Final answer:
An aircraft is clear of the runway when all of its parts, including the tail, have crossed the hold line, making it safe to contact ground control.
Step-by-step explanation:
You are considered clear of the runway when all parts of the aircraft have crossed the hold line. The hold line is often painted on the pavement, indicating where an aircraft must stop to be clear of the active runway. Only once the entire aircraft, including the tail, has passed this line, you may contact ground control as instructed by the tower. This is a safety measure to ensure that the runway is completely unobstructed for other aircraft that may be landing or taking off.