C172 PLT LOST COM AFTER ACFT ELECTRICAL FAILURE IN PCT CLASS B.

Narrative

DURING AN IFR FLT IN VMC CONDITIONS; THE FLT DEPARTED THV NORMALLY AND CLBED TO 4000 FT WHILE IN COM WITH HARRISBURG APCH AND BALTIMORE APCH. DUE TO TFC IN THE BALTIMORE AREA; 2 WINGTIP LNDG LIGHTS WERE TURNED ON IN ADDITION TO THE NAV LIGHTS AND ROTATING BEACON AND STROBE LIGHTS TO ADD TO THE VISIBILITY OF THE C-172. THE FLT WAS HANDED OFF FROM ONE BALTIMORE APCH CTLR TO ANOTHER; BUT WAS UNABLE TO ESTABLISH CONTACT. A SECOND RADIO WAS TRIED; AND STILL NO CONTACT. THE ORIGINAL FREQ WAS TRIED; AND NO CONTACT. THE #1 RADIO WAS SWITCHED OFF AND ON; AND WHEN REPWRING; THE LED NUMBERS COME ON VERY DIM. I CHKED THE AMMETER AND DISCOVERED A DISCHARGE. AFTER RECYCLING THE ALTERNATOR FIELD AND MASTER SWITCH; I WAS UNABLE TO ESTABLISH A POSITIVE CHARGE. I SWITCHED ALL LIGHTS AND RADIOS OFF; AND TURNED ON A SINGLE COM RADIO AND TRIED AGAIN TO ESTABLISH CONTACT WITH BALTIMORE APCH; BUT WAS UNABLE. I THEN SWITCHED THE RADIO OFF; AND THE XPONDER ON AND SQUAWKED 7700 TO DECLARE AN EMER. RATHER THAN PROCEED FURTHER INTO THE SENSITIVE WASHINGTON AREA WITHOUT COM ON A 3 HR FLT TO ISO N CAROLINA; I DECIDED TO LAND THE ACFT AT ITS ORIGIN AT THV; 15 MINS N OF CURRENT POS. UPON REACHING 2000 FT; I USED A CELL PHONE TO CALL FSS TO RELAY MY INTENTIONS. AFTER 5 MINS; I SWITCHED THE XPONDER TO 7600 TO INDICATE THE COM FAILURE; BUT DOUBTED IF THE SIGNAL WAS READABLE DUE TO THE LOW VOLTAGE CONDITION. I USED A PORTABLE GPS TO NAV BACK TO THV. UPON LNDG AT THV WITH NO FLAPS; I IMMEDIATELY CALLED HARRISBURG APCH AND BALTIMORE APCH TO EXPLAIN. THE CTLRS SAID THEY WERE GLAD I TURNED AROUND AND LANDED RATHER THAN PROCEED. IF I HAD INCLUDED THE AMMETER IN MY SCAN; I MAY HAVE NOTICED THE ALTERNATOR FAILURE SOONER; WHILE ENOUGH VOLTAGE WAS AVAILABLE TO XMIT A MESSAGE. THE USE OF LNDG LIGHTS WHILE INFLT ACCELERATED THE BATTERIES' DISCHARGE. AFTER LNDG THE ALTERNATOR BRUSHES WERE DISLODGED; WHICH CAUSED ALTERNATOR FAILURE AND SUBSEQUENT FAILURE OF THE VOLTAGE REGULATOR AND THE LOW VOLTAGE SENSOR. CONTINUING THE FLT ON THE EXISTING FPR AT THE ASSIGNED ALT WAS BRIEFLY CONSIDERED; BUT REJECTED DUE TO LACK OF PWR FOR SUFFICIENT NAV AND THE SENSITIVITY OF THE WASHINGTON AREA. I BELIEVE FEWER PROBS WERE CAUSED BY ME DECLARING AN EMER (THAT NO ONE HEARD OR SAW) AND RETURNING TO THV THAN WOULD HAVE BEEN THE CASE HAD I CONTINUED THE FLT FURTHER INTO THE CLASS B AIRSPACE.