PaulH1 has given a good, clear and simple explanation of why both a left and right hand patterns may be in use. When ATC is present this is very common including other variations such as a join on the base leg or on long final. ATCs job is to maintain a safe separation of aircraft in the circuit.
Helicopters are rarely required to fly the same circuit as fixed wing. Helicopters will preferably fly an opposite circuit tighter and much lower, say 500ft AAL when the fixed wing are at 1000ft. When noise abatement demands a pattern one side of the runway but not the other the helicopter training circuit will be safely flown inside the fixed wing circuit. The joining helicopters will rarely be required to join a fixed wing circuit pattern but rather join below the fixed wing circuit height at 90 degrees and land at the active runway midpoint or cross the active midpoint at a few hundred feet and then hover taxi to parking. Helicopter downwash is extremely dangerous to fixed wing and where the helicopters are required to land on the active threshold then ATC must ensure separation from the rotor downwash. Even the downwash from a small helicopter such as a Robinson R22 can flip a fixed wing.
Last edited by Fl1ingfrog; 24th Dec 2020 at 11:10.