Boeing should ground their 737 MAX 8, according to the former head of the National Transportation Safety Board, who said he won’t fly on one until the mystery behind the crash of Ethiopian Airlines Flight 302 is solved.

“I think Boeing should voluntarily ground the plane and as a result demonstrate that they’re putting safety first in regard to the future of this aircraft,” ex-NTSB chairman Jim Hall told CNBC. “If I was a Boeing investor or a member of Boeing, I’d want to put safety first,” said Hall, who led the NTSB from 1994 to 2001.

Besides Sunday’s crash, Hall cited the October crash of a Lion Air Max 8 in Indonesia in which 189 people lost their lives when the plane plummeted into the Java Sea.

Similarities between the two disasters sparked speculation on a possible problem with the computerized flight controls of the aircraft — a new safety feature designed to protect the plane from stalling — that could confound pilots.

“There has been a lot of discussion and I’m not sure there’s been a lot of transparency in regard to what’s going to be done to fix the error that caused the Lion Air crash,” Hill said.

One of the NTSB’s five board members is Earl Weener, a former Boeing executive who served as chief engineer of airworthiness, reliability maintainability and safety, and then chief of systems engineering, according to his agency bio.