Experience shows that it is easier to remove the MAC 550 from its flightcase if the head is locked in the vertical position. This prevents the head from tilting inside the case, which can make it impossible to pull out. On some early MAC 550s, however, the head locks at a slight angle to vertical, which makes them hard to remove from the flight case. These fixtures should not be locked.
If the head does get stuck, lower the fixture to allow the head to swing back to the vertical position.