Swiss investigators have given an insight into the painstaking work being conducted to reconstruct the flightpath of the Junkers Ju 52 which fatally spiralled into a mountain a year ago.
This work has included difficult attempts to establish wind conditions at the time of the accident, which occurred near Piz Segnas in the Alps. None of the 20 occupants on the Ju-Air aircraft survived.
Owing to the absence of a flight-data or cockpit-voice recorder, investigation authority SUST has been forced to use other fragments of evidence to determine the Ju 52's manoeuvres.
SUST says more than 40 mobile phones, digital cameras and memory cards – as well as other potential data-storage items, from both passengers and crew – were secured at the accident site.
"Most of the found electronic components were heavily damaged and could not be read directly," it states, adding that it sought assistance from French counterpart BEA to retrieve data.
"In the case of some of the damaged components, elaborate procedures made it possible to obtain video and audio recordings of the accident flight and the flight of the day before."
Attempted recovery and reconstructive work on some of the more severely-damaged items is "still ongoing", it adds.
But SUST believes still photographs and video footage, as well as radar information, can be combined with other crucial data sources to rebuild the accident sequence.
The inquiry has used three-dimensional laser scanning and topographical modelling to recreate the valley terrain south-west of Piz Segnas.
It has also scanned a sister Ju 52 to provide an accurate model and position it to match photographs of the ill-fated aircraft taken from the ground during its flight.
"This [aircraft] model can also be used to evaluate images from inside the [Ju 52] to determine the flightpath," says SUST.
"The existing still images and video material should make it possible to determine the positions of the accident aircraft in space, its attitude relative to the terrain and its speed relative to the ground for the critical phase of the accident flight."
SUST adds that audio data might provide further information about the condition of the aircraft's three engines.
But the inquiry is also trying to examine the aerodynamic performance, including the attitude and true airspeed, of the Ju 52 – not only by analysing weight and balance but by assessing small-scale atmospheric movements and disturbances in the valley.
"The wind flows in this valley will be simulated using a sophisticated model, with the real wind and temperature data being incorporated as boundary values," says SUST.
It will take measurements of atmospheric pressure, wind speed, humidity and temperature with a weather station on a ridge in the vicinity of the crash, over the course of several weeks, in order to validate the simulation.
Light-detection and ranging systems will record three-dimensional flow conditions in the area through which the aircraft travelled shortly before entering its spiral dive.
But SUST warns: "The technical and logistical challenges mean that the success of these measurements cannot be guaranteed."
It points out that weather conditions "comparable" to those on 4 August 2018, the day of the crash, must occur during the midsummer measuring period in order for the measurements to be useful in explaining the accident.
SUST is hoping to complete its inquiry in the first quarter of 2020.