A Mid-Air Minute The most critical times for potential mid-air collisions are three minutes after take-off and eight minutes before landing. If an aircraft doesn't appear to be moving but increases in size, the two aircraft are on a direct collision course. Most mid-air collisions occur below 8,000 feet, which is below the altitude of an airliner's flight. -- Information courtesy of Moody's Mid-Air Collision Avoidance Handbook