The domain of air traffic control is the perfect example for the analysis of a linguistic norm. In this field, phraseology is a specialised language created to cover the most common situations encountered in air navigation in order to secure and optimise radiotelephony communications. When phraseology proves inadequate, a more natural form of language, called plain language, is required: it was recently introduced in the field and is a difficult notion to implement. A comparative analysis between a reference corpus and a real-communication corpus allows the description and categorisation of different types of variations used on the radio frequency as well as a discussion on the notions of norm and usage in the field of air traffic control.