The trial in Wiener Neustadt, south of Vienna, stems from an investigation into a local company, Airborne Technologies GmbH, which fits out aircraft with sensors and other equipment.
Prosecutors say that two Ayres Thrush agricultural aircraft were equipped with armour, extra tanks and a special camera that could be used for marking and illuminating targets.
They say one was sent to Malta in 2014 with Kenya as its declared destination and landed in troubled South Sudan, while the other was sent to Bulgaria in 2015.
The defendants are accused of violating Austria's law on war material by exporting such equipment without permission.
One of the defendants, a 53-year-old Australian pilot, is accused of flying the two planes across Austria's borders while the four other defendants allegedly participated in the deal.
They are Blackwater founder Erik Prince, two managers at Airborne Technologies and a trained pilot who allegedly was an adviser.
All pleaded not guilty as the trial started, the Austria Press Agency reported.
Norbert Wess, a lawyer for Prince and two other defendants, argued that the modifications made to the planes did not turn them into war material.
"We maintain with firm conviction the point of view that the categorisation is legally wrong," APA quoted him as saying.
He said all the modifications "are completely innocuous".
He described what happened as transparent export proceedings and said the first plane was always destined for Kenya but made a landing in South Sudan due to technical problems.
Oliver Felfernig, a lawyer for the two Airborne managers and the company, described the prosecutor's accusations as "pure fantasy".
The next court session is scheduled for December 14.
