NMAC BTWN A GOV HELI TRANSITIONING NBOUND THROUGH THE BASE LEG AREA OF THE RWY AT 400 FT AGL AND AN SMA LOW WING ON A CLOSE-IN BASE LEG TO THE RWY TO WHICH THE RPTR WAS XING THROUGH.

Narrative

I WAS IN VNY SURFACE AREA AND I WAS TRANSITIONING IN A NE DIRECTION INTO WHP AIRSPACE. WHP TWR WAS CLOSED AT THE TIME SO I BROADCAST ON 135.0 THAT I WAS 1 MI S OF THE I-5 FREEWAY AND I WAS GOING TO PROCEED NBOUND ACROSS THE ARPT. AN ACFT BROADCAST THAT HE WAS L DOWNWIND. I DON'T RECALL IF HE BROADCAST HIS TAIL NUMBER. AFTER HIS BROADCAST; I BROADCAST THAT I DID NOT SEE HIM. A SHORT TIME LATER; I BROADCAST THAT I WAS NBOUND XING THE APCH END OF RWY 12 AT 1400 FT. I CONTINUED NEBOUND AND APPROX 15 SECONDS LATER; I OBSERVED A LOW WIND; SINGLE ENG AIRPLANE TRAVELING SWBOUND AT APPROX 1300 FT. HE WAS APPROX 300 FT E OF MY COURSE. HE DID NOT BROADCAST THAT HE WAS FLYING A MODIFIED PATTERN NOR DID HE BROADCAST THAT HE HAD TURNED TO HIS BASE LEG. IF HE CONTINUED ON HIS RTE HE WOULD HAVE CROSSED THE RWY 12 CTRLINE APPROX 200-400 YARDS N OF THE THRESHOLD. I DO NOT KNOW IF THE ACFT LANDED AT THE ARPT OR LEFT THE AREA. I DID NOT HEAR ANY OTHER BROADCASTS FROM HIM. ALTHOUGH I DID NOT SEE THE AIRPLANE WHEN IT WAS L DOWNWIND; I HAD NO INTENTION OF XING THE L DOWNWIND RTE. IF THE OTHER PLT HAD BROADCAST HIS TURN TO BASE LEG I MAY HAVE BEEN ABLE TO VISUALLY ACQUIRE HIS POS. IF THE OTHER PLT HAD BROADCAST THAT HE INTENDED TO DEVIATE FROM THE ESTABLISHED PATTERN I WOULD HAVE AVOIDED THE AREA. INSTEAD HE DID NOT FLY OVER THE 118 FREEWAY FOR HIS BASE LEG. HE WAS WELL INSIDE THE PATTERN ON HIS BASE LEG. THE OTHER PLT NEVER ACKNOWLEDGED MY BROADCASTS OR INDICATED IF HE SAW ME.