America’s aviation system is straining under the weight of the longest government shutdown on record: thousands of flight cancellations, long delays at major airports and frustrated travelers nationwide. In an unprecedented move, the Federal Aviation Administration last week ordered airlines to scale back domestic flight schedules, saying the cuts are meant to ease pressure on an overstretched system and help manage air traffic control staffing. Unpaid for more than a month, some air traffic controllers have begun calling out of work, citing stress and the need to take on second jobs — leaving more control towers and facilities short-staffed.