Investigators claim a mis-set altimeter combined with heavy fog led to the fatal crash of a Let L-410 turboprop during an attempt to land at Yirol in South Sudan.
Limited firm evidence for this conclusion has emerged from the inquiry, which had to cope with an absence of flight-recorder information.
The Ukrainian-registered aircraft (UR-TWO) had been leased from Slav-Air to Juba-based South West Aviation.
It was chartered by a company called Baby Air Tours and Travel to fly from Juba to Yirol on 9 September last year, and was transporting 21 passengers and two crew members.
South Sudan's ministry of transport indicates the aircraft approached and passed Yirol airfield from the south-east before crossing Lake Yirol – lying some 1,600m beyond the airfield – before circling clockwise, and then crashing into the western side of the lake.
Yirol airfield has a single runway, designated 03/21, which is 1,400m in length. But the inquiry says there are no navigation aids to support landings.
While the flight-recorder data was unavailable, the inquiry states that examination of the wreckage showed most of the instrument had "frozen in the impact". The altimeter read 1,780ft, it says, and other instruments indicated the aircraft was descending. The elevation of Yirol airfield is approximately 1,430ft.
Investigators state that visibility at the destination was "very poor" as a result of fog, but the pilots did not divert to Rumbek airport or return to Juba.
"The crew circled several times over the town trying to locate the runway," says the South Sudanese inquiry, the finds of which have been released by Ukrainian investigation authority NBAAI.
It claims that the crew erred by not setting the altimeter to the appropriate pressure for Yirol airfield, and consequently were flying lower than the instrument indicated before the accident.
Three passengers were the only occupants to survive the crash.
Several other matters were highlighted during the inquiry including the absence of checks on the crew by flight-safety inspectors. The crew did not check the weather briefing before departure, it says, and the operator could not provide a loadsheet.
Investigators add that the aircraft had undergone propeller maintenance, following a malfunction, a week before the accident but had not informed civil aviation regulators.
