I seem to recall reading in another thread that in addition to those aircraft lost in Crimea, a number of Ukrainian fighter pilots took their planes to defect earlier last year.
Ukraine returns refurbished combat aircraft to service
Gareth Jennings, London - IHS Jane's Defence Weekly
Ukraine has returned to service four combat aircraft that have recently undergone refurbishment work, the office of the president announced on 5 January.
A pair of MiG-29 'Fulcrum' and two Sukhoi Su-27 'Flanker' fighters were handed back to the Ukrainian Air Force (Povitryani Sily/PS) during a ceremony at Ozerne Airbase attended by President Petro Poroshenko.
In addition to the aircraft, about 150 ground vehicles and other weapons systems were handed over to the military during the event.
Having suffered from chronic underinvestment for years, the Ukrainian Air Force was placed at an instant disadvantage when Russian forces entered Crimea in early 2014. The air force's inability to respond with the assets that it did have was compounded when a number of its bases were overrun and its aircraft seized.
It is thought that 45 aircraft, including MiG-29 fighter and Aero L-39 trainer/light strike aircraft, were lost this way when the 204th Tactical Aviation Brigade at Belbek was overrun by Russian Spetsnaz troops on 21 March.
Further to this, when the Ukrainian Air Force did get involved in hostilities in a meaningful way in the east of the country, it found that this endemic underinvestment manifested itself in outdated and poorly maintained combat aircraft, as well as poorly trained pilots. Combined with a lack of combat experience and poor use of intelligence, this meant that the air force has suffered a high loss rate that continues through to today. At least 20 fighters, transports, and helicopters have been shot down in the past year.