Nap-of-the-earth flying was conceived by the U.S. Army to evade enemy detection of rotary wing aircraft, requiring the stressful technique of aircraft movement only inches above the ground terrain. The 100st Airborne Division (Airmobile), Fort Campbell, Ky, tested the nap-of-the earth (NOE) concept from 12 June, 1973, to 30 June, 1974, flying 3267.6 h in NOE training. Four aircraft incidents occurred during this training period, with three of these taking place prior to 1 March, 1974. At that point, after 59.2% of the total hours had been flown, NOE pilot training was curtailed from 8 h/d to 4 h/d. Objective and subjective data infer that pilot (crew) rest and the length of the flying day are important factors in the safety of NOE flying.