Preliminary findings from the fatal Junkers Ju 52 crash probe state that the aircraft entered a left turn before descending in a spiral.
None of the 20 occupants survived the subsequent collision with mountainous terrain, which occurred at 16:56 on 4 August.
Swiss investigation authority SUST has given only sparse details of the final moments of the pleasure flight over the Alps, as the aircraft travelled from Locarno to Dubendorf.
It states that the aircraft flew a north-easterly course into a basin south-west of the Piz Segnas peak, which rises to some 3,100m.
“Towards the north end of the basin, it began a left turn, which developed into a downward spiral trajectory,” it adds.
The JU-Air aircraft was “almost perpendicular” to the terrain at the point of impact, says the inquiry, with the crash site 500m south-east of the Segnaspass Alpine passage.
Switzerland’s civil aviation authority has not found any evidence of technical problems, although it has imposed precautionary operational restrictions on JU-Air’s Ju 52 flights.
Investigators have yet to detail the weather conditions at the time, although the aircraft was operating under visual conditions.