The day after the first prototype was completed, it was trucked to Khodynka Field to begin taxiing trials. On 12 December it caught fire when a defective fuel gasket blew out; the repairs took a fortnight. Immediately before its maiden flight, a pair of experimental 23 mm Shpitalny Sh-3 autocannon replaced the NS-23s. It first flew on 8 January 1947 with Lieutenant Colonel Mikhail Ivanov, at the controls, although he was not the primary test pilot. The Yak-19 was grounded from 2 March to 18 May as it awaited a new engine. On 21 May, the afterburner was tested in flight for the first time. While the fighter was the first Soviet aircraft to be equipped with an afterburner, it was not the first one to use one in flight as the Aircraft 156 had done so over a month earlier. The second prototype first flew on 6 June and was almost identical with the first prototype. Changes included five degrees of dihedral to the horizontal stabilizer, a slightly revised vertical stabilizer and plumbing for a pair of drop tanks underneath the wingtips. This increased the fighter's total fuel load to .