With speed set zero and bearing set zero, you have to make sure there is a non-zero value for range or the TDC freaks out and your torpedo goes wacko. It's a computer thing, not a real TDC thing: a defect in programming the modern PC. It doesn't matter whether you get this range from fiddling with the stadimeter or fiddling with the TDC input dials.
The TDC was a computer too, you know!
