The 1958 Sikorsky S-58 personnel helicopter made a hard landing near the Air Force's Pekanbaru base in Riau province because of engine failure.

The helicopter carried seven civilian passengers, including two Singaporeans, along with four flight officers.

'We suspected a misconduct regarding the presence of two foreigners and five passengers,' Air Force Chief of Staff Vice-Marshall Subandrio told a press conference here which was reported by Xinhua agency on Friday.

Foreign passengers travelling in an Air Force plane must get prior approvals from the Military Headquarters.

'There is neither permit nor security clearance from the headquarters so we concluded the flight has breached the procedures,' said Mr Subandrio, who is in his first weeks to command the air force.

The Air Force will 'take necessary actions against officers responsible for the misconduct according to the existing laws,' he said.

Mr Chandran, CEO of the Singapore-based Chemoil Energy Ltd, was killed in the accident that also injured four other passengers.

It remains unclear why he and his colleague were in that helicopter.