Ukraine has been hard hit by the global recession, and this has led, this year, to a reduction of the defense budget by a third. This means Ukraine is now spending about one percent of GDP on defense. That's very low by world standards, especially for a force of 150,000 troops. Training has been cut drastically, which has been particularly hard on the air force, which has over 800 aircraft. This is large for a country the size of Ukraine (46 million people).
The large size of this aircraft fleet is an accident of geography. Before the Soviet Union dissolved in 1991, it had a disproportionate chunk of its air force in what is now Ukraine, held in readiness for the war with NATO that never happened. But when the Soviet Union dissolved, it was agreed that each of the new fourteen new nations would keep whatever Soviet assets (including military stuff) that was on its territory. So Ukraine ended up suddenly having the second largest air force in Europe. But the Ukrainians knew they could not maintain several thousand aircraft, so they retired, junked or sold off most of them. The ones they now have are the most modern models (Su-27s and MiG-29s). But even these 300 aircraft (similar to American F-15s and F-16s) are too expensive to maintain, especially with this year's budget cut, and more cuts next year.
So the air force is going to put many, if not most, of its combat aircraft into storage, and dismiss many of its 50,000 personnel. That way, the remaining aircraft can be kept operational, and the pilots trained.