Saving TGA with Gimp kill transparency

[perdition]

Hello All

I'm not able to save a TGA file with a transparent background, each time I'm doing it, it put instead a Black bacground.

Info:
I'm using a TGA with transparent background which is working ok , and I just try to save it with gimp (no modification).
I have disable the RLE compression
I have only three channel while I'm opening it whith Gimp (RGB)

Should I have to add another channel as an alpha ? (if yes how can I do it)
Is there something I'm missing ?