Complex sentence: In 2005, the British government announced plans to construct an airfield on Saint Helena, ready to be operational in 2010 when the current "St. Helena" reached the end of its working life.
Split into simpler sentences: In 2005, the British government announced plans to construct an airfield on Saint Helena, which is expected to be operational in 2010. At that time, the current "St. Helena" will have reached the end of its working life.