Bangladesh, Sept. 11 -- Twenty-five years ago, walking through an airport could be a markedly different experience.

Security was generally less intrusive. Friends and relatives could accompany passengers much further into some airports. Many items now barred from hand luggage could still be carried aboard. The elaborate security routines familiar to millions of travellers today had yet to emerge.

The September 11, 2001 attacks changed that.

Four passenger aircraft were hijacked in the United States, with two flown into New York's World Trade Center and another into the Pentagon. A fourth crashed in Pennsylvania after passengers and crew fought back. Nearly 3,000 people were killed.

But the consequences for aviation spread far beyond t...