The National Transportation Safety Board said it has not determined exactly how close a FedEx cargo plane passed over the top of a Southwest Airlines jet last month in Austin, Texas, but there was little margin.

“We still believe the planes were within 100 feet of each other,” NTSB Chair Jennifer Homendy said in an interview.

An air traffic controller had cleared both planes to use the same runway, the NTSB said in a preliminary report. With visibility down to a quarter-mile because of freezing fog, the FedEx pilots didn’t see the Southwest jet until the last seconds.

The NTSB is also investigating an incident Monday night at Boston’s Logan International Airport in which a Learjet pilot who was told by an air traffic controller to wait instead began to take off as a JetBlue plane approached to land on an intersecting runway. The JetBlue pilots pulled up, avoiding a collision.

Those and similar incidents in New York, California and Hawaii led the head of the Federal Aviation Administration to call for a “safety summit” and ignited a debate about whether air safety is declining or the events are just an unusual cluster of serious close calls.

“I don’t know that I can say that it’s a trend, but these are disturbing because it just takes one,” Homendy said. “That is why we investigate incidents — so that we can identify problems, especially when we see trends, and address them before they become a full-blown accident.”

Among the other recent incidents now being investigated:

— Last week pilots of a small airliner aborted their landing in Burbank, California, after a controller cleared another plane to take off from the same runway; the NTSB is investigating.

— A United Airlines jet crossed a runway at Honolulu International Airport in front of a Cessna cargo plane that was landing on the same runway on Jan. 23.

— The NTSB took the rare step of issuing subpoenas for pilots of an American Airlines plane that crossed a runway that a Delta Air Lines jet was using to take off Jan. 13 at New York’s John F. Kennedy International Airport. The American pilots initially refused to sit for recorded interviews, but they complied after getting subpoenas, Homendy said.

— Federal officials are taking another look at an incident in which a United Airlines jet taking off from Hawaii dove to within 800 feet of the ocean before recovering. United says pilots of the December flight are getting additional training.

Except for the United plane that descended sharply after takeoff, the other incidents were “runway incursions” in which a plane wound up on a runway when it was not supposed to be there. A 2017 forum convened by the NTSB found that the most common causes of runway incursions were pilots ignoring orders from air traffic controllers, or miscommunication between pilots and controllers.

“Runway incursions have always been an area of concern, but it looks worse because the system is so safe,” said John Hansman, an aeronautics professor at the Massachusetts Institute of Technology. “We fortunately don’t have many accidents, so we focus on these risk precursors.”

Pilots are the last line of defense in aviation safety. In some of the recent incidents — notably the one in Austin — pilots spotted something wrong and reacted quickly.

The incidents may put an end to any possibility of easing experience requirements for newly hired pilots, which is something that smaller, regional airlines have requested to help them cope with a pilot shortage. President Joe Biden’s nominee for the top job at the FAA, Phillip Washington, said Wednesday that he opposes easing the pilot-qualification standards.

The close calls could also lead to demands for improved technology at airports and on board planes. Kennedy airport has ground-surveillance radar designed to prevent runway incursions. It alerted controllers that the American Airlines plane was crossing an active runway. Only 35 U.S. airports have that technology.

Investigators won’t be able to hear what pilots were doing before most of the recent incidents because the cockpit voice recordings were erased after two hours of flying. Since 2018, the NTSB has asked FAA to require 25 hours of recording capability, which would improve the odds of saving valuable information.

Besides the close calls, there have also been several recent incidents involving severe turbulence causing injuries on planes. In the most recent case, a Lufthansa plane flying from Austin to Germany diverted to Washington Dulles International Airport on Wednesday night; seven people on board were hurt badly enough to go to hospitals. The FAA said it is investigating.

The close calls have attracted the attention of lawmakers, who questioned the acting FAA administrator about them. The official, Billy Nolen, defended the safety of the nation’s air-travel system last month while acknowledging the need for vigilance.

“We are experiencing the safest period in aviation history, but we do not take that for granted,” Nolen told a Senate committee. “Recent events remind us that we cannot become complacent.”

Nolen, like executives of the airline industry, point out that there has not been a fatal crash involving a U.S. airline plane since 2009, an unprecedented stretch.

Still, he said at the hearing and in an internal memo, he was forming a “safety review team” to examine the aviation system, starting with a meeting in March “to examine what additional actions the aviation community needs to take to maintain our safety record.” Nolen said aviation leaders will look into which steps are working “and why others appear to be not as effective as they once were.”

The FAA said Thursday that the summit will be March 15 and include representatives from commercial aviation, airports, labor and aviation experts.