Modern aviation operations demand pilots to process multi-source cockpit instrumentation, creating cognitive fragmentation that degrades situational awareness (SA) and increases mental workload. While conventional head-up displays (HUDs) partially address these challenges, their static symbology and high retrofitting costs limit effectiveness. Augmented Reality (AR) solutions face adoption barriers due to visual conflicts in head-mounted displays and prohibitive waveguide HUD costs. This study proposes a low-cost Projection-Enhanced HUD (PE-HUD) using retroreflective film-screen symbiosis, achieving collimated imagery without complex optics. Neuro-ergonomic validation with functional Near-Infrared Spectroscopy (fNIRS) and eye-tracking were conducted across flight phases with ten cadet pilots under low-visibility simulations. Results demonstrated phase-specific cognitive benefits: PE-HUD significantly enhanced occipital-parietal activation during landing and improved cross-referencing between flight control units and primary displays. Take-off phases showed balanced multi-parameter monitoring, while cruise phases exhibited reduced HUD dependency. Eye-tracking revealed fewer head-down transitions compared to those without HUDs. Critically, pilots’ visual attention patterns varied dynamically across phases, necessitating adaptive interface designs. These findings establish neuro-ergonomic evidence for phase-sensitive HUD optimisation, advancing cost-effective human-machine interface (HMI) design in aviation. Future work will integrate real-time neurofeedback to enable context-aware display adaptation.