There were 6 flights into FREDERIC of 1979 that produced useful data for
the BIGD analysis. However, there were changes of altitude in two of
these flights:
   11I flight - 1 penetration at a standard pressure of 850mb, 2 at 900mb,
      with a return to 850mb for the final 4 penetrations. The 11I flight
      is therefore treated as 3 "logical" flights.
   12H1 flight - the first 2 penetrations were at 900mb, and the final 4
      were at 850mb; it is thus treated as 2 logical flights
The result of this is that the .IDX file lists 9 logical flights.
The first flight, 11F, was at 500mb, and was processed using a different
   track (FRED79_500MB.TRK) than all the others, which were processed using
   FREDERIC79.TRK.