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The UPS freighter that crashed in Louisville was a 34-year-old jet. While that's old for a passenger plane, that's not so unusual in the world of air cargo.There are no initial indications that the age of the McDonnell Douglas MD-11 was in any way a factor in the fatal crash that occurred shortly after takeoff from Louisville, Kentucky Tuesday evening. The cause is still under investigation and will be for some time.But the crash does highlight how planes can have a much longer lifespan than the average flyer realizes. That's because older planes are often retired due to fuel economy, rather than wear-and-tear on the aircraft itself. If a plane isn't flying as much, fuel economy isn't as large a concern.And cargo jets fly about 30% less than passenger jets.The problem of fuel efficiency is especially true in the case of the MD-11, which has a third engine embedded in the tail, as opposed to the majority of passenger jets flying over American skies today, which only have two. The MD-11 was created in a time when companies sought out three-engine aircraft for range and reliability reasons, as planes with three engines were seen as safer than planes with only two, should an engine go out while flying over the open ocean.But as twin-engine jets became more reliable, that became less of an issue. The MD-11, with its three thirsty engines, became an anachronism almost as soon as it was introduced.And while there are still MD-11s flying cargo, the last passenger flight for the jet came more than a decade ago, on a KLM flight from Montreal to Amsterdam in October 2014.McDonnell Douglas ended up building only 200 of the jets, and took its last orders for the plane in 1998 from cargo carriers FedEx and Lufthansa Cargo. By then the company had already merged with Boeing. And it didn't take long for most of the passenger versions of the MD-11 to be converted to freighters.The MD-11 that crashed was initially delivered to Thai Airlines the first year the plane was put into service in 1991. It was sold to UPS and converted to a freighter in 2006.At that point it essentially became a new airplane, undergoing a complete overhaul. Aircraft also undergo overhauls even if they continue in passenger service, being stripped down to their bare frames for inspections, maintenance and upgrades in what is known in the industry as a "D-check.""An airliner is only as old, basically, as its last D-check," aviation consultant Mike Boyd said Wednesday morning. "At UPS, I would not be concerned about airplane age. The main issue for operators is increasing maintenance costs, which U.S. carriers don't take lightly, and fuel burn."But despite those costs, there are still some in use by cargo carriers.UPS has 25 other MD-11s in service, according to data from aviation analytics firm Cirium, and another six that have been in storage for at least the last three months. FedEx, which has been phasing out its MD-11s, has 38 in service, and 34 in storage. And cargo carrier Western Global Airlines has four in service and 12 in storage.