The FAA issued a document dictating additional rules for the airlines on how to operate the 737 MAX jets when the grounded jet returns to the service.

The public has 30 days to comment on the document.

Even as Boeing inches closer to getting the 737 Max back in the air, new problems with the plane are emerging that go beyond the software that played a role in two deadly crashes.

As part of the work to return the Max to service, the company and regulators have scrutinized every aspect of the jet, uncovering new potential design flaws.

The company is looking at whether two bundles of critical wiring are too close together and could cause a short circuit. A short in that area could lead to a crash if pilots did not respond correctly, the people said. Boeing is still trying to determine whether that scenario could actually occur on a flight and, if so, whether it will need to separate the wire bundles in the roughly 800 Max jets that have already been built. The company says that the fix, if needed, is relatively simple.

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Boeing is still working to determine how likely it is that the wires could actually short circuit. The company does not want to make changes to the plane’s wiring if it doesn't have to, fearing that additional damage could be done during a repair.

In recent simulator tests with crews from American, Southwest and United Airlines and Aeromexico, many pilots did not use the prescribed emergency procedures to handle problems with the flights, raising the possibility that regulators could mandate flight simulator training or change the procedures before clearing the plane to fly. The F.A.A. is evaluating Boeing’s analysis of the testing.

Boeing employees mocked federal rules, talked about deceiving regulators and joked about potential flaws in the 737 Max as it was being developed, according to over a hundred pages of internal messages delivered Thursday to congressional investigators.

“I still haven’t been forgiven by God for the covering up I did last year,” one of the employees said in messages from 2018, apparently in reference to interactions with the Federal Aviation Administration.

The most damaging messages included conversations among Boeing pilots and other employees about software issues and other problems with flight simulators for the Max, a plane later involved in two accidents, in late 2018 and early 2019, that killed 346 people and threw the company into chaos.

The employees appear to discuss instances in which the company concealed such problems from the F.A.A. during the regulator’s certification of the simulators, which were used in the development of the Max, as well as in training for pilots who had not previously flown a 737.

“Would you put your family on a Max simulator trained aircraft? I wouldn’t,” one employee said to a colleague in another exchange from 2018, before the first crash. “No,” the colleague responded.

In another set of messages, employees questioned the design of the Max and even denigrated their own colleagues. “This airplane is designed by clowns, who are in turn supervised by monkeys,” an employee wrote in an exchange from 2017.