While trying to land at Kathmandu's Runway 21 in FS98 today, I realised that it's quite funny. When I set the ILS for Runway 21 and turn on final approach, the GS disappears. And when I go to the opposite direction for the approach to Runway 3, the GS for Rwy3 disappears but the GS for ILS Rwy 21 appears. How is this so? Is this due to the Runway's backcourse approach? How does backcourse approaches work?