Mmmmm ....
As Dagw00d pointed out ... the Bo105 had a dump system that allowed for all the fuel to be dumped from the 'main' tanks which left only what was still in the supply tanks at the time of the dump ... IIRC it dumped at a rate of 50-100kg/min (or something like that??) ....
I believe the same dump kit components 'could' be fitted to the BK117... but I don't think it was ever certified for use on that airframe.