To say a plane became "invisible"because it turned its transponder off is fantasy.
Confirm this by visiting the Canadian Defense website again,
"Canada-United States Defense Regulations."
http://www.dnd.ca/menu/canada-us/bg00.010_e.htm
or
http://www.Public-Action.com/911/norad
"NORAD uses a network of ground-based radars, sensors and fighter
jets to detect, intercept and, if necessary, engage any threats to
the continent."
Transponders help to filter out all identifiable aircraft for NORAD
and allow them to focus on those craft that are unidentified. An
aircraft flying without a transponder gets special attention. NORAD
must have known when each of the transponders in the four "suicide"
jets was turned off, and must have known immediately. At all times,
NORAD must have known the location of each of the four planes.
The Christian
Science Monitor says of Flight 11:
"Shortly afterward, as aircraft (sic) was making its turn toward New
York City, the plane's transponder was turned off. With its
transponder off, its altitude became a matter of guesswork for the
controllers, although the plane was still visible on radar ..."
http://www.csmonitor.com/2001/0912/p1s1-usju.html