Flight crew descends below assigned altitude and experiences TCASII RA.

Attributes
ACN	319268
Time
Date	199510
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zny
State Reference	NY
Altitude	msl bound lower : 24500 msl bound upper : 25000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zny
Operator	common carrier : air carrier
Make Model Name	B737-400
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : atp
Experience	flight time last 90 days : 210 flight time total : 9000 flight time type : 6500
ASRS Report	319268
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course other
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 18000 vertical : 0
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation
Situations
ATC Facility	procedure or policy : unspecified

Narrative:

On a flight from bos-phl at FL310 we were given a clearance to cross 70 northeast of brigs intersection at FL200. I programmed the FMC accordingly. Top of descent point was still about 25 NM ahead. A few moments later ATC called and we understood the controller to say, begin descent now through FL250 for traffic. I pressed the capture button on the descent page of the FMC and started down to intercept the 'path' to FL200. Passing FL240 we got a TCASII TA and then RA for traffic at our altitude and less than 10 mi ahead. Simultaneously ATC calls and tells us to 'climb now!!' back to FL250. TCASII was telling us to descend to avoid but we had the traffic visually ahead and off to our right so we stopped the descent and climbed back to FL250. Our subsequent discussion with the controller revealed he said descend now to FL250 for traffic (not through FL250 as both the captain and I heard). Bottom line is there was a miscom between us and ATC. I think if the controller had said 'reclred' to FL250 or 'change clearance to FL250' or better yet, 'cancel clearance to FL200, reclred to descend and maintain FL250' this situation would not have occurred.

Original NASA ASRS Text

Title: FLC DSNDS BELOW ASSIGNED ALT AND EXPERIENCES TCASII RA.

Narrative: ON A FLT FROM BOS-PHL AT FL310 WE WERE GIVEN A CLRNC TO CROSS 70 NE OF BRIGS INTXN AT FL200. I PROGRAMMED THE FMC ACCORDINGLY. TOP OF DSCNT POINT WAS STILL ABOUT 25 NM AHEAD. A FEW MOMENTS LATER ATC CALLED AND WE UNDERSTOOD THE CTLR TO SAY, BEGIN DSCNT NOW THROUGH FL250 FOR TFC. I PRESSED THE CAPTURE BUTTON ON THE DSCNT PAGE OF THE FMC AND STARTED DOWN TO INTERCEPT THE 'PATH' TO FL200. PASSING FL240 WE GOT A TCASII TA AND THEN RA FOR TFC AT OUR ALT AND LESS THAN 10 MI AHEAD. SIMULTANEOUSLY ATC CALLS AND TELLS US TO 'CLB NOW!!' BACK TO FL250. TCASII WAS TELLING US TO DSND TO AVOID BUT WE HAD THE TFC VISUALLY AHEAD AND OFF TO OUR R SO WE STOPPED THE DSCNT AND CLBED BACK TO FL250. OUR SUBSEQUENT DISCUSSION WITH THE CTLR REVEALED HE SAID DSND NOW TO FL250 FOR TFC (NOT THROUGH FL250 AS BOTH THE CAPT AND I HEARD). BOTTOM LINE IS THERE WAS A MISCOM BTWN US AND ATC. I THINK IF THE CTLR HAD SAID 'RECLRED' TO FL250 OR 'CHANGE CLRNC TO FL250' OR BETTER YET, 'CANCEL CLRNC TO FL200, RECLRED TO DSND AND MAINTAIN FL250' THIS SIT WOULD NOT HAVE OCCURRED.

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.