Complex sentence: The exercise was canceled when the first plane struck the World Trade Center, and the E- 4B, operating under call sign "Venus 77", which was waiting on the ramp at Andrews AFB, requested emergency clearance and immediately took off.
Split into simpler sentences: The exercise was canceled when the first plane struck the World Trade Center. The E- 4B was waiting on the ramp at Andrews AFB, then requested emergency clearance and immediately took off.