LOW ALT CIVIL FLT WITHOUT XPONDER TURNED ON UNDER TCA BY PLT OF SMA CESSNA.

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I HAD AS A PAX ON THIS TRIP AN ELDERLY GENTLEMAN WHO IS SUFFERING FROM ALZHEIMER'S DISEASE. HE DOES LOVE TO GO FLYING WITH ME BUT BECAUSE HE IS SOMEWHAT UNPREDICTABLE; I NORMALLY PUT HIM IN THE BACK SEAT. THIS DAY I PLANNED TO SHOW HIM A FRIEND'S HOUSE WHICH WAS IN HEAVILY WOODED AND SOMEWHAT MOUNTAINOUS TERRAIN. I HAVE BEEN TO THIS PLACE SEVERAL TIMES VIA AUTO AND THE NUMBER OF HOUSES AND OTHER BUILDINGS THAT I PASSED WERE SO FEW THAT I CONSIDERED THIS AS 'OTHER THAN CONGESTED AREA.' NAV WAS BY GPS WHICH TOOK ME DIRECTLY TO HIS PLACE; AND THE MINIMUM ALTS SPECIFIED IN FAR 91.119 (C) WERE ADHERED TO. WE MADE 1 CIRCLE ABOVE THE GENERAL AREA; AND THEN DEPARTED THE SAME DIRECTION FROM WHICH WE CAME. DURING THE CIRCLE I NOTICED A SHORT DISTANCE BEYOND MY FRIENDS HOUSE ON THE ROAD THAT I NORMALLY TRAVELED WAS A CLUSTER OF SEVERAL HOUSES IN A SOMEWHAT MORE CLRED AREA. PROBABLY A DEVELOPMENT OF SOME SORT. SINCE THIS COULD POSSIBLY BE CONSIDERED A 'SETTLEMENT' AND THEREFORE BECOME A 'CONGESTED AREA;' THEN OF COURSE HIGHER MINIMUM ALTS WOULD BE REQUIRED. AT ONE TIME I WOULD REGARD EVERYTHING ON THE SECTIONAL CHART THAT WAS NOT IN YELLOW AS UNCONGESTED AREA; BUT IT OCCURS TO ME THAT THIS MAY NOT BE CORRECT. I PLAN TO BE AT A FLT INSTRUCTOR CLINIC LATER THIS MONTH AND IT WILL BE A TOPIC I HOPE TO BRING UP. ALTHOUGH THIS FLT WAS ENTIRELY OUTSIDE OF THE CLASS B AIRSPACE SURROUNDING IAD; IT WAS WITHIN THE 30 MI MODE C RING. WHEN I WAS SECURING THE AVIONICS AT THE CONCLUSION OF THIS FLT; THE XPONDER WAS ALREADY TURNED OFF. HOW IT GOT THAT WAY I CAN ONLY GUESS; BUT MY PAX DEFINITELY GOES IN THE BACK SEAT NEXT TIME.