C560 ENCOUNTERS HELI TFC WHILE LNDG AT FDK DURING MARGINAL WX CONDITIONS.

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WE WERE CLRED FOR A GPS RWY 5 APCH INTO FDK ARPT. THE WX AT THE TIME OF THE APCH WAS AT MINIMUMS. WE MADE GND CONTACT AT APPROX 420 FT AGL. THE CEILING WAS RAGGED; IT WAS RAINING AND FOGGY. WHEN WE MADE VISUAL CONTACT WITH THE RWY; WE NOTICED A ROTATING BEACON ON THE FIRST 1/3 OF THE RWY. I DETERMINED IT TO BE A HELI. I COULDN'T TELL IF HE WAS HOVERING OR WHAT HE WAS DOING. I WAS ABOUT TO MAKE A GAR MISSED APCH WHEN THE HELI WENT STRAIGHT UP AND OUT OF SIGHT. I WENT AHEAD AND MADE A NORMAL LNDG. AFTER LNDG; I SAW THE HELI CIRCLING AT THE EDGE OF THE ARPT. AFTER PARKING; THE 2 PLTS FROM THE CHOPPER APCHED ME AND ASKED TO DISCUSS WHAT HAD HAPPENED. DURING THE DISCUSSION; THEY SAID THEY WERE OUT THERE PRACTICING. AT THAT TIME; I EXPLAINED TO THEM THE WX WAS IFR. THEY CLAIMED THE CEILING WAS 1900 FT. AS I SAID EARLIER; I BROKE OUT AT MINIMUMS. I EXPLAINED THAT THEY SHOULDN'T BE PRACTICING ON THE ONLY INST RWY IN IFR CONDITIONS. IF THEY NEEDED TO PRACTICE; THERE WAS AMPLE ROOM ON THE INACTIVE RWY. I THINK THEY USED POOR JUDGEMENT IN THEIR DECISION TO PRACTICE WITH SUCH POOR WX CONDITIONS. IF I HAD DECIDED TO GO AROUND; THEY COULD HAVE PULLED STRAIGHT UP INTO MY ACFT.