The KT-70 transponder of an aircraft operating in the D.C. Sfra transitioned from the pct assign cod...

Narrative:

Operations inside of the D.C. Sfra had been conducted for approximately 2 hours with no problems; and ops were complete. I was enroute to my base to refuel; park and standby. Approximately 4 miles west of landing I was contacted by potomac approach on 119.700 and asked for my location. I responded. Potomac then asked me to ident. As I reached to press the ident button; I observed my assigned mode 3/a squawk code no longer appeared in the bendix/king KT-70 tso transponder's display window - instead it showed 1200. I cycled the transponder's power knob to off then back to altitude - however 1200 continued to be displayed. At some point during the flight the transponder had dumped the mode 3/a code and switched; uncommanded; to 1200. At no point did I press the 'VFR' button on the transponder. At no point did I touch the knobs to manually set a new transponder code.prior to landing the approach controller gave me a phone number to call after landing. After landing and shutdown; I called the number. They asked several questions about my personal information (name; address; FAA certificate number; etc). They asked if any part of ATC had instructed me to squawk 1200 - I answered no. They also asked if I had purposely or accidentally set my mode 3/a code to 1200 - I answered no. I then explained that the transponder had switched to 1200 uncommanded; with no input or interaction from myself. They informed me that I would likely receive a follow-up phone call from someone on the matter.

Original NASA ASRS Text

Title: The KT-70 transponder of an aircraft operating in the D.C. SFRA transitioned from the PCT assign code to 1200 uncommanded. ATC questioned the pilot who was told to call after landing.

Narrative: Operations inside of the D.C. SFRA had been conducted for approximately 2 hours with no problems; and Ops were complete. I was enroute to my base to refuel; park and standby. Approximately 4 miles West of landing I was contacted by Potomac Approach on 119.700 and asked for my location. I responded. Potomac then asked me to Ident. As I reached to press the Ident button; I observed my assigned Mode 3/A Squawk code no longer appeared in the Bendix/King KT-70 TSO transponder's display window - instead it showed 1200. I cycled the transponder's power knob to OFF then back to ALT - however 1200 continued to be displayed. At some point during the flight the transponder had dumped the Mode 3/A code and switched; uncommanded; to 1200. At no point did I press the 'VFR' button on the transponder. At no point did I touch the knobs to manually set a new transponder code.Prior to landing the Approach Controller gave me a phone number to call after landing. After landing and shutdown; I called the number. They asked several questions about my personal information (Name; address; FAA Certificate number; etc). They asked if any part of ATC had instructed me to squawk 1200 - I answered No. They also asked if I had purposely or accidentally set my Mode 3/A code to 1200 - I answered No. I then explained that the transponder had switched to 1200 uncommanded; with no input or interaction from myself. They informed me that I would likely receive a follow-up phone call from someone on the matter.

Data retrieved from NASA's ASRS site 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.

Attributes
ACN	1293849
Time
Date	201509
Local Time Of Day	0601-1200
Place
Locale Reference	PCT.TRACON
State Reference	VA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Helicopter
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Transponder
Person 1
Function	Captain Single Pilot
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Experience	Flight Crew Last 90 Days 40 Flight Crew Total 4600 Flight Crew Type 500
Events
Anomaly	Aircraft Equipment Problem Less Severe Airspace Violation All Types Deviation - Procedural FAR Deviation - Procedural Security

The KT-70 transponder of an aircraft operating in the D.C. Sfra transitioned from the pct assign code to 1200 uncommanded. ATC questioned the pilot who was told to call after landing.