Boeing Co. Chief Executive Dennis Muilenburg will begin the first of two days of testimony on Tuesday before U.S. lawmakers and will face tough questions on the crashes of two 737 MAX planes, which killed 346 people and sparked calls for reforms.

In an appearance set to begin at 10 a.m. (EDT) before the Senate Commerce Committee, Muilenburg will acknowledge mistakes, according to written testimony released Monday. 

"We have learned and are still learning from these accidents, Mr. Chairman. We know we made mistakes and got some things wrong," the testimony reads. 

Senator Roger Wicker, the committee chairman, said he plans to address at Tuesday's hearing families of the crash victims: "I promise to their loved ones that we will find out what went wrong and work to prevent future tragedies." 

On Monday, Muilenburg visited the Indonesian Embassy in Washington to meet with the ambassador and "pay our respects to those lost aboard Lion Air flight 610 on the first anniversary of the accident," Boeing said in a statement. 

Muilenburg, who was stripped of his title as Boeing chairman by the board this month, will also testify before the U.S. House of Representatives Transportation and Infrastructure Committee on Wednesday.

Michael Stumo, the father of Samya Rose Stumo, who died in the Ethiopian Airlines Flight 302 crash in March, said the victims’ families plan to hold up photos of the "loved ones we lost" to "make sure the focus is on that, rather than political or bureaucratic or engineering issues." 

He questioned why Boeing is only now adding safeguards to a flight control system known as MCAS that investigators have linked to both crashes. 

"When you knew the MCAS system was part of that Lion Air crash, why didn't you act to correct it immediately instead of still withholding information about it and blaming the pilots?" Stumo said of Boeing. 

U.S. airlines have canceled flights into January and February because of the grounding, and the Federal Aviation Administration is not expected to approve the 737 MAX's ungrounding until December at the earliest. 

In March, after the Ethiopian Airlines crash, the plane was grounded worldwide. 

Indonesian investigators reported on Friday that Boeing, acting without adequate oversight from U.S. regulators, failed to grasp risks in the design of cockpit software on the 737 MAX, sowing the seeds for the October 29, 2018, Lion Air 610 crash, which also involved errors by airline workers and crew. 

Muilenburg added that "regulators should approve the return of the MAX to the skies only after they have applied the most rigorous scrutiny, and are completely satisfied as to the plane's safety. The flying public deserves nothing less."

In the third quarter of 2019, according to an earnings report, the plane's continued grounding cost Boeing 900 million U.S. dollars, bringing the total to 9.2 billion U.S. dollars, since two fatal crashes between them killed nearly 350 people in March.