20030227008419I

(-23) On landing, aircraft slid off the end of runway 20 approximately 150 feet. Runway was wet. Weather was 300 feet overcast with a mile and a half visibility. Pilot got weather info from AWOS which reported winds as calm at the specific time AWOS info was received. However, the predominantly recorded winds from the AWOS were 3 to 6 knots from the north. Pilot used laminated landing date card for computing runway length. The card has weight intervals of 1000 feet and pressure altitude intervals of 2000 feet. The pilot took the closest numbers to weight and pressure altitude which meant he rounded down on weight from 22450 to 22000 and down on pressure altitude from 700 to sea level. These figures gave him a needed runway length of 4200 which he then multiplied by 1.15 to give him a wet runway length requirement of 4830. Had he rounded up, he would have used the 23000 and 2000 feet and gotten 4500 which when multiplied by the wet runway factor equals 5175. There are no instructions about rounding up or down on this card. He also touched down somewhere between 1500 and 1700 feet of runway, the FAR end of the first one third of the 5003 foot runway. This was the pilot's third leg ofhis first trip as a brand new pilot in command. The first officer was a very low time pilot. Aircraft dug into mud, but was not significantly damaged. Aircraft was eventually removed from the mud and the engines flushed and run and tires and wheels replaced.