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.