Based on the available information, it would be just as easy to navigate a line of latitude as it would a meridian.
All latitudes are small circles (except the equator which is a great circle) and all meridians are semi great circles - which when considered with their anti meridians make great circles. All meridians converge at the poles and, all lines of latitude and longitute intersect at 90 degrees, therefore any aircraft flying along a meridian or line of latitude will be navigating a (true) cardinal direction becasue they are following a line of constant direction (A Rhumb Line). In practical terms Rhumb Lines are much easier to navigate manually than great circles due the constantly changing direction which is a major characteristic of great circle navigation. So basically just take yer pick - Lat or Long!
Hope this helps
