Assume an audio tape of a lecture, 30 mins per side. Auto reverse. With over 5 mins left on Side One, given instruction on tape to Fast Forward to end of Side One. User busy elsewhere so tape plays to end of Side One then reverses. Nothing on last minutes of tape Side One. Audacity picks up the dead air. No reason to keep the dead air portion in middle of recording. Want to delete the dead air portion. Short of playing tape, even at high speed, to start of dead air, how may we Go To, let us say, 25 mins point? By trial and error should be able in a few attempts to find where dead air starts.
Zoom out so that you can see the whole track http://manual.audacityteam.org/manual/help/manual/man/edit_toolbar.html#zoomproject
You should then be able to see the silent part (flat blue line).
Click, select, zoom in, delete as required.