Six Bulgarian Air Force MiG-29 fighter jets will have their engines overhauled in Poland, according to an intergovernmental letter of intent signed in Sofia on Friday.

Poland’s state-owned Miltary Aviation Works WZL 2, 4 will carry out the repair and upgrade of the engines of three MiG-29s by the middle of next year and a further three by end-2016, according to the Bulgarian Ministry of Defence.

No financial details of the preliminary deal were disclosed. 

The letter of intent was signed by the Bulgarian Defence Minister Nikolay Nenchev and his Polish counterpart Tomasz Siemoniak who is on a one-day working visit to Sofia. They also signed an intergovernmental military cooperation agreement which lays the legal groundwork for the overhaul of the Soviet-made fighter aircraft.

Bulgaria’s agreement with Russia on the maintenance of the Bulgarian MiG-29 fighter jets expires in September. Nenchev has said Poland would repair the fighter jets at a lower cost compared with their Russian manufacturer.

Poland can offer the full range of services relating to the maintaining of the MiG-29s’ reliability and flight-worthiness as well as carry out the modernization of the aircraft, the Defence Ministry said in a statement.

The military cooperation agreement will ensure the resolving of the most urgent problems the Bulgarian military aviation is facing, it added.

In order to maintain high level of combat readiness as a NATO member state, Bulgaria needs to have at least one fighter squadron of 12 aircraft. Currently, there are four flight-worthy MiG-29s with enough flying capacity in the Bulgarian Air Force, therefore the year 2015 is critical from the point of view of military aviation, according to the Defence Ministry. Only two MiG-29s could remain flight-worthy and with enough flying capacity after the middle of next year unless the engines of other MiG-29 are repaired.