Near midair collision between air carrier large transport on approach and GA small aircraft.

Attributes
ACN	84812
Time
Date	198803
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : sea
State Reference	WA
Altitude	msl bound lower : 1500 msl bound upper : 1800
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : sea
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent : approach
Route In Use	approach : visual
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Aircraft, Low Wing, 2 Eng, Retractable Gear
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 210 flight time total : 4900 flight time type : 210
ASRS Report	84812
Person 2
Function	flight crew : single pilot
Qualification	pilot : private
Events
Anomaly	conflict : nmac non adherence : published procedure
Independent Detector	other flight crewa
Resolutory Action	none taken : insufficient time
Consequence	faa : reviewed incident with flight crew
Miss Distance	vertical : 300
Supplementary
Primary Problem	Flight Crew Human Performance

Narrative:

We were cleared the bay visual at sea (16R). On glide slope . Abeam the north end of the runways at boeing field (just inside the 'park' outer marker), we passed directly over a twin small aircraft I immediately asked tower if they had seen the twin on radar. They did not reply, and I asked again. Tower said they did not see the traffic. I was surprised. So I called tower, on the ground and asked why they did not see our traffic. The tower supervisor said that ATC's radar had difficulty picking-up traffic if they were low-level and in our relative position on final. I think that sea has all the ingredients for a midair collision: inadequate radar coverage (insofar as low level VFR traffic is concerned). VFR corridor underlying the floor (1600' MSL) of the TCA. In fact, it underlies the glide slope by only approximately 200 VFR traffic departing nearby boeing field.the field is adjacent to the 16L and 16R approach south to sea. No apparent coordination between towers of either airport where boeing field departures conflict with aircraft making approach to seam. Recommend: lower TCA to surface (vice 1600') all along the 16R ILS final sequence. Get better radar coordination between tower to prevent potential conflicts.

Original NASA ASRS Text

Title: NMAC BETWEEN ACR LGT ON APCH AND GA SMA.

Narrative: WE WERE CLRED THE BAY VISUAL AT SEA (16R). ON GLIDE SLOPE . ABEAM THE NORTH END OF THE RWYS AT BOEING FIELD (JUST INSIDE THE 'PARK' OUTER MARKER), WE PASSED DIRECTLY OVER A TWIN SMA I IMMEDIATELY ASKED TWR IF THEY HAD SEEN THE TWIN ON RADAR. THEY DID NOT REPLY, AND I ASKED AGAIN. TWR SAID THEY DID NOT SEE THE TFC. I WAS SURPRISED. SO I CALLED TWR, ON THE GND AND ASKED WHY THEY DID NOT SEE OUR TFC. THE TWR SUPERVISOR SAID THAT ATC'S RADAR HAD DIFFICULTY PICKING-UP TFC IF THEY WERE LOW-LEVEL AND IN OUR RELATIVE POSITION ON FINAL. I THINK THAT SEA HAS ALL THE INGREDIENTS FOR A MIDAIR COLLISION: INADEQUATE RADAR COVERAGE (INSOFAR AS LOW LEVEL VFR TFC IS CONCERNED). VFR CORRIDOR UNDERLYING THE FLOOR (1600' MSL) OF THE TCA. IN FACT, IT UNDERLIES THE GLIDE SLOPE BY ONLY APPROX 200 VFR TFC DEPARTING NEARBY BOEING FIELD.THE FIELD IS ADJACENT TO THE 16L AND 16R APCH S TO SEA. NO APPARENT COORDINATION BETWEEN TWRS OF EITHER ARPT WHERE BOEING FIELD DEPS CONFLICT WITH ACFT MAKING APCH TO SEAM. RECOMMEND: LOWER TCA TO SURFACE (VICE 1600') ALL ALONG THE 16R ILS FINAL SEQUENCE. GET BETTER RADAR COORDINATION BETWEEN TWR TO PREVENT POTENTIAL CONFLICTS.

Data retrieved from NASA's ASRS site as of August 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.