An air carrier medium large transport descended in response to a TCASII command.

Attributes
ACN	247880
Time
Date	199308
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : lga
State Reference	NY
Altitude	msl bound lower : 3500 msl bound upper : 4000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : n90
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 11500 flight time type : 500
ASRS Report	247880
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

Level at 4000 ft. Being vectored for approach, a 'TA' sounded. We asked ATC if they saw traffic for us at 12 O'clock position. They said no. The traffic continued closer at our altitude and the TCASII sounded 'descend, descend now.' with hazy conditions, I was unable to ascertain if there was a target there or not. I followed the RA to descend. I descended to 3500 ft. When the target was behind us, I climbed back to 4000 ft. ATC said they saw no traffic and we were the third airliner to dodge the phantom target. ATC thought there may have been a ship in the harbor causing the errant signal.

Original NASA ASRS Text

Title: AN ACR MLG DSNDED IN RESPONSE TO A TCASII COMMAND.

Narrative: LEVEL AT 4000 FT. BEING VECTORED FOR APCH, A 'TA' SOUNDED. WE ASKED ATC IF THEY SAW TFC FOR US AT 12 O'CLOCK POS. THEY SAID NO. THE TFC CONTINUED CLOSER AT OUR ALT AND THE TCASII SOUNDED 'DSND, DSND NOW.' WITH HAZY CONDITIONS, I WAS UNABLE TO ASCERTAIN IF THERE WAS A TARGET THERE OR NOT. I FOLLOWED THE RA TO DSND. I DSNDED TO 3500 FT. WHEN THE TARGET WAS BEHIND US, I CLBED BACK TO 4000 FT. ATC SAID THEY SAW NO TFC AND WE WERE THE THIRD AIRLINER TO DODGE THE PHANTOM TARGET. ATC THOUGHT THERE MAY HAVE BEEN A SHIP IN THE HARBOR CAUSING THE ERRANT SIGNAL.

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.