What safety measure was introduced after the World Trade Center incident?

The introduction of locked cockpit doors as a safety measure after the World Trade Center incident was a significant response to enhance aviation security. Following the tragic events of September 11, 2001, the threat of hijacking was dramatically recognized, leading to this pivotal change in cockpit security protocols.

Locked cockpit doors serve as a deterrent against unauthorized access to the flight deck, thereby protecting pilots and maintaining control of the aircraft. This measure is designed to prevent potential hijackers from gaining entry to an area where they could threaten the safety of the flight and its passengers. By ensuring that cockpit access is restricted, airlines can significantly mitigate risks associated with airborne threats.

While the other options may reflect changes or considerations in the aviation industry, they do not directly address the primary security concern raised by the incidents of 9/11 in the same way that locked cockpit doors do.