MINNEAPOLIS — Delta Air Lines is retiring its last DC-9s, the oldest passenger plane in the fleet of the big U.S. airlines.

Delta operated the final passenger flight from Minneapolis to Atlanta Monday evening.

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McDonnell Douglas delivered the first DC-9s in 1965, and eventually built 976 of them. The plane was noteworthy at the time because it was small enough to fly to airports in smaller cities that had previously been served by propeller-driven planes. Its low-to-the-ground profile put its cargo door at about waist height, so ground crews at smaller airports could load it without special equipment.

The plane flew for Delta, Continental and several smaller regional airlines. The one flown on the final scheduled flight on Monday was built in 1978 and went to North Central Airlines. Its fate after that mirrors the merger wave that rolled through the whole airline industry. A combination of North Central and other airlines formed Republic Airlines, which merged with Northwest Airlines in the 1980s. Delta bought Northwest in 2008.

Most airlines retired the DC-9s by the 1990s. But instead of retiring them, Northwest in 1995 refurbished their interiors to squeeze more flying out of them. Federal rules don’t limit how many years a plane can fly, only how many takeoffs and landings. As long as it stayed under those limits, the DC-9s could keep flying.

At one time the planes made up almost one-third of Northwest’s fleet. As of Monday Delta was down to its last six. It’s keeping two planes as spares for a few more weeks.

In an era when planes all have digital instruments, the DC-9 cockpit stands out for its dials. The plane doesn’t have a flight management computer that handles many of the routine flying tasks on newer planes, said Delta’s DC-9 chief line check pilot Scott Woolfrey, who specifically asked to pilot the plane’s last flight. “It’s a pilot’s airplane,” he said before the flight on Monday.

The final flight prompted dozens of aviation enthusiasts to buy tickets, and they lined up at the window to watch the plane come in from LaGuardia airport in New York.

Delta is known for buying used airplanes and flying them longer than other airlines. Even Delta’s DC-9 replacement – used Boeing 717s from AirTran – is a hand-me-down. Delta is giving those planes new interiors and adding Wi-Fi as it brings them into its fleet. The 717, along with the MD-90s that Delta has also been buying used, are both descendants of the DC-9.