UPDATED, March 27 with the European Aviation Safety Agency's new temporary recommendation on having two crew members in cockpits at all times.

There's an axiom in national security circles that if you prepare only for the last attack, you will likely be ill-prepared for the next one. The apparently deliberate crash of Germanwings Flight 9525 at the hands of a co-pilot may prove this true once again, as it appears to have been enabled in part by post-9/11 security reforms that made it harder for intruders to gain access to the cockpit.

Except in this case it wasn't an intruder trying to enter the flight deck, but a pilot who was presumably trying to stop a deadly, intentional descent into the French Alps.

In the wake of the Sept. 11 terrorist attacks, aviation security was dramatically enhanced in the U.S. and Europe. Most of the steps airlines and governments took were aimed at keeping terrorists out of the cockpit, rather than preventing sabotage from within the flight deck. In the U.S., there is even a voluntary program in place that allows pilots to carry firearms in the event an intruder does threaten the safety of their flight.

For example, the steel-reinforced doors with cypher locks that were placed between the flight deck and the cabin are designed to keep out intruders. French investigators said the pilot, who was locked out of the cockpit, tried and failed to open the cockpit door for about eight minutes.

In addition, U.S. airline pilots as well as some on international carriers have received tactical training to block those doors from within the flight deck to secure the cockpit in the event that an intruder attempts to get in. One theory is that co-pilot Andreas Lubitz did this as he commanded the autopilot to force the plane into a descent from which it would never recover.

“Something went wrong with their normal procedures,” Glen E. Winn, an aviation security consultant, told Mashable. “What went wrong here I have no idea.”

In the case of Germanwings Flight 9525, the door worked against safety by preventing the pilot — who presumably could have stopped the plane from descending into the French Alps — from reentering the cockpit after leaving briefly for "natural reasons," as investigators put it on Thursday.

Post-9/11 security reforms in the U.S. have even resulted in armed pilots, known as federal flight desk officers. They bring onto the flight deck loaded firearms meant to foil hijackers. However, in the event of a suicidal pilot, this could enable such a tragedy, rather than prevent it.

“There is unfortunately a bit of irony in this situation, frankly,” said Patrick Cropper, a principal at ICF International and former pilot who consults with airlines, told Mashable in an interview. “In this case the threat was actually inside of the cockpit.”

Gen. David Galtier at a press conference about the crash. Image: Sipa via AP Images/Associated Press

Winn said standard operating procedures for U.S. and international airlines should have still prevented the Germanwings incident. Such procedures call for a high-ranking flight attendant to enter the cockpit whenever one pilot leaves the flight deck for any reason, including to use the bathroom.

Cropper says "nearly all major carriers" in the U.S. have established protocols to use when a pilot needs to leave the cockpit, as well as procedures for allowing them back in. These rules are in the airline's operating manuals, and are considered to be mandatory for pilots and flight crews.

However, Lufthansa CEO Carsten Spohr reportedly said on Thursday that there is no European regulation requiring a flight attendant to enter the cockpit when one pilot is absent. This was backed up by an announcement Thursday from UK-based budget carrier Easy Jet that it is changing its procedures to require two crew members to be in the flight deck at all times.

#Easyjet has announced that the company is to change its procedures from tomorrow to ensure there are always two crew members in the cockpit — Sky News Newsdesk (@SkyNewsBreak) March 26, 2015

Another European carrier, Norwegian Airlines, announced a similar change. "We have been planning to change our procedures, but this accident has sped up the implementation. From now on, two people from the crew will always have to be present in the cockpit," an airline spokeswoman told Yahoo News.

Some other European airlines already followed the two-person policy before the crash.

[UPDATE, March 27: On Friday, the European Aviation Safety Agency announced a temporary recommendation for airlines "to ensure that at least two crew, including at least one qualified pilot, are in the flight crew compartment at all times of the flight."

"Airlines should re-assess the safety and security risks associated with a flight crew leaving the cockpit due to operational or physiological needs," EASA said in the statement.]

The video below explains how a reinforced cockpit door works.

"The regulatory piece of it was that a senior flight attendant or lead flight attendant or purser would open the door for the pilot to come out,” and then step into the flight deck, Winn told Mashable. “The other step was [for them] to be able to open the door for the pilot who would be returning.”

The placement of a flight attendant inside the cockpit is meant to overcome a lock failure as well, Winn said. “There had to be someone there who could open the door,” he said. The flight attendant would also serve as an additional obstacle for a rogue pilot, according to ICF's Cropper.

Winn said U.S. air safety regulations are generally followed worldwide because of airline alliances and the airlines' desire to have regulatory harmony across borders.

The Germanwings flight would appear to have violated the safety protocols in place at Lufthansa, which is Germanwings' parent company. In addition, the pilot's need for a bathroom trip on a short flight is in itself unusual, Winn said, noting that pilots “are normally pre-prepared themselves for not having to go to the lavatory on a short trip.”