Delta Air Lines will institute mandatory diversity training for flight crews following an October 2016 incident in which a Delta steward didn’t believe a black physician who came forward to help an ill passenger was a real doctor.

Starting this spring, Delta’s 23,000 flight attendants must go through the same training that became mandatory for all executives last year.

“As part of Delta’s ongoing diversity and inclusion efforts, the airline launched inclusion training four months ago for Delta leaders,” said Brian Kruse, spokesman, Delta Air Lines. “Cultural sensitivity training has long been part of Delta’s flight crew curriculum.”

Delta is the last of the four major U.S. airlines to make such training mandatory. American Airlines requires a refresher course every two years, and United Airlines requires all employees dealing with customers to receive diversity training. Southwest Airlines trains all new employees through anti-discrimination and anti-harassment courses.