Rescue helicopters near the crash area. (Claude Paris/AP)

Flying is safer than ever before, yet in this era of locked cockpit doors and pilot screening, authorities say a single aviator was able to crash a commercial airline in the French Alps. This disaster has raised questions about how pilots are evaluated and just how sure airlines can be that such a horrifying but rare scene won’t recur.

Aviation security experts say that what unfolded on Germanwings Flight 9525 could not have happened on a U.S. airline due to strict security procedures issued in the wake of the Sept. 11, 2001, terrorist attacks. Cockpit doors were strengthened, and airlines now lock them at all times, with most doors requiring security codes known only to a handful of people. In addition, no pilots can be left alone in the cockpit, which investigators say is what happened on the Germanwings flight.

High-profile aviation disasters such as the crash of Flight 9525 in the Alps or the disappearance of Malaysia Airlines Flight 370 may draw frenzied media coverage, but they belie the reality in the air: Flying has never been safer. The era of planes going down because of engine failures or collisions has given way to state-of-the-art technology and vastly improved radar networks. The last plane crash in the United States that killed more than 50 passengers occurred in 2001.

It is still very early in the investigation, and many things remain unknown about the crash and the co-pilot.  If it is determined that the co-pilot intentionally brought down the plane, as French officials said Thursday, it will be extremely unusual. Investigations into airline crashes almost never find that deliberate pilot action was responsible.

Of course, flying carries inherent risks, and some of the danger emanates from the cockpit. Safety advocates and transportation officials have worried about pilot fatigue and distractions, among other things. But an intentional action, rather than an accidental one, is much rarer, particularly on a jetliner, experts say.

“To do that in an airliner is just pretty darn rare,” said Robert Benzon, who spent 27 years as a National Transportation Safety Board lead crash investigator. “You could tweak a database on suicide and you’d get a lot of little planes, but airliners, not much would pop up at all.”

If this crash is confirmed as deliberate, Benzon said it would be only the third suicide by airliner in memory, not counting the planes that were hijacked on Sept. 11, 2001. Investigators looking into the Malaysia Airlines flight that disappeared last year have pointed to pilot action as a likely explanation, but that is still an unconfirmed theory.

The most recent commercial airline crash aviation officials blamed on a deliberate action by a pilot occurred near Nantucket, Mass., in 1999. EgyptAir Flight 990, which was leaving John F. Kennedy International Airport in New York en route for Cairo, crashed into the ocean, killing the 217 people on board. The National Transportation Safety Board’s investigation said the first officer had guided the plane down, though it did not speculate on a motive.

However, that determination was not unanimous. EgyptAir contended that the plane was not intentionally crashed, arguing that the first officer may have been trying to maneuver the plane “out of a perceived dangerous situation” when it crashed.

Two years earlier, a SilkAir flight from Jakarta to Singapore crashed. The Indonesian transportation agency that led the investigation said that while it was possible that the crash was intentional, it was unable to determine that conclusively. (There was also a 2013 crash in a Namibian national park investigators believed to be intentional, though it involved a smaller number of passengers.)

The tally of intentional crashes does not count the terrorist attacks on Sept. 11, 2001, which were carried out by hijackers who took control of four planes; two of them were crashed into the World Trade Center towers in New York, while one was piloted into the Pentagon. The fourth plane crashed in a field near Shanksville, Pa., after passengers and crew revolted in midair.

Intentional crashes occur more frequently with small planes, Benzon said.

“In the light-plane business, unfortunately, there have been quite a few suicides, but they only take the pilot down and there’s no passengers on board,” he said. And there have been times when passengers got onto planes with bombs and brought down the aircraft, he added.

Ultimately, despite the safeguards in place in the hiring process as well as on the plane, there is little that can be done if a pilot or another person looking to bring down a plane is alone in the cockpit. Still, the inability of the pilot and other crew members to get back into the cockpit stands out as an unusual factor in this crash. Benzon said the rest of the flight crew generally have the means to get back into the cockpit.

“Most airlines have procedures to unlock that door,” he said. “I’m a little surprised that, if the scenario we’re talking about is true, that the captain couldn’t get back up forward.”

He said the methods for reentry are “confidential, because if you tell everybody, there goes the plan.” Speaking in general terms, though, he said some crew members may have a key or a code that would grant them access.

Glenn Winn, an aviation security expert, said the Germanwings crash will probably put a greater focus on psychological screening of pilots and crew members, who already go through multiple background checks as part of their jobs. Pilots and crew members are screened by the airlines that employ them as well as the airports where they operate. The screenings include fingerprinting and criminal history.

“As evidenced by our safety record, the U.S. airline industry remains the largest and safest aviation system in the world as a result of the ongoing and strong collaboration among airlines, airline employees, manufacturers and government,” Melanie Hinton, managing director of the industry group Airlines for America, said in a statement. “Our pilots undergo rigorous evaluations in the hiring process, which helps to ensure the safety of the U.S. aviation system, and its passengers, crew, cargo and aircraft. While working at an airline, all pilots have to regularly undergo thorough medical examinations to maintain their license.”

In 2002, the Federal Aviation Administration issued new rules regarding cockpit doors, requiring the doors to be strengthened so they could not be opened by physical force and could block shrapnel fire from small arms or explosives. FAA officials also required the door to be locked at all times and be equipped with an internal locking device.

According to the Airline Pilots Association, every airline in the United States has procedures designed to ensure that there is never a situation in which a pilot is left alone in the cockpit. The same, however, is not required by Lufthansa and many other airlines, as Europe has no regulatory requirement that two crew members must be in the cockpit at all times.​

“We could never have imagined that a tragedy like this could occur within our company,” Germanwings said in a statement Thursday. “Yet even after this terrible event, we have full faith in our pilots. They remain the best in the world; this event is an extremely tragic isolated incident.”

Norwegian Airlines announced Thursday that it would implement a rule to require that, as did Air Canada and EasyJet.

“This means that if one of the pilots leaves the cockpit, one crew member must replace him/her during this time,” Norwegian said in a statement.​

Lori Aratani contributed to this report.

[This post has been updated.]