concern about the updated training materials and protocols Boeing has suggested.
Carey said that at a meeting between pilots and the FAA in April, the FAA highlighted a checklist Boeing had directed pilots to use in the event of an MCAS misfire that an agency official said had not been validated since 1967.
Some Aviation Subcommittee members during Wednesday's hearing questioned the qualifications of foreign pilots and the standards of the International Civil Aviation Organization, the United Nations' aviation agency.
"The pilots of Ethiopia Air, their hours give me a great deal of question," said Rep. Paul Mitchell, a Michigan Republican, referring to the experience of the pilots involved in the Ethiopia crash. "We need to look at the ICAO standards versus our standards in North America for pilot qualifications and training and reconcile that."
Rep. Sam Graves, a Missouri Republican, cautioned against jumping to predetermined conclusions about the factors that contributed to the crashes, though he listed his main concerns.
"In reading the preliminary accident reports I, as well as, obviously, many others with a lot of flying experience, many have also raised concerns with the pilot training, with pilot experience, with aircraft maintenance and definitely with airline operations. All of these issues have to be investigated," Graves said.
Muilenburg has said his company made a mistake in its disclosures related to a 737 Max safety light that was not operating on all aircraft, though he said in April that the aircraft's systems were properly designed and the pilots did not "completely" follow procedures Boeing had outlined.
Carey, the union president, said he was "offended" by insinuations that pilots were to blame for the crashes and described Ethiopian Airlines pilots as "world-class."
"To make the claim that these accidents would not happen to US-trained pilots is presumptuous and not supported by fact," Carey said in prepared remarks. "Vilifying non-US pilots is disrespectful and not solution-based, nor is it in line with a sorely needed global safety culture that delivers one standard of safety and training."
The Boeing 737 Max has been grounded worldwide since March. While Boeing said in May that it had finalized the development of a fix for the plane's MCAS system, no time frame for the aircraft's return to the skies has been announced.
A source with knowledge of the process says the FAA certification flight of the Boeing 737 Max is expected to take place in the next two weeks. The certification flight is the next step to getting the planes back in the air, but not the final step.
The source says it will take four to six weeks to validate the findings of the flight once it's complete. If the changes to the 737 Max are approved, US airlines are expected to fly the aircraft again after September, the source said.
CNN's Jacob Rosen contributed to this report.