A DA40's right tire went flat during taxi out. Maintenance replaced the tube and tire. The left tire was checked and found properly inflated. On taxi out the second time the left tire went flat. The aircraft was determined to be 250 pounds overweight and 0.5' forward of the aft CG limitation prior to departure.

Narrative

In May 2009; while taxiing for departure; the right main tire on my aircraft went flat. The aircraft was towed to a maintenance facility; the tube and tire were replaced; the pressure in the remaining tires was checked and it was confirmed that they had been properly inflated. The old tire was unevenly worn and the old tube had a 1/8 inch crack in the side about 2 inches below the rim of the tire. Having confirmed that the aircraft was in airworthy condition; we continued as planned. Later that day; while taxiing for the second time; the other main tire went flat in the same manner. Further investigation revealed that we were about 250 LBS overweight and 1/2 inch forward of the aft center of gravity limitation. The planned trip was concluded successfully; but with one less passenger. Though I'm still baffled why the tubes and tires failed while taxiing but not while landing; it's clear to me that the overweight condition was neither safe nor prudent. Upon reflection; I realized that most of my cross country experience has been in heavier; multi-engine aircraft with dramatically greater capacity; allowing me to become somewhat complacent about loading considerations. This occasion has served as a wake-up call -- knowing the load and performance data for a flight is imperative regardless of the aircraft's capabilities. I was fortunate this time. I will choose to be meticulous next time.