Indonesian investigators have recommended that the country's civil aviation authority review its approval process for aircraft modifications, after finding deficiencies in a converted Aviastar British Aerospace 146-300 destroyed after hitting terrain last April.
The operator modified the passenger aircraft in September 2008 to create a combi version capable of transporting 42 passengers plus a freight load. The aircraft's manufacturer had not issued an approved combi modification for the type, and the changes were approved by the Indonesian DGCA.
But a National Transportation Safety Committee investigation into the fatal terrain strike, during a go-around at Wamena, found that the DGCA approval "did not appear to consider all appropriate matters relating to airworthiness and safety relating to that configuration change".
It points out that the modified aircraft had no weight and balance trim sheet, and that the operator was using an incorrect version for the fatal flight. The centre-of-gravity chart used was not designed for the combi variant but a 110-passenger configuration.
The NTSC found that the planned landing weight at Wamena was 38,319kg but that modification reduced the maximum design landing weight from 38,328kg to 37,008kg.
This meant that the aircraft was "not being operated within the maximum weight limitations" for the DGCA-approved modification, says the NTSC.
While the deficiencies did not contribute to the accident, the investigation has recommended that the DGCA review the processes by which it approves modified aircraft.
It states that the review should include a range of issues, such as type certificate data, weight and balance charts, loading manuals, airworthiness, maintenance and fire-detection.