Date: 201705
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Citation X (C750)
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Landing
ATC / advisory: Tower ZZZ
Aircraft component: Hydraulic Main System
Component problem: Failed
Reporter organization: Corporate
Reporter function: Captain; Check Pilot; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Sea
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Ground Excursion Runway
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I was piloting a Citation X+ business jet. I was acting as pilot in command and flying the aircraft from the left seat. My First Officer was acting as second in command and monitoring from the right seat. We departed on a beautiful morning with 9 passengers. We departed with 10;300 lbs of fuel for the 1 hr 15 min flight (estimated 3400 lb fuel burn); 1800 lbs of passenger weight; and 270 lbs of baggage weight. Weather at ZZZ was forecast to have southerly winds with an overcast layer that was to quickly burn off. As we approached ZZZ; we realized the overcast was still lower than forecast which would require an instrument approach to get through the layer. We requested vectors for the VOR rwy XX approach; with the plan to level off at traffic pattern altitude once visual and circle to land on the longest favorable runway. All of this went smoothly and we were well prepared for the approach.Once established inbound; we began configuring for landing. Flaps were selected to 2 (full flaps is 3 which we were planning to set later); and the gear was selected down. Shortly after the gear was confirmed down and locked (approximately 10-15 seconds) we received an amber CAS (Crew Alerting System) message alerting us 'HYD VOLUME LOW A'. The Citation X has two independent hydraulic systems; with A being the most critical as it operates the gear; wheel brakes; and nose wheel steering... all of which have non hydraulic backup systems. We immediately leveled and called ATC to request termination of the approach and vectors for ZZZ1. ZZZ1 was ideal because it had much longer runways; good weather; and a Service Center. Knowing that the A side hydraulics controlled landing gear actuation; we left the gear extended and flaps set in position 2. ATC gave us a heading of 060 and an altitude of 5000 ft. We flew slow (170 kts) towards ZZZ1 and ran through the appropriate checklists. The first checklist; for the loss of hydraulic volume; had us unload the appropriate pump and pull the PTU (Power Transfer Unit) circuit breaker so the PTU wouldn't adversely affect the remaining B side hydraulics. We were then directed to the checklist for landing procedures with no hydraulic system A pressure. Flaps 2 was the maximum flap input allowed and normal flap 2 landing distance was to be increased by a factor of 1.7. Our normal flap 2 distance was computed to 3310 ft and increased with the appropriate factor to 5630 ft. We chose the runway which was nearest the service center. Our landing weight was to be just under 30;000 lbs. We considered reducing our weight further; but reduction in distance required wouldn't have been very significant. Our computed Vref speed was 134. Braking was going to be accomplished via a pressurized nitrogen bottle which is used in lieu of hydraulic fluid. The biggest disadvantage with this system is that it doesn't allow for anti-skid braking. I've been attending recurrent simulator training events twice a year for [many] years on Citation models. They all use the same nitrogen system and its imperative during simulator training that you ease back on the braking handle gently and smoothly or the wheels will lock up; resulting in blown tires and lateral runway excursions at high speed. After the airplane slows to 80 kts or so; the handle can be pulled more aggressively and the airplane comes to a stop within the computed landing distance. Pilots are also trained not to release or pump the handle as nitrogen pressure will be expelled overboard. A single smooth and steady pull is what is ingrained and also advised in the checklist. Because of this; we called and explained our situation requesting that they be prepared to meet us on the runway with a tug to pull us from our stopping point to the Service Center; as we didn't want to let go of the handle. We also let ATC know of the situation and requested that emergency vehicles be standing by to assist if need be. Most X pilots are also aware of a runway excursion which happened at JFK where the crew had to utilize the backup system and deployed the right thrust reverser (with thrust; which is prohibited); resulting in a high speed lateral excursion and major aircraft damage. With this on our minds; and an additional 3000 ft of runway available; I elected to leave the right thrust reverser and speed brakes stowed. I didn't want to impede directional control as I felt that we had more than sufficient runway available and the landing distance calculations are not based on the usage of either of these systems anyway. I also train with leaving them stowed and it's never adversely affected the stopping performance; but the thrust reverser in particular can adversely affect directional control. My primary focus on landing was to not lock up the wheels (again; no anti-skid protection) and to keep the airplane tracking straight. My training experience led me to believe the braking system would be more than adequate with the runway we had available. As we joined final approach; I noticed my speed was a little fast... in the 150 range. I only fly flap 2 approaches in training; so it took a little while to find a power setting which kept