The NTSB has recommended that Center Weather Service Units be staffed at all times when significant weather is broadcast and that the lockout feature of ASOS broadcasts be eliminated. In its investigation of the fatal crash of an American Airlines MD-82 after it overran the end of Runway 4R during landing in a thunderstorm at Little Rock (Ark.) National Airport on June 1, 1999, the NTSB discovered that the Center Weather Service Unit had closed about two hours before the 11:50 p.m. crash. The Safety Board also said that a lockout feature of ASOS, activated at specific times after each hour to allow for reediting of data, prevented the system from issuing pertinent weather information.