The 767-300F proven in efficiency and economics. The 767 Freighter keeps ton-mile costs to a minimum with its two-person flight deck, and its twin high-bypass-ratio engines offer excellent fuel economy. The cargo airplane’s operating costs, per available ton-mile, are projected to be 20 percent lower than those of its closest competitors.
August 20, 2007. On August 20, 2007, a Boeing 737-800 operated by China Airlines departed from Taiwan, Taoyuan International Airport on a regularly scheduled flight to Naha Airport, Okinawa, Japan. Following landing and leading-edge slat retraction, a failed portion of the slat track assembly was pressed through the slat track housing and
The most obvious exterior feature of a freighter is the big cargo door. Main cargo doors are about 11 feet (3.4 m) wide and 7-10 feet (2-3 m) tall depending on fuselage diameter. Door widths are similar on most aircraft to accommodate standard loading equipment, pallets, and containers. UPS 767-300F ready for loading.

Modifications to the 737-800 airframe include installing a large cargo door, a cargo handling system, and additional accommodations for non-flying crew or passengers. The aircraft is designed to fly up to 1,995 nmi (3,695 km; 2,296 mi) at a MTOW of 174,100 lb (79,000 kg).

Boeing 737-800 (738) Maximum dimensions for an assistive device carried in the cargo hold: 27 in (height) x 51 in (width)
Boeing has developed a freighter conversion of the -800 which is dubbed 737-800BCF. It has a large cargo door in the left side, forward of the wings. The BBJ2 is the corporate jetliner version of the 737-800. It is not recognisable from the standard model. Finally Boeing builds a maritime surveillance aircraft that is based on the 737-800.
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boeing 737 800 cargo door dimensions