Aircraftman (AC) (or Aircraftwoman (ACW)) is the lowest rank in the Royal Air Force, ranking below Leading Aircraftman and having a NATO rank code of OR-1. The rank was introduced on the formation of the RAF in 1918, replacing the Royal Flying Corps rank of Air Mechanic 2nd Class. There used to be two grades: Aircraftman 1st Class (AC1) and Aircraftman 2nd Class (AC2; also colloquially known as an AC plonk). Aircraftmen are generally known as erks. Note that Aircraftsman, despite being commonly seen even in official documents, is incorrect.