Narrative:

Navy helicopter was under gca control at NAS willow grove, PA. We were on base leg heading 060 at 2000 ft MSL at 90 KTS. Gca advised us of traffic at 11 O'clock and 2 mi. We naturally looked and saw nothing (we did note traffic later at 4000- 5000 ft over 3 mi in that position heading away). We were turned onto 'facility' at 350 degree heading and were still at 2000 ft when we received another traffic call at 9 O'clock. I noted traffic at 8-9 O'clock at 'about' our same altitude. We were then 'on GS' and began descent. With the distraction I may have stayed 'slightly above GS' but was about 4 mi from landing at 1700 ft when gca asked if we 'had the traffic at 9 O'clock'. Looking left (I was in the left seat) I noted that traffic now at less than a mi at our altitude in my estimation. I lowered the collective watching the traffic close us and pass over our position with 300-400 ft of clearance maximum (aircraft passed 50 ft behind us). Having just completed a bay formation training flight I feel confident in my closure and distance estimates. Had I not reacted, I feel there would have been a midair accident. After landing I contacted the TCA controller and tower supervisor. Tower indicated the traffic was out of northeast philadelphia airport VFR and had us 'in sight'. They noted no mode C altitude readout. If the small aircraft had us visually why didn't they climb? Why didn't they change headings to pass well behind us so as not to conflict with our ground controled approach? 1) ensure better coordination between northeast philadelphia tower and NAS willow grove to ensure adequate separation when operating runway 33 operations. 2) require adequate check of mode C in TCA by 'pilots' prior to takeoff. 3) require radar control of all aircraft out of northeast philadelphia when they launch north into runway 33 at NAS willow grove. 4) a lot of traffic on a clear night can become confusing. Things farther away could be confused with closer traffic calls. City light can confuse low traffic. I don't think the small aircraft had us at all.

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Original NASA ASRS Text

Title: MIL HELI ON GCA APCH HAS NMAC WITH SMA.

Narrative: NAVY HELI WAS UNDER GCA CTL AT NAS WILLOW GROVE, PA. WE WERE ON BASE LEG HDG 060 AT 2000 FT MSL AT 90 KTS. GCA ADVISED US OF TFC AT 11 O'CLOCK AND 2 MI. WE NATURALLY LOOKED AND SAW NOTHING (WE DID NOTE TFC LATER AT 4000- 5000 FT OVER 3 MI IN THAT POS HDG AWAY). WE WERE TURNED ONTO 'FAC' AT 350 DEG HDG AND WERE STILL AT 2000 FT WHEN WE RECEIVED ANOTHER TFC CALL AT 9 O'CLOCK. I NOTED TFC AT 8-9 O'CLOCK AT 'ABOUT' OUR SAME ALT. WE WERE THEN 'ON GS' AND BEGAN DSCNT. WITH THE DISTR I MAY HAVE STAYED 'SLIGHTLY ABOVE GS' BUT WAS ABOUT 4 MI FROM LNDG AT 1700 FT WHEN GCA ASKED IF WE 'HAD THE TFC AT 9 O'CLOCK'. LOOKING L (I WAS IN THE L SEAT) I NOTED THAT TFC NOW AT LESS THAN A MI AT OUR ALT IN MY ESTIMATION. I LOWERED THE COLLECTIVE WATCHING THE TFC CLOSE US AND PASS OVER OUR POS WITH 300-400 FT OF CLRNC MAX (ACFT PASSED 50 FT BEHIND US). HAVING JUST COMPLETED A BAY FORMATION TRAINING FLT I FEEL CONFIDENT IN MY CLOSURE AND DISTANCE ESTIMATES. HAD I NOT REACTED, I FEEL THERE WOULD HAVE BEEN A MIDAIR ACCIDENT. AFTER LNDG I CONTACTED THE TCA CTLR AND TWR SUPVR. TWR INDICATED THE TFC WAS OUT OF NE PHILADELPHIA ARPT VFR AND HAD US 'IN SIGHT'. THEY NOTED NO MODE C ALT READOUT. IF THE SMA HAD US VISUALLY WHY DIDN'T THEY CLB? WHY DIDN'T THEY CHANGE HDGS TO PASS WELL BEHIND US SO AS NOT TO CONFLICT WITH OUR GND CTLED APCH? 1) ENSURE BETTER COORD BTWN NE PHILADELPHIA TWR AND NAS WILLOW GROVE TO ENSURE ADEQUATE SEPARATION WHEN OPERATING RWY 33 OPS. 2) REQUIRE ADEQUATE CHK OF MODE C IN TCA BY 'PLTS' PRIOR TO TKOF. 3) REQUIRE RADAR CTL OF ALL ACFT OUT OF NE PHILADELPHIA WHEN THEY LAUNCH N INTO RWY 33 AT NAS WILLOW GROVE. 4) A LOT OF TFC ON A CLR NIGHT CAN BECOME CONFUSING. THINGS FARTHER AWAY COULD BE CONFUSED WITH CLOSER TFC CALLS. CITY LIGHT CAN CONFUSE LOW TFC. I DON'T THINK THE SMA HAD US AT ALL.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.