Russian airframer Irkut is finalising installation of equipment on a second prototype of the MC-21-300, while three other prototypes are still undergoing construction.
Irkut has not indicated a maiden flight date for the second aircraft, however, as it continues to prepare the first flight-test MC-21 for a new round of trials.
The initial MC-21 first became airborne on 28 May and the twinjet performed nine preliminary test flights before being parked for modification.
During these flights the aircraft's stability and controllability in various wing and landing-gear configurations was analysed.
The tests also examined the accuracy of systems measuring altitude and speed parameters, and explored different modes of powerplant operation – both in flight and on the ground.
Irkut says the aircraft has since been undergoing "adjustment" of software to take into account the results of the preliminary flights.
It is also being prepared for stress analysis which involves fitting sensors to measure in-flight loading on the aircraft's structure.
"Comparison of calculated and actual loads will verify the correctness of the initial aircraft design data," says Irkut. Over 500 miniature strain gauges will be fitted to the fuselage, wing, undercarriage, and empennage.
This preparatory work is a labour-intensive exercise, it says, and takes "several weeks".
Irkut and United Aircraft have provided an update on the programme to deputy prime minister Dmitry Rogozin.
United Aircraft chief Yuri Slyusar says that the company aims to reach production levels of 70 MC-21s annually by 2024.