Europe’s aviation hubs face strikes, aircraft shortages and congested skies—what travelers need to know before booking summer flights.

BRUSSELS, Belgium — With summer only a few months away, travelers eyeing European getaways may find the journey as challenging as the destination is alluring. Brussels is one of many European capitals bracing for renewed labor actions, squeezed aircraft supplies and detours around closed airspace that could snarl schedules from June through September.