A.  There are a few reasons why some flights might not be trackable.  Number 1, only flights that are on an active IFR flight plan are tracked and displayable.  VFR flights or any flight not on an IFR flight plan do not have tracking information available.  Number 2, some aircraft owners have specifically requested that their tail numbers not be listed in public tracking mechanisms such as this one.  In those cases, those aircraft tail numbers are filtered out and not displayed.  And lastly, on occasion, some data is missed on our traffic data, and flights fail to register with our systems.  This last item is infrequent and we try our best to insure that dropped data is kept to a minimum level.

Q.  I can't track a particular commercial flight that I'm looking for.  Is it blocked?

A.  Nope... in general, commercial flights are not blocked.  There are a number of reasons it might not be available at the time you're trying to track it.  1) It might be that you have the flight number/three letter ID incorrect.  2) It might not have taken off yet.  3) If the aircraft isn't in US airspace, it won't be trackable. (Unless you have access to the London data).  4) You might be trying to track a flight that is actually a different airline.  Some commuter airlines confuse people with their names... for example, Delta 5893 is actually a Comair flight 893.  Other airlines can do things similarly, which makes tracking them difficult.