Center (DLR) as a reduced-gravity aircraft for conducting research into microgravity; the A300 is the largest aircraft to ever have been used in this capacity. A typical flight would last for two and a half hours, enabling up to 30 parabolas to be performed per flight. By the 1990s, the A300 was being heavily promoted as a cargo freighter. The largest freight operator of the A300 is FedEx Express, which has 65 A300 aircraft in service as of May 2022. UPS Airlines also operates 52 freighter versions of the A300. The final version was the A300-600R and is rated for 180-minute ETOPS. The A300 has enjoyed renewed interest in the secondhand market for conversion to freighters; large numbers were being converted during the late 1990s. The freighter versions – either new-build A300-600s or converted ex-passenger A300-600s, A300B2s and B4s – account for most of the world freighter fleet after the Boeing 747 freighter. The A300 provided Airbus the experience of manufacturing and selling airliners competitively. The basic fuselage of the A300 was later stretched (A330 and A340), shortened (A310), or modified into derivatives (A300-600ST Beluga Super Transporter). In 2006, unit cost of an -600F was $105 million. In March 2006, Airbus announced the impending closure of the A300/A310 final assembly line, making them the first Airbus aircraft to be discontinued. The final production A300, an A300F freighter, performed its initial flight on 18 April 2007, and was delivered to FedEx Express on 12 July 2007. Airbus has announced a support package to keep A300s flying commercially. Airbus offers the A330-200F freighter as a replacement for the A300 cargo variants. The life of UPS's fleet of 52 A300s, delivered from 2000 to 2006, will be extended to 2035 by a flight deck upgrade based around Honeywell Primus Epic avionics; new displays and flight management system (FMS), improved weather radar, a central maintenance system, and a new version of the current enhanced ground proximity warning system. With a light usage of only two to three cycles per day, it will not reach the maximum number of cycles by then. The first modification will be made at Airbus Toulouse in 2019 and certified in 2020. As of July 2017, there are 211 A300s in service with 22 operators, with the largest operator being FedEx Express with 68 A300-600F aircraft. Variants A300B1 The A300B1 was the first variant to take flight. It had a