us close to Vref. The pilot monitoring gave a 'ref plus 10' call at 100 ft and I pulled the throttles back to idle. The flare and touchdown were smooth; if maybe a little long. As soon as the nose was grounded; I reached for the emergency brake handle and applied smooth and steady pressure. The airplane did not seem to decelerate much. At the midpoint of the runway; I began increasing pressure; but again did not feel the responsiveness that I was used to in the simulator. The aircraft gave a 'three thousand' foot call to announce runway remaining. I now pulled much harder and the airplane finally began to slow; but did not ever lock up the wheels or provide the aggressive braking that I was accustomed to and expecting. With 1000 ft remaining; it was clear that we were going to roll off the end; but we had lost enough momentum that I knew there would be no damage; other than to my pride. It felt akin to sliding on an icy road; when you know you're not getting stopped but that you'll not hit anything either... just along for the ride. The nose wheel has accumulator pressure to provide limited steering inputs; so I used the tiller to avoid edge lighting and the ILS structures straight ahead. I veered slightly left and the aircraft exited the end of the asphalt at maybe 10-15 mph. We rolled through a layer of gravel separating the asphalt from the dirt and came to rest in a grass field approximately 15 ft from the edge of the pavement. Vehicles were on the scene and Cessna also responded rapidly. The PM escorted the passengers from the aircraft while I secured the cockpit. There were no injuries; dents; or even scratches on the paint. Surprisingly; passengers were all smiles as they exited the aircraft and the whole event began to feel surreal. Despite the positive outcome; I still had a pit in my stomach that we must have done something wrong. Pride was getting the better of me. The hydraulic leak was the result of a failed O-ring in the slat control valve. When we retracted the slats after departure; fluid began leaking at a slow rate. It did not leak enough in the first 45 minutes to get noticed; and during the last 45 minutes we were so busy preparing for our approach and circling procedure that it wasn't noticed either. The CAS message doesn't alert the crew until it reaches 16%. The flight parameter data was downloaded from the aircraft's monitoring systems. It showed that we did in fact land a little fast... 6 kts over Vref and approximately 500 ft beyond the normal touchdown zone. [We] also felt that the landing weight contributed to our difficulties stopping the aircraft. The checklist makes no mention of landing weight limitations outside of the normal structural weight. There were a couple of pilots present who acknowledged that the simulator training promotes cautious use of the brake lever due to it's propensity to blow tires and felt that it may not be an accurate simulation of the system. We left the meeting in agreement that if the simulator isn't accurate; then pilots need to be educated on how to actually operate the handle as it's intended; since they'll only have a few seconds to recalibrate their rooted habits when they actually need to use it (8 seconds from our nose wheel touchdown to departing the runway). We also urged them to reconsider the 1.7 factor if it is based on maximum braking and being perfectly on speed to which they agreed to consider. Lessons learned and objectives going forward:First and foremost; Citation pilots need to be educated that the emergency brake system does not produce the results that pilots have become accustomed to during simulator training.  I will hold a safety meeting within our department to explain the event with our own company pilots; so that they know what to expect and what they could do to help themselves out in a similar scenario. Namely; go slow. I feel that we were plenty prepared for what we expected to happen; but not well prepared for the unexpected... which had a much higher possibility of occurrence; given the circumstances. If I had it to do over; I would have requested a holding pattern north of ZZZ1. I would have taken 30 minutes or more to burn off excess fuel; brief a plan of action if everything works out as expected; discuss what's likely to go wrong and a plan of action if it does; and also a plan of action if things go horribly wrong. We advised ATC of our situation and intentions; but we could have gone deeper. I'd take that time to call our operations and maintenance team so that everyone is aware of what is happening and to expect our call upon landing for either good news or bad news. I'd give consideration to even longer runways; even if they're inconvenient for passengers or maintenance. However; given the close proximity to water that most nearby airports are; I'm not sure they'd be as preferable as ZZZ1 was. Rolling into the water at another nearby airport would've been very bad news. Lastly; I'd fly the approach as precisely as possible and touch down with as much remaining runway as possible so that my performance can't be judged or cited as a contributor to a potential incident.Despite the very fortunate results of this incident; there is always room for improvement.There is a very unique opportunity for myself and many others in the Citation community to learn and grow from this experience.


Second narrative: [Report narrative contained no additional information.]


Synopsis: C750 flight crew reported a hydraulic failure that required use of the emergency brake system on landing which did not slow the aircraft enough. This resulted in a runway excursion of about 15 ft.