Be35 pilot reports acquiring the runway late during the GPS 18 approach to GTU and landing long. This alarms the Tower controller who questions the maneuver.

Narrative

I am reporting the events during a GPS approach to landing at GTU that appeared to have concerned the tower controllers about my ability to stop before the end of the runway. I was on an IFR flight plan to GTU with AUS as an alternate; as the sole occupant of my Bonanza. Forecast weather for AUS when I left was to improve to 2000 OVC by XA:00 CST and continue improving. This forecast turned out to be highly inaccurate; and GTU ATIS Charlie reported a 400 foot ceiling (1200 MSL) and wind calm from about 40 minutes before my arrival. At 4000 MSL I was in the clear and able to see lights through the undercast; so the layer was at most 2000 feet thick. Some lights were clearer than others; suggesting the undercast varied in thickness. I requested the GPS 18 approach which has a 400 foot MDA; and was advised by GTU tower when I checked in with them from Austin approach that the AWOS was now reporting 300 OVC. I had 3 hours of fuel remaining and decided to try the approach; with a first backup plan of the ILS at AUS and a second backup plan of AQO; for which the AWOS was reporting clear. I established a stabilized approach; dropped the gear at the FAF; and descended to the MDA of 1200 MSL (400 AGL). I turned off my strobes and landing light; leaving only my position lights on so I would not be blinded by reflected light in the clouds. I leveled off well before the MAP at approximately 100 kts and through the last approximately 2 miles saw various lights at times below me; suggesting the bottom was ragged. While attempting to acquire the runway I drifted somewhat left of course. Shortly before I was about to initiate a missed approach at the MAP; I acquired the runway lights ahead and slightly right. Since I had a clear view of the runway and good visibility; I pulled off power; corrected right; and began a power off descent to the runway. This descent increased my speed slightly; but I put the wheels on the runway at about the mid-point and stopped after the 29/11 intersection but before the next taxiway intersection; leaving about 1500 feet of runway remaining out of 5000. The tower controller called me during this descent asking whether I would have enough runway to land. In hindsight; his concern was entirely understandable. Since the MAP for the GPS 18 is the runway threshold; by initiating a descent from 400 feet just before the MAP there was no chance I would land in the normal touchdown zone. Since I routinely fly power off descents; my visual picture of the runway told me I would have adequate room to land safely. Still; in executing this descent; I effectively gave up about 2000 feet of a 5000 foot runway; decreasing my safety margin. I more often fly and practice ILS approaches; where the MAP is typically some distance before the runway threshold and at 200 feet AGL. From that point; a landing can be made on a 3 degree glide slope in the normal touchdown zone. On any GPS approach with the MAP at the runway threshold and 400 AGL; a steeper descent will be necessary and the landing will be further down the runway; if the descent is initiated just before the MAP. I did not fully factor this into my approach planning; and had I done so; I could have flown the approach 10-15 knots slower. While I still would not have landed in the touchdown zone; I could likely have touched down and stopped earlier. This would have left me a greater margin of error to deal with anything unexpected; and also would be particularly important on an approach to a shorter runway. I learned several things from this event. First; I should practice GPS approaches to a full landing more often. Second; I should fly from the MDA to the MAP at as slow an airspeed as practical. Third; if I haven't acquired the runway well before the MAP on a GPS approach; I should more carefully consider whether a descent is the best course of action; even if I acquire the runway late in the approach.