Thousands of passengers scrambled to update their travel plans on Saturday as air traffic slowly unsnarled after a nationwide ground stop on Friday, but a number of travelers had already arrived at their destinations on flights left unaffected by the order.
Dozens of international flights took to the skies even as airports across the United States coped with only the third nationwide ground stop issued by the Federal Aviation Administration this century. Certain flights were also permitted to depart from the West Coast to Hawaii during the stoppage.
Here’s why some air travel was allowed to proceed even as flight systems were hampered.
Some international flights were able to depart.
More than 8,200 flights were canceled nationwide on Friday — over 31 percent of those scheduled — according to Cirium, an aviation data firm.
But more than 60 flights bound for overseas left the United States during the stoppage, according to Flightradar24, an aviation tracking site.
There are 21 air traffic control facilities, 20 in the contiguous United States and one in Alaska, that oversee swaths of regions throughout the country, according to the F.A.A.
As some of those centers recovered, flights were “allowed to depart through coordination with the Air Traffic Control System Command Center,” the F.A.A. said in an advisory less than two hours after the ground stop was ordered.
Flights departing for Europe from the Midwest or the East Coast can communicate with the Canadian authorities, said Dan Bubb, a former commercial pilot and an aviation history professor at the University of Las Vegas, Nevada.
Additionally, air traffic over the open ocean is less congested compared with that of the contiguous United States, he said.
Nick Daniels, the president of the National Air Traffic Controllers Association, said that for planes traveling such a large distance over oceanic airspace, “they have process procedures, different standards of separations, than what we do to run the volume of air traffic” in the lower 48 states.
What about domestic flights to Hawaii or Alaska?
At 3 p.m. Pacific Standard Time on Friday, about two hours before the nationwide ground stop was lifted, San Francisco International Airport said that Pacific-bound flights, including to Hawaii and Asia, had been cleared for departure.
Why were passengers flying west so fortunate?
Those pilots generally will talk to one of the regional centers until reaching cruising altitude, Dr. Bubb said.
Centers handling air traffic headed to Hawaii or Asia are outside the contiguous United States, and therefore could avoid long delays from the ground stop, he said. At a certain altitude, a pilot could swap communications with California controllers for those in Hawaii or Alaska, he added.
Flights within Alaska, those from the West Coast to Hawaii and international flights from Honolulu were also exempt from the ground stop, Alaska Airlines said in statement.
Areas like Alaska, Guam and Hawaii are far enough removed that they can remain stable even as systems are affected in the contiguous United States, Mr. Daniels said.
“If they would have seen any kind of risk in the system there as well, then they would have reacted the same to reduce the capacity,” he added.
Can medical aircraft fly during a ground stop?
Generally, yes.
During ground stops and delays, emergency and medical aircraft are given clearance to take off.
Life Flight Network, a medical transport service, said in a social media post on Friday that “EMS aircraft are exempt from this restriction,” and its services were operating normally.
“Above all is medevac and law enforcement,” Mr. Daniels said. “They get a priority movement throughout the system, and we move other aircraft out of their way, versus, they need to get in line with everyone else.”
Communications continued despite the stoppage.
Pilots and controllers have many tools to communicate, Dr. Bubb said.
Pilots can talk to one another, their companies and ground control through different messaging systems even if one fails.
“There are redundant systems where pilots can talk to each other, and we can sequence ourselves, but we follow the instructions that we were given by air traffic control,” Dr. Bubb said.