Narrative:

While descending to make an assigned 15000 ft crossing restr at jaybo intersection on the dupont 3 arrival into philadelphia, we failed to reset our altimeters as we passed through the FL180 transition level. Shortly after level off at jaybo the ATC controller queried our altitude noting a 300 ft difference between our transponder reported altitude and verbal reported altitude. I verified us at 15000 ft assigned. The controller then called traffic below us crossing left to right which we observed and reported in sight. Almost simultaneously then the following events occurred. I realized our error in failing to reset the altimeters from 29.92 inches to 29.51 inches and we reset them. The approaching traffic triggered a TCASII RA calling for a climb which we did, and the ATC controller asked us to switch to our other transponder which we also did. The crossing traffic passed by below us in sight at all times. I think the following were contributing factors in this situation. We had previously been told to expect holding ahead at terri intersection, although we had not yet received clearance. As a result we slowed to near holding airspeed with ATC concurrence. We were then cleared to cross jaybo at 15000 ft. First officer was flying and expressed concern about our ability to comply with the restr. I told him to increase speed as needed to make the restr and was closely monitoring his transition from the rapid descent rate to level flight. I was not very familiar with the arrival (I'd flown it once before, I think) so I was also concerned with just basic navigation to some extent as well as trying to think ahead to how long we could afford to hold. When the first officer, PF, elected to disengage the autoplt and hand fly the rapid descent and level off I was distracted enough by monitoring him that I failed to note our passage through the transition level until the controller's call.

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Original NASA ASRS Text

Title: B737 ACFT IN DSCNT TO MAKE XING RESTR. FLC FAILED TO RESET ALTIMETER LEAVING FL180. FLC DISTRACTED BY TCASII TFC AND MAINTAINING VISUAL ON TFC.

Narrative: WHILE DSNDING TO MAKE AN ASSIGNED 15000 FT XING RESTR AT JAYBO INTXN ON THE DUPONT 3 ARR INTO PHILADELPHIA, WE FAILED TO RESET OUR ALTIMETERS AS WE PASSED THROUGH THE FL180 TRANSITION LEVEL. SHORTLY AFTER LEVEL OFF AT JAYBO THE ATC CTLR QUERIED OUR ALT NOTING A 300 FT DIFFERENCE BTWN OUR XPONDER RPTED ALT AND VERBAL RPTED ALT. I VERIFIED US AT 15000 FT ASSIGNED. THE CTLR THEN CALLED TFC BELOW US XING L TO R WHICH WE OBSERVED AND RPTED IN SIGHT. ALMOST SIMULTANEOUSLY THEN THE FOLLOWING EVENTS OCCURRED. I REALIZED OUR ERROR IN FAILING TO RESET THE ALTIMETERS FROM 29.92 INCHES TO 29.51 INCHES AND WE RESET THEM. THE APCHING TFC TRIGGERED A TCASII RA CALLING FOR A CLB WHICH WE DID, AND THE ATC CTLR ASKED US TO SWITCH TO OUR OTHER XPONDER WHICH WE ALSO DID. THE XING TFC PASSED BY BELOW US IN SIGHT AT ALL TIMES. I THINK THE FOLLOWING WERE CONTRIBUTING FACTORS IN THIS SIT. WE HAD PREVIOUSLY BEEN TOLD TO EXPECT HOLDING AHEAD AT TERRI INTXN, ALTHOUGH WE HAD NOT YET RECEIVED CLRNC. AS A RESULT WE SLOWED TO NEAR HOLDING AIRSPD WITH ATC CONCURRENCE. WE WERE THEN CLRED TO CROSS JAYBO AT 15000 FT. FO WAS FLYING AND EXPRESSED CONCERN ABOUT OUR ABILITY TO COMPLY WITH THE RESTR. I TOLD HIM TO INCREASE SPD AS NEEDED TO MAKE THE RESTR AND WAS CLOSELY MONITORING HIS TRANSITION FROM THE RAPID DSCNT RATE TO LEVEL FLT. I WAS NOT VERY FAMILIAR WITH THE ARR (I'D FLOWN IT ONCE BEFORE, I THINK) SO I WAS ALSO CONCERNED WITH JUST BASIC NAV TO SOME EXTENT AS WELL AS TRYING TO THINK AHEAD TO HOW LONG WE COULD AFFORD TO HOLD. WHEN THE FO, PF, ELECTED TO DISENGAGE THE AUTOPLT AND HAND FLY THE RAPID DSCNT AND LEVEL OFF I WAS DISTRACTED ENOUGH BY MONITORING HIM THAT I FAILED TO NOTE OUR PASSAGE THROUGH THE TRANSITION LEVEL UNTIL THE CTLR'S CALL.

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.