The all-composite wing box of Russia’s next-generation Irkut MC-21 narrowbody jetliner is back in testing, according to the Central Aerohydrodynamic Institute (TsAGI) near Moscow. In January the first wing box cracked under excessive loading, the institute reported, but in February the testing continued on the second specimen.

A TsAGI spokeswoman told AIN that arrival of the third wing box was postponed from January to March to allow time to introduce changes reflecting earlier test results. The second specimen is to undergo a continuous cycle test, while the third will be subjected to a wide range of trials covering structural integrity, frequency and strength.

Structural integrity and strength testing of the first specimen was intended to demonstrate mastery of the technology and its readiness for approval. The assembly was subjected to preliminary frequency and stiffness tests, and was also used in separate research examining the effects of wing damage typically seen in airline service.

The MC-21’s wing boxes are manufactured by AeroComposite of Russia in cooperation with FACC of Austria under contract from Irkut and its parent, United Aircraft. The MC-21 wing was designed by AeroComposite and its subcontractor, the Sukhoi design bureau.

The experimental prototypes are made of Prism EP2400/BNCF IMS 24K carbon-fiber reinforced plastics. Irkut chose untried “vacuum infusion” technology in 2010, hoping to avoid the higher manufacturing cost incurred by reliance on large autoclaves (as used for the Boeing 787 and Airbus A350 XWB).

TsAGI is responsible for testing and issuing approvals for flight-testing of the MC-21 development prototypes, due to start in 2015. Also, the institute leads a multi-tiered scientific research effort on building “a comprehensive database on composite [airframe] parts.” Under a contract with Boeing, the institute conducted extensive composite structure research and testing for the 787 Dreamliner project.