Flight crew of an light transport air carrier aircraft descended below assigned approach vectoring altitude in response to the visual sighting of conflicting traffic.

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Attributes
ACN	256441
Time
Date	199311
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : ewr
State Reference	NJ
Altitude	msl bound lower : 5600 msl bound upper : 6000
Environment
Flight Conditions	Mixed
Light	Night
Aircraft 1
Controlling Facilities	tracon : n90
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Route In Use	arrival other enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 220 flight time total : 2200 flight time type : 220
ASRS Report	256441
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : took evasive action flight crew : returned to intended course or assigned course
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 30000 vertical : 2000
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While on radar vectors for the ILS 22L at ewr, ATC assigned a turn to approximately 110 degrees. When the turn was nearly completed, both crew members saw what appeared to be a 'heavy' aircraft within a few hundred ft of our altitude and rapidly approaching in our direction. I called ATC several times and was 'blocked' by other xmissions. Impact seemed imminent and the captain decided to descend approximately 400 ft below assigned altitude. IMC conditions were then encountered and the traffic was no longer visible. ATC then responded and replied that the nearest traffic was approximately 6 mi away and 2000 ft above us. We then immediately climbed back to our assigned altitude and saw that it was 2 aircraft positioned such that it appeared it was 1 aircraft that was very close. A remedy for these visual illusions would be to have TCASII in all operations involving airline/commuter operations. The illusion was so real that I would respond the same way in attempt to avert a collision if the scenario occurred again.

Original NASA ASRS Text

Title: FLC OF AN LTT ACR ACFT DSNDED BELOW ASSIGNED APCH VECTORING ALT IN RESPONSE TO THE VISUAL SIGHTING OF CONFLICTING TFC.

Narrative: WHILE ON RADAR VECTORS FOR THE ILS 22L AT EWR, ATC ASSIGNED A TURN TO APPROX 110 DEGS. WHEN THE TURN WAS NEARLY COMPLETED, BOTH CREW MEMBERS SAW WHAT APPEARED TO BE A 'HVY' ACFT WITHIN A FEW HUNDRED FT OF OUR ALT AND RAPIDLY APCHING IN OUR DIRECTION. I CALLED ATC SEVERAL TIMES AND WAS 'BLOCKED' BY OTHER XMISSIONS. IMPACT SEEMED IMMINENT AND THE CAPT DECIDED TO DSND APPROX 400 FT BELOW ASSIGNED ALT. IMC CONDITIONS WERE THEN ENCOUNTERED AND THE TFC WAS NO LONGER VISIBLE. ATC THEN RESPONDED AND REPLIED THAT THE NEAREST TFC WAS APPROX 6 MI AWAY AND 2000 FT ABOVE US. WE THEN IMMEDIATELY CLBED BACK TO OUR ASSIGNED ALT AND SAW THAT IT WAS 2 ACFT POSITIONED SUCH THAT IT APPEARED IT WAS 1 ACFT THAT WAS VERY CLOSE. A REMEDY FOR THESE VISUAL ILLUSIONS WOULD BE TO HAVE TCASII IN ALL OPS INVOLVING AIRLINE/COMMUTER OPS. THE ILLUSION WAS SO REAL THAT I WOULD RESPOND THE SAME WAY IN ATTEMPT TO AVERT A COLLISION IF THE SCENARIO OCCURRED AGAIN.

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.