Flight crew of large transport on approach to dca observed traffic that appeared to be too close in altitude. Approach controller advised transponder was reading 1000' low.

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Attributes
ACN	128843
Time
Date	198911
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : dca
State Reference	DC
Altitude	msl bound lower : 7000 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	cruise other descent : approach descent other
Route In Use	arrival other enroute airway : dca
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : cfi pilot : commercial pilot : flight engineer pilot : instrument
Experience	flight time last 90 days : 180 flight time total : 4800 flight time type : 180
ASRS Report	128843
Person 2
Affiliation	company : air carrier
Function	flight crew : second officer
Qualification	pilot : commercial pilot : flight engineer pilot : instrument
Experience	flight time last 90 days : 200 flight time total : 2000
ASRS Report	129093
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot conflict : airborne less severe
Independent Detector	other controllera other flight crewa
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Received a clearance from FL230 to 15,000, given altimeter setting. Given a crossing restriction 5 west of armel at 8000'. Altimeter given and read back. Due to strong winds (tail) clearance required quick descent to meet. Winds were tailwind at 130 KTS. They gave us a frequency change to approach at dca and were unable to contact initially DOE to frequency congestion. We reported leveling 8000 and I then pointed out traffic to rest of crew and we questioned controller about a 12 O'clock near our altitude. Controller questioned our altitude and we reported level 8000'. Controller then said our mode C was reporting 1000' low and to turn it off. That we did and normal landing dca. The pressure in the area was much lower than standard due to a low pressure area moving rapidly from great lakes to northeast USA.

Original NASA ASRS Text

Title: FLT CREW OF LGT ON APCH TO DCA OBSERVED TRAFFIC THAT APPEARED TO BE TOO CLOSE IN ALT. APCH CTLR ADVISED TRANSPONDER WAS READING 1000' LOW.

Narrative: RECEIVED A CLRNC FROM FL230 TO 15,000, GIVEN ALTIMETER SETTING. GIVEN A XING RESTRICTION 5 W OF ARMEL AT 8000'. ALTIMETER GIVEN AND READ BACK. DUE TO STRONG WINDS (TAIL) CLRNC REQUIRED QUICK DSCNT TO MEET. WINDS WERE TAILWIND AT 130 KTS. THEY GAVE US A FREQ CHANGE TO APCH AT DCA AND WERE UNABLE TO CONTACT INITIALLY DOE TO FREQ CONGESTION. WE REPORTED LEVELING 8000 AND I THEN POINTED OUT TFC TO REST OF CREW AND WE QUESTIONED CTLR ABOUT A 12 O'CLOCK NEAR OUR ALT. CTLR QUESTIONED OUR ALT AND WE REPORTED LEVEL 8000'. CTLR THEN SAID OUR MODE C WAS REPORTING 1000' LOW AND TO TURN IT OFF. THAT WE DID AND NORMAL LNDG DCA. THE PRESSURE IN THE AREA WAS MUCH LOWER THAN STANDARD DUE TO A LOW PRESSURE AREA MOVING RAPIDLY FROM GREAT LAKES TO NE USA.

Data retrieved from NASA's ASRS site as of August 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.