You can only repair a helicopter on a helipad. That is oke. But when one part sticks out a litle like the tail rotor of te MI8 which is a bit bigger you cannot repair it.
I would suggest increasing the zone or when the wheels are on the helipad or in the helipad zone you should be able to repair it.
Park helicopter on helipad with broken tail rotor sticking a little bit out from the pad.
What is also a thing. the mi8 helicopter always keeps turning when the engine is running. and breaks are very soft. So on an incline it does not even hold its position. But the main thing is. The repair of the tail is because of this so difficult. You should stay close to the sweet spot to do the repair for a couple of seconds while you hold F while the helicopter is moving. This is very tedious. So why not make it like pushing vehicles. Something that just graps the repair anymation (the F to repair with progress bar) so that it doesn't matter that the heli moves a bit.