NEW DELHI, Sept. 8 -- Aircraft maker Hindustan Aeronautics Limited (HAL) on Sunday put the spotlight on the importance of maintenance for the continued airworthiness of the locally made Dhruv advanced light helicopters, adding that it was working closely with the Indian Army to ascertain the root cause of a defect in the chopper's tail drive shaft --- a vital component of its transmission system. The development came a day after HT cited an official communication and reported that the army recorded a worrying incident involving damage to the helicopter's tail drive shaft (TDS) during flying operations, forcing it to immediately issue orders for a one-time, fleet-wide safety check. "One-Time Check (OTC) is a routine maintenance practice issued subsequent to a defect, in this case, of the TDS. HAL is working closely with the Indian Army and has deputed expert teams to ascertain the root cause. HAL reiterates that maintenance aspects are critical to the continued airworthiness of helicopters and emphasises that all maintenance directives must be followed scrupulously," HAL said in a statement. The statement was issued on Sunday in response to a September 6 HT story titled 'Safety alarm triggers fresh ALH fleet check' that also highlighted that the root cause analysis will explore different angles including strict adherence to the maintenance schedule. The TDS transfers engine power to the tail rotor to balance torque of the main motor and its robustness has a direct bearing on the directional stability of a helicopter. The incident involving the army advanced light helicopter (ALH) took place on September 4. And within hours the Directorate General of Electronics & Mechanical Engineers (Aviation) issued directions for the safety check on priority to all ALH units, including those of the air force and the navy. HAL, which has designed and developed the ALH, was also informed of the development, with a request to help identify the root cause of the TDS breakdown. "It is intimated that an incident of TDS bearing mount broken at station #9A during flying has been reported on IA-1134 (helicopter tail number) of an Army Aviation squadron on 04 Sept 25. In order to ensure flight safety, it is directed that a One Time Check is to be carried out on all ALH heptrs for any abnormality in the TDS," the Directorate General of Electronics & Mechanical Engineers (Aviation) wrote in the September 4 letter, a copy of which is with HT. The workhorse ALH fleet has logged more than 4.5 lakh flying hours and has been in service for more than two decades, across the Indian armed forces, coast guard, and civil operators, the HAL statement said. "It has flown in extreme terrains, from high-altitude Himalayan regions to coastal environments. In the ongoing flood rescue and casualty evacuation operations, the Indian Army also undertook high-risk helicopter rescue operations using ALHs to evacuate stranded civilians and CRPF personnel, which stands as testimony to their trust placed in the platform," it said, calling the ALH one of its flagship indigenised platforms. The three services and the coast guard operate around 330 twin-engine ALHs. The 300 ALHs operated by the army and air force were declared airworthy after comprehensive safety checks only on May 1, after being grounded for months following a fatal coast guard ALH crash at Porbandar on January 5. Weeks ago, HAL carried out extensive tests on the ALH in a maritime setting in an attempt to pinpoint and fix a lingering flaw in helicopters operated by the navy and coast guard that have now been grounded for more than eight months following the Porbandar crash in which two pilots and an aircrew diver were killed. The navy and the coast guard together operate around 30 ALHs. The problem with the navy and coast guard choppers appears to be linked to sustained operations in a maritime environment. A swashplate fracture caused the January 5 coast guard crash, but the reason for the breakdown of the critical component that compromised the ability of the pilots to control the helicopter's motion has not yet been determined. The ALHs of the army and air force were cleared for flying duties in May based on the recommendations of the defect investigation committee (DIC) consisting of officials from the Bengaluru-based Centre for Military Airworthiness and Certification (CEMILAC), the Directorate General of Aeronautical Quality Assurance and HAL. The helicopter has been involved in around 15 accidents during the last five years, putting the spotlight on its safety record. The ALH underwent a design review followed by the replacement of a defective control system only in 2023-24. The review, which involved installing upgraded control systems on the helicopters to improve their airworthiness, came after the ALH fleet was grounded several times in 2023 after a raft of accidents....