The Uniformed Firefighters Association and the Uniformed Fire Officers Association has filed an action with the city's Office of Collective Bargaining seeking to keep staffing at the current level.
Under an agreement that expires Monday, 60 engine companies in busy areas have five-person crews. The rest have four-person crews.
The unions say the changes could result in trapped or unconscious people not being found in fires.
Fire Commissioner Salvatore Cassano says all engines will be staffed with four firefighters and one officer beginning Tuesday. He says the FDNY just recorded "the fewest fire deaths on record" and has the fastest response time ever.