By federal law, all passenger airliners now feature cockpit doors that are locked by the pilots from inside the cockpit. Likewise, pilot training now includes the specific instructions not to open the door in response to any occurrence or emergency in the passenger cabin; instead, pilots are under instructions to land the aircraft immediately in response to any perceived emergency that would have previously been cause to investigate beyond the flight deck (Sperry 2003).

Fortunately, by itself, this single set of rules has effectively ended any realistic threat of terrorists (or anybody else) ever hijacking a passenger airliner or otherwise gaining access to the flight controls for the purposes of commandeering it. In fact, it is now virtually impossible for anyone to hijack an American airliner again. Unfortunately, all of the subsequent passenger restrictions and billions of dollars in expenditures for sophisticated screening equipment is largely considered a complete waste...
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