On October 26, 2023, during an aviation incident, a pilot-in-command (PIC) received an ATIS advisory indicating visibility was 1/2-mile with snow and fog, and the ILS Runway 06 approach was in use. The pilot observed that the hydraulic pressure gauge was normal, and performed a brake test. During the approach, the PIC visually observed that the hydraulic pressure gauge was normal, and performed a brake test. The tower controller advised the flightcrew that a Navajo had just landed and reported a 250-foot ceiling, 3/4 miles of visibility, and that the runway braking action was ‘good.’ Upon final approach, the airplane broke out of the overcast clouds about 400 feet above the ground. The runway appeared dry, with only blowing snow across it. After touchdown, the pilot selected the ‘dump flaps’ to slow the airplane. The airplane continued down the runway and did not appear to be slowing to a safe speed. The flightcrew observed the 1,000-foot marker approaching and selected the emergency brakes. After a lack of deceleration, the pilot selected the parking brake. The airplane continued off the end of the runway, impacted a non-franigible fence, and came to rest with about 4 feet of the airplane protruding onto a public access road. Examination of the airplane revealed a reference speed set on the pilot’s airspeed indicator of 117 knots. According to the Hawker Siddeley HS-125 Airplane Flight Manual (AFM) performance section, the uncorrected landing distance, on a dry runway, at the estimated landing weight of the airplane was approximately 2,610 feet. The AFM performance section also had a chart for the ‘Effect of Slipsery Runway on Landing Distance.’ The chart determined that the ‘equivalent scheduled landing distance available,’ was approximately 2,150 feet. According to airport records, a NOTAM was issued the day of the accident, which referenced the condition of the runway surfaces as having ‘Thin wet snow all surfaces.’ An air traffic controller observed slush ‘spraying’ from the airplane, which extended 5 feet from the airplane, as the airplane rolled out. The airplane’s braking system was tested after the accident and no abnormalities were noted. Probable Cause: The pilot-in-commands inadequate preflight planning resulting in an approach to a runway with insufficient length. A factor related to the accident was the slush covered runway which resulted in a hydroplaning condition.On October 26, 2023, during an aviation incident, a pilot-in-command (PIC) received an ATIS advisory indicating visibility was 1/2-mile with snow and fog, and the ILS Runway 06 approach was in use. The pilot observed that the hydraulic pressure gauge was normal, and performed a brake test. During the approach, the PIC visually observed that the hydraulic pressure gauge was normal, and performed a brake test. The tower controller advised the flightcrew that a Navajo had just landed and reported a 250-foot ceiling, 3/4 miles of visibility, and that the runway braking action was ‘good.’ Upon final approach, the airplane broke out of the overcast clouds about 400 feet above the ground. The runway appeared dry, with only blowing snow across it. After touchdown, the pilot selected the ‘dump flaps’ to slow the airplane. The airplane continued down the runway and did not appear to be slowing to a safe speed. The flightcrew observed the 1,000-foot marker approaching and selected the emergency brakes. After a lack of deceleration, the pilot selected the parking brake. The airplane continued off the end of the runway, impacted a non-franigible fence, and came to rest with about 4 feet of the airplane protruding onto a public access road. Examination of the airplane revealed a reference speed set on the pilot’s airspeed indicator of 117 knots. According to the Hawker Siddeley HS-125 Airplane Flight Manual (AFM) performance section, the uncorrected landing distance, on a dry runway, at the estimated landing weight of the airplane was approximately 2,610 feet. The AFM performance section also had a chart for the ‘Effect of Slipsery Runway on Landing Distance.’ The chart determined that the ‘equivalent scheduled landing distance available,’ was approximately 2,150 feet. According to airport records, a NOTAM was issued the day of the accident, which referenced the condition of the runway surfaces as having ‘Thin wet snow all surfaces.’ An air traffic controller observed slush ‘spraying’ from the airplane, which extended 5 feet from the airplane, as the airplane rolled out. The airplane’s braking system was tested after the accident and no abnormalities were noted. Probable Cause: The pilot-in-commands inadequate preflight planning resulting in an approach to a runway with insufficient length. A factor related to the accident was the slush covered runway which resulted in a hydroplaning condition.