There is a terrible truth about 9/11: It didn’t have to happen. Something as simple as a hardware store lock could have deterred the perpetrators.

The arguments against hardening cockpits were numerous, and now seem unconvincing to the point of being risible.

They included: It would cost too much; the public had to see the captain go walkabout during the flight; first responders couldn’t get to the pilots after a crash; air pressure wouldn’t be stable between the cockpit and the passenger cabin; and the pilots wouldn’t be able to go to the toilet.

Long before 9/11, airplane hijackings were a well-established tool for terrorists, asylum-seekers, extortionists, or simply those who were unbalanced.

Generally the hijackers made their way unhindered to the cockpit, with or without a hostage, and forced the pilots to fly to a country estranged from the one they started from — like Cuba for hijackers from the United States or Western Countries for Soviet defectors.

There was no mystery about hijacking and the vulnerability of pilots. What differentiated the 9/11 hijackings was the fiendish planning, complete with flying lessons and, finally, the ease with which they did their evil.

After all the deaths, the untold suffering, the billions of dollars that have gone into airport security, including the establishment of the Transportation Security Administration, the question remains why something wasn’t done before 9/11 to secure the cockpits. They are now impenetrable, they are bulletproof and can even withstand a small bomb.

A White House science adviser told me long after the tragedy, “We should have done more.”

Pilots and informed people suggested over the years that cockpits should be hardened, others went further and petitioned the Federal Aviation Administration. But the cockpits remained open, accessible and vulnerable, inviting heinous crime.

The failure to harden cockpits after early hijackings is a tragedy of multiple derelictions, from the airlines through the FAA, the White House, the FBI (which investigates terrorists and airline crime); the National Transportation Safety Board; the aviation unions, even the Occupational Safety and Health Administration — an airliner is a workplace.

The system failed because the voices from numerous places calling for hardened cockpits were too soft, too scattered, and not united. A strong statement, a loud voice, a repetitive complaint or more media coverage might have made the difference.

Since 9/11 public service announcements on public transport of all kinds have cried out, “If you see something, say something.” People saw something from many angles but said little.

General aviation is a catch-all phrase for everyone who flies everything, from crop dusters to corporate jets, from aerial-mapping aircraft to single-engine Cessnas, even those who fly cadavers overnight, a small industry.

They hangout between flights in dismal places called “pilot lounges.” In these lounges — usually a room with a few couches and vending machines for coffee and snacks — the talk is about aviation and how to fix some aspect of it.

The thing about the men and women of general aviation is that they are in a sense the heart of aviation: They fly because they would rather do that than anything else. They can’t imagine another life. And they, for assorted reasons, haven’t landed jobs with airlines or major overnight delivery services.

In these lounges, long before 9/11, there was always a groundswell of opinion about hijackers, and they all asked rhetorically, “Why don’t they lock the damn cockpits?” I heard a version of that phrase often when I was a private pilot.

Of all of those who could share some blame, the most culpable are the airlines. They didn’t want to alarm the passengers, spend the money or inconvenience the flight crew. But mostly, no airline wanted to be first.

From time to time, there was talk of doing something about the ease with which ill-wishers could penetrate the cockpit on commercial flights. Often doors were open and passengers could go to the flight deck, if they asked.

I have happy memories of being allowed into the cockpit of the French Concorde and being cheerily greeted by the pilot, who was reading Le Monde, “At 60,000 feet and Mach 2, what am I going to do?” Sensibly, he was leaving the flying to the autopilot.

We will forever mourn the 2,977 innocent people who died on 9/11, and all those who have died in its aftermath.

Aviation is very safe because it prides itself on its culture of safety. On Sept. 9, 2001, that culture was absent. That culture reduced to hoping for the best rather than locking the damn door.