Indonesian investigators have determined that both power levers on a Merpati Nusantara Xian Aircraft MA60 had been retarded to the beta range before the aircraft landed hard at Kupang’s El Tari airport and broke up.

Beta mode is a critical throttle setting on turboprops, below flight-idle, which is normally used only for ground manoeuvring.

Selecting this setting during flight can be extremely hazardous, and the MA60 has an electromagnetic locking mechanism designed to prevent the pilots’ inadvertently moving the throttle levers below flight-idle.

But the National Transportation Safety Committee says the power lever lock was opened during the approach. While this was consistent with the operator’s approach checklist, it was not part of the airframer’s operating manual.

The carrier had experienced earlier problems with locking systems failing to open after landing, and had chosen to revise the checklist in 2008.

But investigators state that they could not find “any evidence” of a safety assessment or risk analysis associated with the revision.

While the aircraft also has a mechanical stop-slot, which is also designed to prevent beta-range selection, the inquiry says the first officer, who was flying, also overrode this system on approach – possible to avoid a delay in selecting ground-idle after landing.

Flight-data recorder information showed that the left-hand power lever entered the beta range at a height of 112ft with the right-hand lever following at 90ft.

Simulations determined that, with both levers below flight-idle, the resulting propeller pitch angle would cause significant drag. The aircraft would lose lift and descend rapidly.

Investigators state that the flight-data recorder showed the aircraft had undergone “several high vertical speed events” less than 1,000ft above ground as it made its approach. The aircraft’s descent rate increased to nearly 1,300ft/min, it says, during this critical period.

The Merpati MA60 struck the runway with an impact of almost 6g, separating the entire wing structure from the fuselage and causing the cabin section to fracture. It slid for just over 200m before coming to rest.

Five of the 50 occupants were seriously injured during the accident on 10 June 2013.

Source: Cirium Dashboard