The most likely reason for the Russian deployment is to refine the Su-57 design for production and to gain as much operational wartime experience as possible.
Russia appears to have deployed two prototype Sukhoi Su-57 PAK-FA fighters to its Khmemeim airbase in Syria.
While the reports of the stealth fighters arriving in Syria are unconfirmed, it would be highly unusual to deploy a developmental asset into a combat zone before it is ready for war. Indeed, the Russian move—if confirmed—would be the equivalent of deploying the Lockheed Martin F-22 Raptor into combat during the late 1990s or early 2000s while the jet was still in the engineering manufacturing development (EMD) phase. However, deploying a developmental jet into combat to gather operational experience and data might not be unusual from the Soviet/Russian perspective.
“This is testing in actual war. The Soviets did that,” Vasily Kashin, a senior fellow at the Center for Comprehensive European and International Studies at Moscow’s Higher School of Economics, told The National Interest.
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The main purpose of the deploying the Su-57 to Syria is to gather as much operational experience and performance data as possible on the jet’s avionics. Not only can Russia test the performance of the Su-57’s active electronically scanned array radar and electronic intelligence packages, there is an opportunity to perform some limited combat missions.
“It can carry strike weapons although it is not the main purpose,” Kashin said. “[The Russian Air Force] can also use the radar on actual foreign aircraft. It depends on how the plane designers evaluate the tests.”