Just a month after regaining positive momentum, ­business aviation flight activity slipped into negative territory in September, falling 1.1 percent year-over-year, ­according to TraqPak data released by aviation services company Argus. Activity was down nearly across the board, though midsize jet flying did increase 3.7 percent and fractional ­flying inched up by 1.1 percent. By aircraft category, large-cabin jet activity fell the most (-5.7 percent) from a year ago, followed by turboprops (-3.7 percent) and light jets (-0.6 percent). Among operators, Part 135 flying dropped by 3.5 percent, while Part 91 activity slid slightly by 0.5 percent. Argus’s TraqPak data “is serial-number-specific aircraft arrival and departure information on all IFR flights in the U.S.”