 The third prototype, designated as the VT-13 and inscribed with "A Gift for the XVIth Congress of the Party", was powered by a Soviet-built M-15 engine with a NACA cowling, but this proved to be unreliable and was not put into production.The second prototype passed its State acceptance trials on 13 August 1931 and was ordered into production a month later on 13 September.One problem noted during the trials was a tendency to make an uncontrolled 180° turn when landing in light winds.