Narrative:

Scheduled service from bwi to syr level at FL370 and requesting lower due to turn, another company flight was at FL310 and swbnd (heading toward us). The suspected conflicting company traffic was cleared to FL280. We shortly received a traffic alert on our aircraft's TCAS unit. This was followed very quickly (7-8 seconds) by a resolution advisory (RA). The resolution advisory commanded an approximately 1500 FPM descent on the RA/vsi. I, the PF pushed over to follow the RA command, and at approximately 26500' the TCAS unit called, 'clear of traffic.' we immediately returned to FL270 and advised the center of the RA incident and also performed a TCAS function test. The unit was found to be in normal operational condition. The ZNY controller seemed to be confused as to what had occurred and did not give us positive confirmation that he understood that we were responding to a TCAS RA. The center sector was very busy and many aircraft were attempting to change altitudes due to turbulence and precipitation. We were almost immediately handed off to ZBW. The ZBW controller gave no indication that he was aware of any conflict. TCAS units may be sensitive to high closure rate conditions where altitude sep is only 1000'. Both aircraft were approximately head-on at high true airspds. The newer digital flight management systems normally are capable of capturing preselected altitudes from very large rates of climb or descent. In the altitude capture from vertical rates of 3000 FPM or more do not occur until 300' prior to selected altitude. I don't know what type of aircraft the suspected company traffic was, but it may have had an advanced autoplt or FMS system. The resolution advisory was of very short duration and the captain reported seeing only the #9 next to the target symbol on the TCAS indicator indicating 900' of vertical sep. The clear of traffic call occurred at approximately 200' below our assigned altitude. The manual pull out consumed approximately 200-250'. As I understand the system, the ATC altitude reporting equipment at the controller's station requires a deviation of 250' or greater to recognize a conflict, particularly when high vertical rates of climb or descent immediately prior to level-off are involved. We, the crew afterward did not feel at significant risk or that a true conflict had occurred. The RA was of very short duration and involved a very small altitude change to produce an all clear. NTSB NASA and the FAA should look into the possibility of this type of incident and also advise controllers to be particularly aware of the term 'RA.' also advised ATC controllers that the TCAS units may be more sensitive than their equipment.

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Original NASA ASRS Text

Title: ALT EXCURSION OCCURS AT TCAS II EQUIPMENT INDICATES A POTENTIAL CONFLICT LESS THAN STANDARD SEPARATION WITH OPPOSITE DIRECTION TRAFFIC THAT EXPERIENCED A SLIGHT ALT DEVIATION ALT OVERSHOT WHILE DESCENDING.

Narrative: SCHEDULED SVC FROM BWI TO SYR LEVEL AT FL370 AND REQUESTING LOWER DUE TO TURN, ANOTHER COMPANY FLT WAS AT FL310 AND SWBND (HDG TOWARD US). THE SUSPECTED CONFLICTING COMPANY TFC WAS CLRED TO FL280. WE SHORTLY RECEIVED A TFC ALERT ON OUR ACFT'S TCAS UNIT. THIS WAS FOLLOWED VERY QUICKLY (7-8 SECS) BY A RESOLUTION ADVISORY (RA). THE RESOLUTION ADVISORY COMMANDED AN APPROX 1500 FPM DSNT ON THE RA/VSI. I, THE PF PUSHED OVER TO FOLLOW THE RA COMMAND, AND AT APPROX 26500' THE TCAS UNIT CALLED, 'CLR OF TFC.' WE IMMEDIATELY RETURNED TO FL270 AND ADVISED THE CENTER OF THE RA INCIDENT AND ALSO PERFORMED A TCAS FUNCTION TEST. THE UNIT WAS FOUND TO BE IN NORMAL OPERATIONAL CONDITION. THE ZNY CTLR SEEMED TO BE CONFUSED AS TO WHAT HAD OCCURRED AND DID NOT GIVE US POSITIVE CONFIRMATION THAT HE UNDERSTOOD THAT WE WERE RESPONDING TO A TCAS RA. THE CENTER SECTOR WAS VERY BUSY AND MANY ACFT WERE ATTEMPTING TO CHANGE ALTS DUE TO TURB AND PRECIPITATION. WE WERE ALMOST IMMEDIATELY HANDED OFF TO ZBW. THE ZBW CTLR GAVE NO INDICATION THAT HE WAS AWARE OF ANY CONFLICT. TCAS UNITS MAY BE SENSITIVE TO HIGH CLOSURE RATE CONDITIONS WHERE ALT SEP IS ONLY 1000'. BOTH ACFT WERE APPROX HEAD-ON AT HIGH TRUE AIRSPDS. THE NEWER DIGITAL FLT MGMNT SYSTEMS NORMALLY ARE CAPABLE OF CAPTURING PRESELECTED ALTS FROM VERY LARGE RATES OF CLB OR DSNT. IN THE ALT CAPTURE FROM VERT RATES OF 3000 FPM OR MORE DO NOT OCCUR UNTIL 300' PRIOR TO SELECTED ALT. I DON'T KNOW WHAT TYPE OF ACFT THE SUSPECTED COMPANY TFC WAS, BUT IT MAY HAVE HAD AN ADVANCED AUTOPLT OR FMS SYS. THE RESOLUTION ADVISORY WAS OF VERY SHORT DURATION AND THE CAPT RPTED SEEING ONLY THE #9 NEXT TO THE TARGET SYMBOL ON THE TCAS INDICATOR INDICATING 900' OF VERT SEP. THE CLR OF TFC CALL OCCURRED AT APPROX 200' BELOW OUR ASSIGNED ALT. THE MANUAL PULL OUT CONSUMED APPROX 200-250'. AS I UNDERSTAND THE SYS, THE ATC ALT RPTING EQUIP AT THE CTLR'S STATION REQUIRES A DEVIATION OF 250' OR GREATER TO RECOGNIZE A CONFLICT, PARTICULARLY WHEN HIGH VERT RATES OF CLB OR DSNT IMMEDIATELY PRIOR TO LEVEL-OFF ARE INVOLVED. WE, THE CREW AFTERWARD DID NOT FEEL AT SIGNIFICANT RISK OR THAT A TRUE CONFLICT HAD OCCURRED. THE RA WAS OF VERY SHORT DURATION AND INVOLVED A VERY SMALL ALT CHANGE TO PRODUCE AN ALL CLR. NTSB NASA AND THE FAA SHOULD LOOK INTO THE POSSIBILITY OF THIS TYPE OF INCIDENT AND ALSO ADVISE CTLRS TO BE PARTICULARLY AWARE OF THE TERM 'RA.' ALSO ADVISED ATC CTLRS THAT THE TCAS UNITS MAY BE MORE SENSITIVE THAN THEIR EQUIP.

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.