Rare transmission system failure led to ALH crash, shows inquiry

Investigators have found that a rare failure of a critical part in the transmission system caused the crash of a coast guard Dhruv advanced light helicopter (ALH) at Porbandar in Gujarat on January 5, and a high-powered panel has been constituted to determine the reason for the breakdown of the component before steps can be initiated to fix the problem and declare the fleet airworthy again, two officials familiar with the matter said on Monday.

The ALH is unlikely to return to service soon.

The military’s fleet of around 330 locally produced ALHs, which was grounded a month ago for a thorough safety inspection following the latest accident, is unlikely to be cleared for flying for several more weeks, the officials said.

The multi-mission helicopter has been designed and developed by Bengaluru-based Hindustan Aeronautics Limited (HAL).

A detailed analysis by the Council of Scientific and Industrial Research-National Aerospace Laboratories (CSIR-NAL), Bengaluru, found that the device that malfunctioned compromised the ability of the pilots to control the helicopter’s motion, said the first official. It is most likely the swashplate assembly in the ALH’s transmission system, HT has learnt. Two pilots and an aircrew diver were killed in the coast guard crash.

A top committee has been set up to find out what caused the material failure and how to address it to achieve safe flight operations.

The defect investigation committee (DIC) will consist of officials from the Bengaluru-based Centre for Military Airworthiness and Certification (CEMILAC), the Directorate General of Aeronautical Quality Assurance and HAL, and is expected to submit its report in four weeks after which remedial action will be taken, said the second official.

The ALH has been flying for more than 20 years, and a failure of this nature has never occurred before, he said. “The DIC will determine whether the issue is related to quality, inspection or maintenance. If it’s a straightforward issue and can be fixed easily, the helicopters will be cleared to fly in batches after mandatory safety checks involving the transmission system. If not, the ALH will stay grounded longer,” the second official added.

Both officials asked not to be named.

The ALH underwent a complete design review followed by a replacement of a defective control system only in 2023-24.

The ALH’s armed version Rudra was also grounded after the January 5 crash. The army and the Indian Air Force account for more than 90 Rudra helicopters.

The ALH has been a workhorse for the three services and the coast guard. For instance, the army flew its ALHs for more than 40,000 hours last year, with the helicopters mostly operating at altitudes of more than 15,000 feet. The army operates around 90 ALHs.

The wreckage of the coast guard helicopter was flown to Bengaluru on January 15. CSIR-NAL analysed the ALH’s integrated drive system, including the transmission system, gearbox and rotor hub, before pinpointing the failure.

Flight safety hinges on a helicopter’s transmission and control systems, said Air Marshal Anil Chopra (retd), a former director general of the Centre for Air Power Studies.

“Any failure can result in a catastrophic accident. Normally, the reliability of the transmission and control systems is extremely high. Any failure requires detailed analysis of the problem so that appropriate measures can be taken to rectify it. The ALH should resume flying only after the safety issue is resolved,” Chopra added.

The ALH has been involved in around 15 accidents during the last five years, putting the spotlight on its troubling safety record.

The coast guard suspended ALH operations following an accident last September when a helicopter crashed into the Arabian Sea near Porbandar. Then too, two pilots and an aircrew diver were killed. The grounding was for a one-time check. The three services did not ground their fleets then.

The Coast Guard cleared the helicopters for flying a few weeks later, after a safety inspection involving HAL, CEMILAC and all coast guard units.

Last September’s accident, too, came after the design review that culminated in a critical safety upgrade on the military’s ALH fleet, initiated by HAL. The upgrade involved installing upgraded control systems on the helicopters to improve their airworthiness. The safety inspection after last September’s crash focused on flying controls and the transmission system. Accidents can happen due to several reasons, including technical defect, human error (aircrew), and human error (servicing).

When the coast guard grounded its fleet last September, the focus of the inspection was on the safety, security, integrity and crack detection checks of several parts, including the main drive flexible shaft and its attachments, main and tail rotor assemblies, upper and lower control systems, and roll, pitch, collective and tail rotor actuators. The inspection found nothing amiss.

The comprehensive design review came after the ALH fleet was grounded several times in 2023 too after a raft of accidents called into question its flight safety record.