Imagine flying at 4500' indicated altitude, 4500' AGL, heading 270, directly towards a steep mountain ridge that tops out at 4000 feet. Imagine the mountain being almost like a flat wall.
When can I change altitudes to >4500' and still be in compliance with 91.159? Do I have to wait until I get close to the ridge when I'll be under "3,000 feet above the surface"? Can I interpret "above the surface" in a more liberal way?
§ 91.159 VFR cruising altitude or flight level.
Except while holding in a holding pattern of 2 minutes or less, or while turning, each person operating an aircraft under VFR in level cruising flight more than 3,000 feet above the surface shall maintain the appropriate altitude or flight level prescribed below, unless otherwise authorized by ATC:
(a) When operating below 18,000 feet MSL and -
(1) On a magnetic course of zero degrees through 179 degrees, any odd thousand foot MSL altitude 500 feet (such as 3,500, 5,500, or 7,500); or
(2) On a magnetic course of 180 degrees through 359 degrees, any even thousand foot MSL altitude 500 feet (such as 4,500, 6,500, or 8,500).
(b) When operating above 18,000 feet MSL, maintain the altitude or flight level assigned by ATC.