The decay of the free neutron into a proton, electron, and antineutrino is the simplest beta decay system. The electron-antineutrino correlation (a-coefficient) is one of several important experimental parameters of neutron decay. Together these can be used to determine the weak decay couplings GA and GV, measure the CKM matrix element Vud, and conduct precision low energy tests of physics beyond the Standard Model. The aCORN experiment uses a novel “wishbone asymmetry” method that, unlike previous experiments, does not require detailed proton spectroscopy. The first aCORN run in 2013–2014 produced an improved measurement of the neutron a-coefficient. The second run in 2015–2016 on the high-flux beamline NG-C at the NIST Center for Neutron Research collected a ten-times larger data set and now has a preliminary result. Details and results of the experiment will be presented.