Pressure mounted on Boeing Co in Washington as U.S. lawmakers called for executives to testify about two crashed 737 MAX jets, even as the world's biggest plane maker worked to return the grounded fleet to the skies.

A Senate panel plans to schedule a hearing with Boeing at an unspecified date, officials said, the first time a U.S. congressional committee has called the company's executives to appear for questioning over the crashes. The same panel, the Senate Commerce subcommittee on aviation and space, will also question FAA officials on March 27, likely about why the regulator agreed to certify the MAX planes in March 2017 without requiring extensive additional training.

The Ethiopian Airlines crash has shaken the global aviation industry and cast a shadow over the Boeing model intended to be a standard for decades to come. The crash on March 10 that killed all 157 on board has set off one of the widest investigations in aviation history. Initial reports from investigators say there are clear similarities between the crash and the Lion Air accident that killed all 189 crew and passengers in November.

While no direct link has yet been established, the MCAS flight control software and related pilot training are at the center of the investigation, and U.S. lawmakers are questioning the Federal Aviation Administration's certification of MAX's safety.

Boeing has promised a swift update to the MCAS, and the FAA said the installation of new software and related training was a priority. However, extra computer-based training will be required after the software update, the pilot union of MAX's biggest customer, Southwest Airlines Co, said on Wednesday, becoming the first major airline union to comment.