As well as their entire fleet of 12 Su-25 ground assault aircraft.
They left behind all of their Su-30 and Su-35 air superiority fighters and also flew in at least 4 more additional Mi-28 and Ka-52 attack helicopters. Overall since January through March the number of Russian helicopters in its Syria air base went up from some 4 machines to about 14.
Thus the Russian drawdown did not only mean a reduction of Russian forces, it also meant a change in its composition. Fixed-wing assault aircraft were withdrawn completely and rotary-wing attack aircraft were brought in their place.
A Su-25 has different capabilities than an attack helicopter but their roles are the same. They are meant to provide close air support to frontline ground units. So why change one for the other?
The Su-25s the Russians had in Syria were old airframes. It is likely that in a space of six months the Russians flew so many sorties on these machines that they needed to be flown back for factory-level overhauls.
However, this can not be the reason why they were replaced by attack helicopters. The Russians could have easily flown in a new rotation of Su-25s but they opted not to.
Here is where it gets interesting. Syrian rebels were always known to possess some anti-aircraft capability including shoulder-fired anti-aircraft missiles. (Also "MANPAD" as in "man-portable air defense system".)
However, they were believed to have only a very small number of these as there was a taboo among their backers against introducing such weapons (which could be used in terrorist attacks) to the battlefield.
Recently, however, it has become apparent this taboo has been broken as two Syrian jets were brought down with such weapons this month and pictures of rebels posing with such weapons begun popping up on the internet.
So Russians either knew, or very quickly realized that risks would increase for their planes in Syria's skies. Why pull out the Su-25s?
A Su-25 is a ground attack plane similar in concept to the American A-10 Thunderbolt II, albeit the Russians really have a longer tradition of building such planes going back to the legendary Ilyushin Il-2 of World War II.
It is supposed to attack the enemy in a dive run and from a close proximity. That naturally exposes it to short-range anti-aircraft fire far more than a Su-24 or a Su-34 which generally attack from a higher attitude and further from the front line.
Thus it makes every sense for the Russians to withdraw Su-25s rather than their other planes, but why then fly in attack helicopters?
A Su-25 is a far more survivable airframe than a helicopter. It is armored, able to vacate the battlefield faster, drops down only for the attack, and most of all – can pull off high g-force evasive maneuvers that a rotary-wing can not.
Russian wonder weapon
But here is the catch. Russian attack helicopters come with the Russian MANPAD-jamming wonder system and Su-25s do not.
Russians have developed an "airborne defense system"President-S which their tests show defeats their Strela-2, Strela-3 and Igla shoulder-fired anti-aircraft missiles with ease. It makes an aircraft equipped with it practically completely immune to all such weapons except the very new ones, like the Russian Verba missile.
According to the manufacturer this has been demonstrated on the battlefield in Syria as well:
"In Syria, jamming systems mounted on the Mi-28N, were easily able to suppress the guidance systems of early examples of man-portable air defense systems at the disposal of the of terrorists, in particular, the Soviet Strela-2 and Igla-1, as well as the Chinese HN-5."Modern Russian helicopters are equipped with the President-S system of countermeasures, but so far the Su-25 bombers are not. The modernized Su-25SM3 variant will boast a similar Vitebsk-25 system but so far there are none in service.
So here is the scary thought: Russians have developed a MANPAD-jamming system which in their opinion is so effective that it makes a delicate rotary-wing like the Ka-52 more survivable than a no frills flying tank like the Su-25.
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