On September 21, 1999, at 1830 eastern daylight time, a Cessna 172P, N63325, was substantially damaged during a runway overrun while landing at the Potomac Airfield Airport (VKX), Friendly, Maryland. The certificated flight instructor and private pilot were not injured. Instrument meteorological conditions prevailed for the instrument training flight that originated at VKX approximately 1815. An instrument flight rules flight plan was filed for the flight conducted under 14 CFR Part 91. In a written statement, the flight instructor said the purpose of the flight was to provide instrument flight training to the private pilot. He said they experienced delays in receiving their clearance from Air Traffic Control (ATC) on the ground, and decided to pick up the clearance while airborne in the traffic pattern at Potomac Airfield. The flight instructor stated: "After performing the run-up, we took off on [runway] 6 and by 3/4 of the runway my student reported no airspeed on the dial. By now, the airplane was already airborne. At that point, an abort was impossible. We climbed normally by keeping a low angle climb and I decided to take over the controls. The traffic pattern was a little bumpy but the visibility was OK; the airport was always in sight. I decided to come back and land on [runway] 6...My first attempt was fast with about 20 [degrees] of flaps so I did a go-around. My second was a little slower but the plane was floating too long and I applied power and went back up. On the third one, I did successfully touch down but the plane was hydroplaning to the end of the runway without stopping or slowing down. I saw the ravine at the end of the runway but couldn't stop and the left wheel caught the edge of the bank and the plane skid to a stop on the left wing." In a written statement, the private pilot said: "During the takeoff I divided most of my attention to maintaining directional control of the aircraft, so I am uncertain whether or not I may [have] had a reading on the airspeed indicator before rotation. After N63325 became airborne and I had positive directional control of the aircraft, I noticed the airspeed indicator was at [zero] mph and not moving at all. "[The flight instructor] attempted to land twice on runway 6. Both times we did not have enough runway left to land safely and he aborted the landings. I suggested we land at National (DCA) or Andrews (ADW) since we were fast and had no way of determining our airspeed on final. He agreed we would do that if a third landing attempt was unsuccessful." Examination of the wreckage by the operator and a mechanic revealed the pitot tube was obstructed. In a telephone interview, the operator stated he was able to clear the pitot tube with a piece of safety wire and the airspeed indicator worked when tested. The flight instructor reported 700 hours of total flight experience, 60 hours of which were in the 90 days prior to the accident. He reported 100 hours of experience in make and model. The private pilot reported 390 hours of total flight experience, 20 hours of which were in the 90 days prior to the accident. He reported 25 hours of experience in make and model. The weather reported at ADW, 5 miles northeast of Potomac Airpark, was: visibility 4 miles in light rain showers and fog. There were few clouds at 300 feet with a broken ceiling at 500 feet and an overcast layer at 1,800 feet. The winds were from 330 degrees at 14 knots.