At the start of the 21st century, the 727 remained in service with a few large airlines. Faced with higher fuel costs, lower passenger volumes due to the post-9/11 economic climate, increasing restrictions on airport noise, and the extra expenses of maintaining older planes and paying flight engineers' salaries, most major airlines phased out their 727s; they were replaced by twin-engined aircraft, which are quieter and more fuel-efficient. Modern airliners also have a smaller flight deck crew of two pilots, while the 727 required two pilots and a flight engineer. Delta Air Lines, the last major U.S. carrier to do so, retired its last 727 from scheduled service in April 2003. Northwest Airlines retired its last 727 from charter service in June 2003. Many airlines replaced their 727s with either the 737-800 or the Airbus A320; both are close in size to the 727-200. , a total of 109 Boeing 727s (5× 727-100s and 104× -200s) were in commercial service with 34 airlines; three years later, the total had fallen to 64 airframes (4× 727-100s and 60× -200s) with 26 airlines.