The tail section of the plane was 30 feet off the ground, yet someone was able to write in the oil residue – much like a dirty car window, except this didn’t appear to be funny in the vein of "Clean Me."

No, this was two faces — one smiling and one “menacing” — with the words "Bye-Bye" written. That was enough for 13 flight attendants on a United Airlines flight last July scheduled to leave San Francisco for Hong Kong to refuse to fly over what they called a security threat.

They were subsequently fired.

Now they’re suing.

All 13 have filed a federal complaint against United on Wednesday with the U.S. Department of Labor and the Occupational Safety and Health Administration (OSHA), saying they were terminated unjustly. They are asking to be reinstated and to have back pay retroactive to July 14, the day of the incident.

The flight attendants noticed the drawings and the words on the plane and, according to the complaint, wanted the 300+ passengers taken off the plane for a security sweep. When that didn’t happen, they refused to fly that day.

"Given the gravity of the risk involved — the lives of passengers and crew alike — we were not willing to bow to United's pressure to ignore an unresolved security threat even though the company made clear that we risked losing our jobs," flight attendant Grace Lam said in a statement.

The flight attendants' lawyer, David J. Marshall, told the Chicago Tribune that his clients “did exactly what the flying public would expect from a group of highly experienced airline professionals. ... Our clients are entitled to legal protection for doing what was right."

Each flight attendant had a minimum of 18 years experience with United and a cumulative total of 299 years as flight attendants. The complaint says they were fired in retaliation for alleged insubordination; the flight was eventually cancelled.

In a written statement provided to Fox News, United said

"All of FAA's and United's own safety procedures were followed, including a comprehensive safety sweep prior to boarding, and the pilots, mechanics and safety leaders deemed the aircraft entirely safe to fly.”