NEW DELHI, June 26 -- Plane maker Hindustan Aeronautics Limited (HAL) is struggling to pinpoint and fix a lingering flaw in the Dhruv advanced light helicopters operated by the navy and coast guard, and a question mark hangs over when the locally produced choppers - grounded for almost six months now - will return to service, officials aware of the matter said on Wednesday. The military's advanced light helicopter (ALH) fleet was grounded following a fatal coast guard crash at Porbandar in Gujarat on January 5, but the army and air force choppers were declared airworthy after comprehensive safety checks on May 1. HAL has instrumented two ALHs - one each from the navy and coast guard - to gather critical data on the performance of the helico...