Inventory of pre-owned turbine and piston helicopters remains below 7 percent of the in-service fleet, making it a seller’s market, according to data compiled by Utica, N.Y.-based aviation information firm JetNet. At the end of April, the turbine helicopter inventory stood at 6.9 percent (1,160 aircraft), slightly higher than the 6.2 percent seen a year ago but still below the 10 percent that denotes market equilibrium. The number of pre-owned sale transactions for turbine helicopters increased by 13.5 percent, to 394, in the first four months compared with the same period last year. But the average number of days on the market for turbine helicopters increased by 82 days, to 314, in the first trimester compared with 2009, while the average asking price increased by 3.2 percent. Meanwhile, piston helicopter inventory inched down from 7 percent in April 2009 to 6.5 percent in the same month this year. Pre-owned sale transactions for piston helicopters decreased by 17.7 percent in the first four months versus the same period last year, though average asking prices still increased by 3.1 percent.