The interesting case was the 737-100/200, for which ALPA contracts required the FE, and non-union airlines flew without FE. In 1980 as the DC9-80 neared certification, McDouglas, having promised to never certify another non-FE airplane after the DC-9, was getting hit hard by ALPA. McD called the DC9-80 a follow-on, and not a new plane.
A study was then released showing the two-crew 737 had by far the best safety record of any plane, followed by the DC-9 and the three-crew 737.
ALPA dropped its objection to no FE, and within a year McDuck changed the name to MD-80.
GB