The reliability of a vital component of the Dhruv advanced light helicopter’s transmission system has come into question after the Indian Army recorded a worrying incident involving damage to the tail drive shaft (TDS) during flying operations, forcing it to immediately issue orders for a fleet-wide one-time check to ensure flight safety, officials aware of the matter said on Friday.
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 Dhruv took place on Thursday. 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, the officials said, asking not to be named.
Aircraft maker Hindustan Aeronautics Limited (HAL), which has designed and developed the ALH, has also been informed of the development, with a request to help identify the root cause of the TDS breakdown.
The reason for the TDS breakdown is not immediately known but different angles will be explored, including strict adherence to maintenance schedule, people aware of the matter said.
“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,” the Directorate General of Electronics & Mechanical Engineers (Aviation) wrote in the letter, a copy of which is with HT.
{{/usCountry}}“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,” the Directorate General of Electronics & Mechanical Engineers (Aviation) wrote in the letter, a copy of which is with HT.
{{/usCountry}}The photographs of the broken parts are appended to the letter.
“In order to ensure flight safety, it is directed that a One Time Check is to be carried out on all ALH heptrs (helicopter) for any abnormality in the TDS,” it said. To be sure, the ALH fleet has not been grounded, but each helicopter will be cleared for flying only after the one-time check.
The development is significant as the army and air force ALHs were declared airworthy after comprehensive safety checks only on May 1, after being grounded for months following a fatal coast guard crash at Porbandar in Gujarat on January 5. The navy and coast guard ALHs are still grounded.
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.
The 300 ALHs operated by 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 navy and the coast guard together operate around 30 ALHs.
The army’s September 4 communication has laid down the checks that must be conducted on priority. These include checks on TDS bearing and elastomeric bushes, check for crack on tail boom top face sheet at three different stations, and visual inspection of the TDS bracket at three stations for slackness and shearing of rivets by using a 10X magnifying glass.
The army has requested HAL to “communicate modalities of root cause analysis of the incident on IA-1134 on top most priority.”
HAL has instrumented two ALHs --- one each from the navy and coast guard --- to gather critical data on the performance of the helicopter’s integrated dynamic system, including the transmission system, gearbox and rotor hub, as well as test the loads some systems can withstand in different operating conditions.
The problem with the navy and coast guard appears to be linked to sustained operations in a maritime environment. HAL is trying to determine the exact problem and fix it. The DIC will scrutinise the data compiled by the Rotary Wing Research & Design Centre and recommend the next steps.
HAL had earlier broadened the scope of the investigation after the Porbandar crash by involving Bengaluru-based Indian Institute of Science (IISc) to perform fatigue testing of a critical part in the helicopter’s transmission system to get to the bottom of the matter.
This was after a high-powered panel found that a swashplate fracture caused the January 5 coast guard ALH crash in which two pilots and an aircrew diver were killed, but the reason for the breakdown of the critical component that compromised the ability of the pilots to control the helicopter’s motion could not be determined.
A fleet-wide inspection conducted after the January 5 crash revealed that some navy and coast guard ALHs were facing the same problem --- cracks in the swashplate assembly.
The ALH underwent a design review followed by a replacement of a defective control system only in 2023-24. The helicopter has been involved in around 15 accidents during the last five years, putting the spotlight on its safety record.
The comprehensive design 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 too after a raft of accidents called into question its flight safety record.