To call Aliens: The Series a pilot might be going a bit too far. Although the show was commissioned by Fox TV in late 86, one of the first shows the fledgling network put money into, the producers behind Alien and Aliens Gordon Carroll,David Giler and Walter Hill were split over the decision to make a TV show of the franchise.
Julian Wall directed what is best described a mood setter to show the Fox executives, no real script had been developed to form a full episode, it was a series of sequences with no real main cast. Wall had directed 2nd Unit on Aliens for James Cameron the year before. Various FX scenes were re-used from Cameron's picture. The rumour was that H.R Giger's Alien wouldn't have been the only alien creature to appear in the TV show, as it followed Weyland-Yutani's colonization of space.
Now as far as I know this footage has only ever been viewed at a UK Alien convention in 1998. A friend of mine who's a huge Alien and Aliens fan aquired a DVD transfer of a VHS version of the footage in the early 2000's. He gratefully provided me with some grabs. From what I remember of when I see this footage it had a very TV/Video feel to it. Scenes were very boringly staged with extras stood around in some scenes looking like they have no idea why they are there. The footage makes heavy use of James Horner's Aliens score. Reports that the sets came from a variety of stock studio sets including Outland, though from what I gather this pilot was shot in the USA not at Shepperton or any other UK studio.
Visual FX are credited to Suzanne M. Benson and John Lee, both who worked on the miniatures in Aliens, though in an interview with Benson I read in an Aliens fanzine she said she had nothing to do with the show.
Who decided that this show should be made and then who decided it wasn't worth pursuing is unknown. Like the Red Dwarf USA pilot I mention below it has become a bit like a holy grail to some fans, but to most like the ring of power in the time between Isildur having the ring and Gollum owning it, it has fallen into legend and become forgotten about.