Consumer advocate Ralph Nader said the larger engines mounted to the Boeing 737 Max represented a design flaw and called for the plane to be permanently grounded.

The 737 Max “must never fly again,” Nader said. “It’s not a matter of software. It’s a matter of structural design defect: the plane’s engines are too much for the traditional fuselage.”

Speaking at an aviation safety event in Washington, Nader lambasted Boeing Co. for designing the 737 Max as yet another revision to an airframe that was first built in the 1960’s, rather than designing a new plane from scratch.

Those larger engines—mounted higher on the 737’s wing than the previous version of the jet—altered how the plane flies in certain circumstances. That led Boeing to install the automated flight system that malfunctioned in two fatal crashes by the 737 Max since October, killing 346 people, including Nader’s grandniece.

Nader also said Boeing’s top leaders should resign, adding, “Good heavens, they would’ve resigned in 24 hours in Japan out of shame.”

Boeing didn’t immediately return an email seeking comment. The company is working with regulators on a software fix that will alter the flight control system to keep malfunctions such as the one linked to the two crashes from sending planes into uncontrollable dives.