The Syrian Air Force has suffered major losses in the last year, as the aircraft and helicopters were unleashed on rebels (and civilian supporters) and took a beating. Of the 370 usable fixed wing war planes the Syrian Air Force had two years ago, about half are now out of action because of combat losses or wear and tear. Nearly two-thirds of the 360 helicopters are gone, for the same reasons.
Syrian Air Force losses have been heavy, with some 400 aircrew dead, captured, or missing. Nearly a hundred fixed wing and over a hundred helicopters have been lost. About half of these aircraft were captured or destroyed on the ground as rebels attacked, and often captured, air bases. The jets (and a few transports) were hit while landing and taking off, and this threat often led to airbases being abandoned, with aircraft incapable (because of damage or lack of spare parts) of flying out being destroyed or just left behind. The rebels have about a dozen flyable helicopters and some helicopter pilots have defected, but there is not really a rebel “air force” just yet.
The air force is rapidly disappearing because of combat and operational (accidents and poor maintenance) losses. At this point the government has nothing to lose and simply regards the remaining aircraft as similar to diminishing ammo supplies. Use it or lose it to advancing rebels.