An internal investigation by the U.S. Federal Aviation Administration found The Boeing Co. was planning to strengthen protective engine covers on its 777 jets years before one experienced a serious failure.

An administration document showed Boeing and the regulator had been discussing ways to fix and improve the company's engine cowlings. Sources familiar with the matter said talks about the potential weaknesses of the cowlings began in 2018.

"Boeing has decided to redesign the fan cowl instead of trying to modify existing fan cowls to address both the structural strength concerns," the administration document said.

Boeing planned to build new fan cowlings and provide its customers service instructions on how to replace the old cowlings on 777 jets. Aviation experts have said they were concerned about possible damage from weak engine covers. If the covers can't withstand the impact of a fan blade breaking off it might result in a serious accident, they said.

Experts said pilots were trained to land a plane with just one engine. While it was rare, fan debris could damage other parts of a plane and put passengers at risk.

The recent incidents involving United 777 jets has sparked a new investigation into Boeing manufacturing practices. The administration has ordered new inspections on Boeing 777 jets and has even proposed a temporary grounding of the planes.

A former chairperson of the National Transportation Safety Board, Jim Hall, criticized the administration for failing to "aggressively" look at the issue of Boeing's engine covers. He said there was no indication the administration had adequately looked into the problem.

Boeing said it would continue to follow all of the administration's guidance involving its 777 engine cowlings. The company said it was currently engaged in efforts to develop new safety protocols and component enhancements across its fleet.

The administration said it was closely monitoring Boeing's efforts involving its planned improvement to its engine fan blades and engine covers. The agency said it would be expanding on its work with the company following an engine cowling failure on a Southwest flight that killed a passenger in 2018.

"Any proposed design change to a critical piece of the structure must be carefully evaluated and tested to ensure it provides an equivalent or improved level of safety and does not introduce unintended risks," the administration said.