If you are talking about airplanes then it varies on the airport, But in football the "touchdown-zone" is normally refered to as the "end-zone" and is ten yards.

== == == == When the ball is advanced past the end zone line it is called a touchdown. It is worth 6 points. A touchdown can be scored on offense or defense by any player. A touchdown can also be scored if there is a fumble in the end zone, and the team that scored on that end zone recovers the fumble.

No, it is an interception and a defensive touchdown.

the end zone

No, such a scenario would result in a safety. However, if you fumble the ball in your own end zone, and the opposing team recovers it, that would yield a touchdown.