The short-to-medium-range 727 jet, which first flew in 1963, was the only Boeing-built trijet in its time. It was designed to operate out of small airports with shorter runways than were used by 707s. The 131-passenger trijet also was the first Boeing commercial jetliner to use an auxiliary power unit (APU), a small gas turbine that eliminated the need for ground power or starting equipment at small airports when the main engines were shut off. All 727s carried self-contained, hydraulically operated stairs, which, combined with the APU, could make the airplane independent of ground equipment. The 727 jet also was built as a freighter and as a ''quick change'' version, which airlines could convert from a passenger transport to a freighter, or a combination of both, as they chose.