This is from the FAA Office of Aviation Medicine:
"The number of serious errors committed by pilots dramatically increases at or above concentrations of 0.04% blood alcohol. This is not to say that problems don't occur below this value. Some studies have shown decrements in pilot performance with blood alcohol concentrations as low as the 0.025%".
This might be related to this from the NIAAA:
In a typical study of the effects of pilot impairment, aircraft pilots completed eight sessions of simulated flight between San Francisco and Los Angeles in a Boeing 727-232 simulator (16). Planning and performance errors, procedural errors, and failures of vigilance each increased significantly with increasing BAC. Serious errors increased significantly at the lowest BAC, 0.025 percent, compared with performance at 0 percent BAC.