Narrative:

During an IFR flight in VMC conditions, the flight departed thv normally and climbed to 4000 ft while in communication with harrisburg approach and baltimore approach. Due to traffic in the baltimore area, 2 wingtip landing lights were turned on in addition to the navigation lights and rotating beacon and strobe lights to add to the visibility of the C-172. The flight was handed off from one baltimore approach controller to another, but was unable to establish contact. A second radio was tried, and still no contact. The original frequency was tried, and no contact. The #1 radio was switched off and on, and when repwring, the led numbers come on very dim. I checked 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 communication radio and tried again to establish contact with baltimore approach, but was unable. I then switched the radio off, and the transponder on and squawked 7700 to declare an emergency. Rather than proceed further into the sensitive washington area without communication on a 3 hour flight to iso north carolina, I decided to land the aircraft at its origin at thv, 15 mins north of current position. Upon reaching 2000 ft, I used a cell phone to call FSS to relay my intentions. After 5 mins, I switched the transponder to 7600 to indicate the communication failure, but doubted if the signal was readable due to the low voltage condition. I used a portable GPS to navigation back to thv. Upon landing at thv with no flaps, I immediately called harrisburg approach and baltimore approach to explain. The controllers 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 transmit a message. The use of landing lights while in-flight accelerated the batteries' discharge. After landing the alternator brushes were dislodged, which caused alternator failure and subsequent failure of the voltage regulator and the low voltage sensor. Continuing the flight on the existing fpr at the assigned altitude was briefly considered, but rejected due to lack of power for sufficient navigation and the sensitivity of the washington area. I believe fewer problems were caused by me declaring an emergency (that no one heard or saw) and returning to thv than would have been the case had I continued the flight further into the class B airspace.

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Original NASA ASRS Text

Title: 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.

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.