Date: 201512
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: Fog; Rain
Light: Dawn
Aircraft: Gulfstream IV / G350 / G450
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Landing
ATC / advisory: Tower ZZZ
Aircraft component: Brake System
Component problem: Malfunctioning
Reporter organization: Corporate
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Flight Engineer; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Ground Excursion Runway; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Airport; Weather
Primary problem: Weather

Narrative: I was Captain on a repo flight from with just pilots. It was IFR flight plan. The weather [at departure airport] was 1/4 mile in fog. [Destination airport] was fog with RVR values up and down with report of 1;200 touchdown 1;000 rollout. I was the flying pilot. We accepted the ILS 16 using the auto pilot and Enhanced Vision System (EVS) / Heads Up Display (HUD). We were vectored 3 miles outside the FAF and cleared for approach with clearance to land where ATC advised of RVR of 1;200 touchdown 1;000 roll out. The non flying pilot completed the checklist and made the proper callouts. At Decision Height (DH) he called the approach lights; I saw them also; we continued down another 100 feet. We then had runway environment in sight where I clicked off the autopilot and landed normally on the wet runway in the touchdown area. Applying the reverses and brakes we continued down the runway. As the centerline lights turned from white to red/white then red non flying pilot called for more brakes at which time I am standing on them. It seemed the aircraft was not decelerating like it normally does.The aircraft continued down the center of the runway and skid off the end into the grass. It look like we went about 3-4 feet into grass with the mains. The aircraft did not sustain any damage nor was the pilots hurt.This is hard IMC not to mention waking up [early in the AM]. The runway and brakes were wet which resulted in lack of braking. [My Suggestion is to] repo the night before in better weather [and] stand on the brakes harder.


Second narrative: I was the SIC on this ferry flight and the PIC was flying the airplane using the autopilot and Heads Up Display (HUD) with Enhanced Vision Display (EVS). We were cleared for the ILS approach to Runway 16 and vectored onto the localizer about 3 miles outside the final approach fix where we completed the before landing checklist including arming the ground spoilers. When I saw the lights on the MALSR I called 'I have the lights'. At about 150 feet (AGL) I saw the runway centerline lighting and green threshold lights. We touched down just past the first 2 stripe runway touch down zone marker. The PIC immediately applied reverse thrust and braking. About 500 feet from where the centerline lights alternated from solid white to red and white I called for more brakes. I didn't know it then; but the PIC was already standing on the brakes at that time. Even with heavy braking and reverse the airplane wasn't decelerating like it usually does. We went off the end of the runway at a slow speed. When we stopped in the grass the rear wheels were about 3 to 4 feet from the pavement. The Runway was wet and the weather was down to the minimums. There was no damage to the aircraft and no one was injured.


Synopsis: The flight crew of a corporate aircraft reported insufficient deceleration during landing on 6;500 foot runway in poor weather that led to a runway excursion.