Animate a child object in local space

I’m trying to make a trap door open on a tilted plane. Can’t seem to animate the door swinging open on just it’s local y axis. Since it’s parented to the tilted plane, it deviates from it’s own y rotation. I tried a constraint, but I don’t think I’m using it correctly.

Any help would be appreciated.

Thanks!

you may have to post an example file for us to see …