Air Canada joins airlines ruling that two crew members will now be required to be in the cockpit at all times in the wake of the Germanwings plane crash that killed all 150 people aboard. (Reuters)

Flying is safer than ever before, yet in this era of locked cockpit doors and pilot screening, authorities said Thursday that a single aviator was able to deliberately crash a commercial airliner in the French Alps.

The disaster has raised questions about how pilots are evaluated and how airlines can be sure that such a horrifying, if rare, event won’t reoccur.

Aviation security experts say what unfolded on Germanwings Flight 9525 could not have happened on a U.S. airliner because of strict security procedures adopted after 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 onboard.

Moreover, U.S. pilots cannot be left alone in the cockpit — the fatal error that investigators say doomed the Germanwings flight.

“It’s just a common-sense issue,” said aviation security expert Glenn Winn. “If you have a two-person cockpit, you don’t leave [one of] them alone up there.”

In Europe, however, there is no requirement that two crew members be in the cockpit at all times. After the crash, European carriers were moving swiftly to adopt such rules; on Thursday, Norwegian Airlines became the first to announce that its flights would adhere to those guidelines.

Although high-profile disasters such as the crash of Flight 9525 and the disappearance last year of Malaysia Airlines Flight 370 draw frenzied media coverage, flying has, in reality, never been safer. The era of planes going down because of engine failure, wind shear or midair collision has given way to state-of-the-art technology and vastly improved radar networks. The last plane crash in the United States to kill more than 50 passengers occurred in 2001.

The grim story told by investigators probing this latest crash — a pilot desperately trying to reenter the cockpit after the co-pilot locked him out — seems to fit a pattern for recent air disasters: The causes have been unusual, possibly unprecedented, making them difficult to predict and tough to prevent.

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

Flying carries inherent risk, and some of the danger emanates from the cockpit. Safety advocates and transportation officials usually worry about pilot fatigue and distractions. But pilots almost never intentionally crash their planes, experts say.

“To do that in an airliner is just pretty darned 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 the third suicide by airliner in recent memory, not counting the four planes hijacked by terrorists 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 remains an unconfirmed theory.

The most recent commercial crash 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 to Cairo, crashed into the ocean, killing all 217 people on board. The NTSB’s investigation said the first officer had guided the plane down; it did not speculate on a motive.

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

Two years earlier, a SilkAir flight from Jakarta to Singapore crashed. The Indonesian transportation agency said that although the crash may have been intentional, it was unable to determine that conclusively.

Late late year, investigators found that a 2013 crash in a Namibian national park bore similarities to what French officials say happened on the Germanwings flight. But that crash involved a smaller number of passengers than the crowded commercial flights.

Airlines have safeguards in place both onboard their planes and throughout the hiring process, but experts say there is little they can do to stop a suicidal pilot if he or she is left alone at the controls. What stands out as an unusual factor in the Flight 9525 crash is the inability of the pilot and other crew members to get back inside the cockpit.

New rules issued by the Federal Aviation Administration in 2002 require that cockpit doors be enhanced so they cannot be opened by physical force and could block shrapnel fire from small arms or explosives. These doors also must be equipped with an internal locking device and locked at all times.

But “most airlines have procedures to unlock that door,” Benzon 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.”

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

Winn, the 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 checks include fingerprinting and criminal history.

U.S. airlines put pilots through “rigorous evaluations” before hiring them, Melanie Hinton, managing director of the industry group Airlines for America, said in a statement.

“While working at an airline, all pilots have to regularly undergo thorough medical examinations to maintain their license,” she said. “In addition, all U.S. airlines can and do conduct fitness-for-duty testing on pilots if warranted.”

According to the FAA, airline pilots are required to undergo a medical exam with an approved physician every six or 12 months depending on their age. The screening typically includes questions about a pilot’s psychological condition. Physicians can order additional psychological testing if they think it is necessary.

Pilots who fail to disclose or falsify information about physical or psychological conditions and medications face fines of as much as $250,000. As part of the screening, pilots must report any visits to a health professional during the previous three years.

While the focus of the Germanwings investigation has shifted to the co-pilot, there are probably multiple factors that contributed to this crash, according to Thomas Anthony, director of the aviation safety and security program at the University of Southern California.

For example, he said, investigators might consider workplace culture. Germanwings is one of a new breed of low-cost carriers that have proliferated in the industry as travelers look for more economical ways to fly.

“It’s a mistake to look at a single factor when we’re looking at an aircraft mishap,” he said.