Investigating monotony in air traffic control towers — Comparing a small and medium-sized airport

Abstract

This study investigates the development of monotony in air traffic control (ATC) using a multi-modal measurement setup applied in a field study at two operational towers of differing traffic intensity: Tartu (low-traffic) and Tallinn (medium-traffic). The results provide a first step toward establishing a reference baseline for assessing monotony in uneventful low-traffic tower environments using an evidence-based approach. Monotony is conceptualized as a state of reduced physiological and cognitive activation resulting from low task stimulation, repetitive and uneventful operational contexts. We operationalized this concept using indicators of monotony based on eye tracking to measure blink duration, PERCLOS and frequency, as well as saccade velocity. Other measures included electrocardiography (ECG), the Karolinska Sleepiness Scale (KSS), and the Psychomotor Vigilance Task (PVT). These measures were collected across multiple shifts over a period of eight days from seven air traffic controllers.Linear mixed-effects models (LMM) were fitted to each indicator, revealing consistent time-on-task effects at Tartu across several modalities, including blink duration, percentage eye closure (PERCLOS), heart rate (HR), root meansquare of successive differences (RMSSD), and standard deviation of NN intervals (SDNN). These findings support a pattern of growing parasympathetic activation and reduced arousal associated with monotony. In contrast, Tallinn showed signs of active physiological regulation under higher workload conditions, such as elevated baseline HR and decreasing SDNN, indicative of sustained engagement. The sympathovagal balance (LF/HF) ratio increased over time at both sites, but in the context of stable or increasing RMSSD/SDNN at Tartu, this may reflect compensatory physical activity rather than sympathetic dominance. Monotony indicators differed significantly between towers, with up to a 7.0-fold stronger temporal change in PERCLOS and a 1.4-fold difference in SDNN at Tartu compared to Tallinn.The study further discusses methodological feasibility in operational ATC work environments and highlights how operational differences, such as task rotation and self-directed off-position activities, impact data collection and interpretation. Findings underscore the need for tailored monotony mitigation strategies, particularly in low-traffic towers, and advocate for continued research in operational environments to capture complex real-world phenomena.