IIRC they need to fly the cold war jets like this - below radar - as their electronic capabilities are too bad, so a more modern russian jet could launch a missile at them just before the old Ukrainian jet sees a threat.
That is one of the reasons Ukraine urgently needs more modern material like enough numbers of F-16. While also being not the most modern jets those at least have some decent radar capabilities and can also be used as interceptors for air defense.
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