Refer to figure.
Rhumb lines are lines of constant track, i.e. cut the meridians at the same angle. The rhumb line track between two points having the same latitude would follow a parallel of latitude and the track would be either 090° (easterly) or 270° (westerly). 

This would not be the shortest route, which is a Great circle track, unless the latitude is the Equator. All other parallels of latitude (apart from the Equator) are only rhumb lines.

Therefore, if an aircraft is flying from Point A (032ºE) towards an easterly direction to Point B (038ºE) along the 18ºS parallel of latitude, then the aircraft is flying along a rhumb line track.

Great circles are lines, which if extended would form a circumference on the Earth and these tracks mark the shortest distance.

Meridians of longitude are lines that run from south to north between the poles. There is an infinite number, one of which is the Greenwich or Prime meridian. A meridian and its opposite on the other side of the Earth, its anti-meridian, combine to make a circumference around the Earth.

All meridians and the Equator are both rhumb lines and great circles. If you are tracking along a meridian your track would be consant either 000° (northerly), or 180° (southerly), and along the Equator either 090° (easterly), or 270° (westerly), and at the same time the shortest route would be followed.