CESSNA 172, N738TV, & CESSNA R182, N4445R, WERE BEING OPERD BY CONGRESSIONAL AIR AT MONTGOMERY COUNTY AIRPARK. THE 172 WAS RTRNG TO THE ARPT AT AN ALT OF 2500 FT DRG A DUAL TRNG FLT. THE R182 HAD DEPD FROM BALTIMORE & WAS ALSO INBOUND TO THE ARPT, BUT AT 1500 FT. THE CFI IN THE 172 ADZD THE R182 PLTS THAT HE WOULD CIRCLE A LANDFILL NEAR THE ARPT, THEN FOLLOW THE R182 TO THE ARPT. THE R182 PLT RPRTD HE SAW THE 172 PASSING IN FRONT OF HIS ACFT IN THE OPPOSITE DRCTN (AT A DSTC OF ABT 1/2 TO 3/4 MI); 'AFTER A MINUTE' HE LOOKED OUT THE RGT WINDOW & SAW THE 172 CONVERGING IN A DSCNDG RGT TURN. HE TRIED TO AVOID A COLLISION, BUT WAS UNABLE. THE 172 WENT INTO AN UNCTLD DSCNT & CRASHED; THE R182 PLT MADE A FORCED LNDG IN A WHEAT FLD. AN EXAM OF THE 172 REVEALED THAT ABT 3-1/2 FT OF THE LEFT OUTBOARD WING SECTION WAS MISSING. THIS SECTION WAS FOUND EMBEDDED IN THE RGT SIDE OF THE R182'S FUSELAGE, JUST BEHIND THE ENG.