World air travel is down 6% on pre-11 September attack levels, travel data firm OAG has said on the second anniversary of the hijackings.
Aircraft have been put out of commission
Airlines worldwide are flying 5,000 fewer flights than they did two years ago and flights in the United States are down 16%, the company said.
Some airlines, however, insist that their business has started to recover over the past 12 months.
The airline sector has been hit by the economic downturn, travel fears following the attacks, the Sars virus outbreak and the war in Iraq.
The growth in low-fare carriers such as
Southwest Airlines in the US and Europe's EasyJet has not stemmed the drop in air traffic.
American Airlines, United Airlines, Continental Airlines and Lufthansa Airlines each said they planned
to reduce the number of flights on the 11 September anniversary.
Continental said it intended to cancel 13 of its roughly
2,200 scheduled daily flights on Thursday, all of which depart from Newark International Airport.
Newark was the departure
point for United Flight 93, the San Francisco-bound
jetliner that crashed in Shanksville, Pennsylvania.
Germany's Lufthansa airline cancelled four flights on Thursday to the US.
"We think it's because it's not a day where people go on
business trips and have business meetings," a spokeswoman said.
"It's more of a memorial day."