In a context of weakening social dialogue in general and the legitimacy of trade unions in particular, we propose in this article to take a fresh look at the manager/trade union pairing through a change of perspective. In the framework of an in-depth qualitative research in a French aeronautical industry, we are interested in the daily informal dialogue between middle-managers and union representatives at the shop floor level (Kochan, Katz and McKersie, 1994). This reversal allows us to develop a new perspective on the role and legitimacy of unions in an organisation. To anchor this change of perspective, we will use the concept of relational dialogue (Cunliffe and Eriksen, 2011) to go beyond the traditional approach of social dialogue and to test its effect in another way. We will then show that relational dialogue, which is based on relational quality, a processual approach to dialogue and its polyphonic dimension, enables middle managers to anticipate tensions in teams and improve their managerial decisions. However, this relational dialogue is not perceived positively by the whole organisation, especially by human resources managers, who will try to reduce its impact by implementing management tools that prevent the more informal arbitrations resulting from the dialogue between managers and unions. As part of the recent evolution of IR research, enriched by conceptual frameworks coming from organization studies, this article contributes to test the concept of relational dialogue not between the manager and his employees but with trade union officials. We were thus able to identify the organisational conditions of this dialogue, which had not yet been done. The application of the concept of relational dialogue in an industrial relations situation enriches the pluralist approach by underlining both its potential and the importance of the conditions necessary for this type of dialogue.