This research aims to investigate how air traffic controllers (ATCOs) contribute to landing instability from the perspective of situation awareness and shared situation awareness. Seven (7) air traffic controllers were involved in semi-structured interviews to study how their decisions were made during the landing phase. Based on the results obtained, the themes identifed were articulated respectively under both proactive and reactive scenarios. In proactive scenarios, ATCOs will make a plan to expedite the air traffic flow to manage the workload in the future, which may decrease the time and the distance for pilots to operate. Thus, the task-related Shared-Mental Model (SMM) enables ATCOs to expedite the landing process within acceptable margins. In addition, the team-related SMM allows ATCOs to intervene in the unstabilized landing state in advance. On the other hand, as for reactive scenarios, unexpected weather and pilots' improper performance due to fuel conservation are the most prominent scenarios for ATCOs to be reactive. To tackle unexpected changes, the task-related SMM facilitates ATCOs in providing extra time and distance for pilots to operate the aircraft. To conclude, for a team in a complex system to achieve the required performance, proper SMM is compulsory to mitigate requirements conflicts between different operators. In this case, it enables ATCOs to achieve a proactive mangement in ways which can be accepted by pilots. Furthermore, proper SMM also helps ATCOs to actively provide extra instructions for pilots under contingencies.