The Apparent Intersect object snap is very useful in 2D, but it sometimes fails in 3D, but there are workarounds one may employ in these cases.

Show below is a Blue line that runs at an elevation of 0m and a red line that has an elevation of 1m.

It is required, from the top view, to draw a line from the apparent intersection of the two lines (at the elevation of the red line), to one of the endpoints of the Blue line.

Work in the Top View and start the line command.

Shift + RMC > Apparent Intersect

LMC Red Line

LMC Blue Line

LMC Endpoint of Blue Line and complete the line command.

Consider a yacht hull that was developed using 3D splines and onto which a spline must be developed which indicates the waterline on a horizontal plane:

First create a horizontal line that indicates the waterline and position the UCS and view so that it may be attempted to draw a 3D Spline by using apparent intersects.

Unfortunately, unlike in the simple line example above, this method fails. Instead, copy the yacht polylines and then use the trim command, making sure to select the option to project the edges:

As can be seen the perpendicular splines do not trim. Set the UCS to the Front View and work from there.

Use the trim command as before to get the desired result.

Draw a Spline (Fit) through the relevant endpoints and copy it across to the full yacht.

Alternatively a surface could have been created and the intersection of it with a plane could be found.

The surface already approximates a smoother curvature, and it can be seen below how the green line is a much better line than the cyan line.