What HD said. Yes it's true, unless the helicopter operator has a Non Standard Flight authorisation to carry out a specific task, and is able to alight clear of the built up area in the event of a power unit failure. The helicopter routes follow the Thames from Barnes to the Isle of Dogs. In the event of a power unit failure, the pilot is supposed to ditch into the river. Whether he would or not is a different matter. Twin engined helicopters can follow more direct routeings depending on other conflicting traffic. Common routeings for these are via Brent, Alexandra Palace and Crystal Palace, or due South from Battersea Heliport.