Among the listed barriers, which cannot be managed easily while flying?

Glare and sun spots present a significant challenge for pilots while flying because they can impair vision and obstruct the pilot's ability to see other aircraft, the ground, or crucial instruments clearly. Unlike scanning techniques, which can be improved through training and practice, glare from sunlight can be a sudden and unpredictable factor that is not easily mitigated.

While pilots might adjust their head position, use sun visors, or change their flight path slightly to avoid direct sunlight, these actions often do not completely eliminate the hazard. It can be particularly disorienting during critical phases of flight, such as takeoff or landing, when maintaining visual awareness is essential.

On the other hand, aircraft blind spots can be managed by using proper scanning techniques and awareness of the aircraft's design, while altitude variations are navigable through adjustments in flight plans and altitude settings. Thus, glare and sun spots are less manageable in-flight conditions than the other factors presented.