At the moment the West is reluctant to send the most modern warplanes and attack helicopters. Firstly, they are very expensive and any training for Ukrainian pilots to use them would take a long time. Then there is the problem of routine maintenance and repairs on these very sophisticated machines and supplying them with ammunition. Another factor is that the US and its allies are wary of sending F16 or Apache helicopter as if such machines were shot down it would very embarrassing and the fear of this technology falling into Russia hands. Then again never say never. Warplanes maybe sent in the future if Ukraine's position on the battlefield gets significantly worse.