A C3550's transponder switched to a 1200 code from a discrete code while inbound to the potomac ADIZ...

Attributes
ACN	638975
Time
Date	200412
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : dca.airport
State Reference	DC
Altitude	msl single value : 15000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : pct.tracon tower : zzz.tower
Operator	other
Make Model Name	Citation II S2/Bravo
Operating Under FAR Part	Part 91
Flight Phase	descent : approach
Route In Use	approach : visual
Flight Plan	IFR
Person 1
Affiliation	government other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 120 flight time total : 8200 flight time type : 6000
ASRS Report	638975
Person 2
Affiliation	government other
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe non adherence other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft Airspace Structure
Primary Problem	Aircraft

Narrative:

While navigating on the mount vernon visual approach to dca's runway 1, our #2 transponder (which was actively squawking our assigned beacon code) suddenly switched to a '1-2-0-0' code. (We later learned that this particular unit occasionally reverted to a VFR squawk code spontaneously.) potomac approach immediately questioned us and assigned a heading off of the cvfp. After being radar idented on the #1 transponder and explaining the anomaly, we were permitted to continue to an uneventful approach and landing on runway 1. Ground control asked us to phone them, and he in turn gave us a telephone number to the command center. We explained our situation and the controller offered his perspective on 'a high speed jet descending toward the capitol and suddenly switching discrete codes while inbound to the ADIZ boundary.' the conversation was amiable and informative. The controller offered that the matter was closed and no further inquiries were required.

Original NASA ASRS Text

Title: A C3550'S XPONDER SWITCHED TO A 1200 CODE FROM A DISCRETE CODE WHILE INBOUND TO THE POTOMAC ADIZ.

Narrative: WHILE NAVING ON THE MOUNT VERNON VISUAL APCH TO DCA'S RWY 1, OUR #2 XPONDER (WHICH WAS ACTIVELY SQUAWKING OUR ASSIGNED BEACON CODE) SUDDENLY SWITCHED TO A '1-2-0-0' CODE. (WE LATER LEARNED THAT THIS PARTICULAR UNIT OCCASIONALLY REVERTED TO A VFR SQUAWK CODE SPONTANEOUSLY.) POTOMAC APCH IMMEDIATELY QUESTIONED US AND ASSIGNED A HDG OFF OF THE CVFP. AFTER BEING RADAR IDENTED ON THE #1 XPONDER AND EXPLAINING THE ANOMALY, WE WERE PERMITTED TO CONTINUE TO AN UNEVENTFUL APCH AND LNDG ON RWY 1. GND CTL ASKED US TO PHONE THEM, AND HE IN TURN GAVE US A TELEPHONE NUMBER TO THE COMMAND CTR. WE EXPLAINED OUR SIT AND THE CTLR OFFERED HIS PERSPECTIVE ON 'A HIGH SPD JET DSNDING TOWARD THE CAPITOL AND SUDDENLY SWITCHING DISCRETE CODES WHILE INBOUND TO THE ADIZ BOUNDARY.' THE CONVERSATION WAS AMIABLE AND INFORMATIVE. THE CTLR OFFERED THAT THE MATTER WAS CLOSED AND NO FURTHER INQUIRIES WERE REQUIRED.

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.