MOSCOW — Russian aviation authorities have grounded the country’s fleet of its newest model of civilian airliner, the Sukhoi Superjet 100. Metal fatigue, a problem usually associated with older airplanes, was discovered in the tail section of a new Sukhoi plane, the Russian regulator Rosaviatsia said Friday.

The grounding was the latest setback for the jet, which has already had a rough introduction.

The plane model is important as a test of whether Russia’s military aerospace prowess can translate into a successful civilian product. The manufacturer, Sukhoi, is best known as the maker of military jets now flying over Syria.

Most other Russian-made civilian jets, once a workhorse fleet of wide- and narrow-body planes, are flown only by regional airlines, and the aging planes are plagued by safety problems.

After the latest announcement, the Russian national airline Aeroflot, a major Sukhoi customer, on Saturday canceled 21 domestic flights.