Narrative:

While on the milton 7 arrival for lga, between proud intersection and dials intersection on the expressway visual, ATC cleared us to 2500 ft and cleared us for the visual approach. About 2 mi south of dials intersection, he told us to climb to 3000 immediately to avoid traffic. He then told us he had only cleared us to 3000. The captain and I clearly heard the clearance for our aircraft to descend to 2500. Supplemental information from acn 213291: the approach controller then cleared our flight, air carrier 2, for the expressway visual, cross dial at 2500. Nearing dials, the TCASII TA indicated 2 O'clock traffic. The traffic was sighted and a second TCASII alert sounded. At the same time, approach control pointed out 2 O'clock traffic and an urgent clearance to 3000 with a comment about him previously clearing flight, air carrier 2, to 3000. We both observed the single engine aircraft pass off to the right at a moderate distance. The single engine aircraft had turned west toward us and then made a turn to the south to avoid our flight path. A telephone call to ATC to discuss the traffic conflict was enlightening. The conflict traffic was an 'accommodation' of that airplane by ATC. The inference was that it was some sort of law enforcement surveillance. While it is nice to be accommodating, having an aircraft in an arrival area conducting ground surveillance does not seem to make very good sense. It would seem to add workload to the approach controller that he really doesn't need. Further, a surveillance operation of the ground might divert those pilot's attention away from a very busy sky.

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Original NASA ASRS Text

Title: AN ACR MLG DSNDED BELOW ASSIGNED ALT.

Narrative: WHILE ON THE MILTON 7 ARR FOR LGA, BTWN PROUD INTXN AND DIALS INTXN ON THE EXPRESSWAY VISUAL, ATC CLRED US TO 2500 FT AND CLRED US FOR THE VISUAL APCH. ABOUT 2 MI S OF DIALS INTXN, HE TOLD US TO CLB TO 3000 IMMEDIATELY TO AVOID TFC. HE THEN TOLD US HE HAD ONLY CLRED US TO 3000. THE CAPT AND I CLRLY HEARD THE CLRNC FOR OUR ACFT TO DSND TO 2500. SUPPLEMENTAL INFORMATION FROM ACN 213291: THE APCH CTLR THEN CLRED OUR FLT, ACR 2, FOR THE EXPRESSWAY VISUAL, CROSS DIAL AT 2500. NEARING DIALS, THE TCASII TA INDICATED 2 O'CLOCK TFC. THE TFC WAS SIGHTED AND A SECOND TCASII ALERT SOUNDED. AT THE SAME TIME, APCH CTL POINTED OUT 2 O'CLOCK TFC AND AN URGENT CLRNC TO 3000 WITH A COMMENT ABOUT HIM PREVIOUSLY CLRING FLT, ACR 2, TO 3000. WE BOTH OBSERVED THE SINGLE ENG ACFT PASS OFF TO THE R AT A MODERATE DISTANCE. THE SINGLE ENG ACFT HAD TURNED W TOWARD US AND THEN MADE A TURN TO THE S TO AVOID OUR FLT PATH. A TELEPHONE CALL TO ATC TO DISCUSS THE TFC CONFLICT WAS ENLIGHTENING. THE CONFLICT TFC WAS AN 'ACCOMMODATION' OF THAT AIRPLANE BY ATC. THE INFERENCE WAS THAT IT WAS SOME SORT OF LAW ENFORCEMENT SURVEILLANCE. WHILE IT IS NICE TO BE ACCOMMODATING, HAVING AN ACFT IN AN ARR AREA CONDUCTING GND SURVEILLANCE DOES NOT SEEM TO MAKE VERY GOOD SENSE. IT WOULD SEEM TO ADD WORKLOAD TO THE APCH CTLR THAT HE REALLY DOESN'T NEED. FURTHER, A SURVEILLANCE OP OF THE GND MIGHT DIVERT THOSE PLT'S ATTN AWAY FROM A VERY BUSY SKY.

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.