Work on reducing this to 1,000 feet at all levels began almost immediately .
It was not until the 1990s that air data computers ( ADCs ) , altimeters and autopilots became sufficiently accurate to make this a reality .
Work on reducing this to 1,000 feet at all levels began almost immediately , but it was not until the 1990s that air data computers ( ADCs ) , altimeters and autopilots became sufficiently accurate to make this a reality .