Learjet 35
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Aircraft
Around 40 minutes outside of ZZZ; while in cruise flight at FL360; the LO HYD light illuminated. The hydraulic pressure gauge confirmed that the pressure was below normal. We monitored the gauge and realized it was losing pressure. As a crew we prepared for the possibility of a complete hydraulic failure. We prepared the cabin and medical crew for a possible [forced] landing. TRACON was informed of our hydraulic system failure and assisted us in our descent. Once on approach to Runway XXR we tried to lower the gear with no success. At this point the hydraulic pressure gauge indicated zero pressure. We then performed the Hydraulic System Failure/Alternate Gear Extension procedure which blew the gear down and 3 green; 2 red lights confirmed the gear was down. At this time we [received priority handling] with ZZZ Tower. After the auxiliary hydraulic pump was energized we were able to lower the flaps to 20 degrees and adjusted our target speed accordingly. On touch down to Runway XXR; First Officer released the parachute and we felt the drag for 1-2 seconds before the chute separated from the aircraft. I was then able to use the thrust reversers and also had regular breaking. We then exited the runway and made it to the ramp under our own power. Upon inspection of the aircraft we found hydraulic fluid on the left flap; left main gear wheel well and on the left side if the [empennage].
During the initial preflight; the hydraulic reservoir had fluid to the top of the sight glass and the indicating ball was at the top. I did not visually recheck the fluid level after that but during post-flight and preflight walk arounds before and after each leg there was no indication of a hydraulic leak. About 40 minutes from landing at ZZZ; the Low Hydraulic Caution light illuminated; and the gauge was showing under 1;000 psi. The PF (Pilot Flying) and I reviewed the QRH Yellow Tabs 10 and 13 a couple of times to ensure we understood what was going to happen and found the related circuit breakers. We discussed our time schedule and decided that we would [have priority handling] if we needed to blow the gear down. We did inform Center and Approach of our issue and they were vectoring us to give us time to descend and slow down for the gear extension. We informed the medical crew of the situation and told them to ensure that all the gear in back was stowed and secured tightly. We followed the QRH and when the gear failed to fully extended; we followed through and blew the gear down. After which we [received priority handling] with the ZZZ Tower and was cleared to land on Runway XXR. We turned on the electric pump on short final per the QRH and once the main gear touched down; we deployed the drag chute; thrust reversers; spoilers; and found we had normal hydraulic braking but stayed ready to use the emergency air brakes if needed. We turned off the runway and advised the tower that we were dropping the drag chute; but it had already detached right after deployment. We taxied back to the FBO and inspected the aircraft. Hydraulic fluid was present on the left flap upper and lower surfaces; and the underbelly of the plane from the gear doors back being heavier on the left side of the aircraft. The drag chute was found on the runway around the point of touchdown and deployment on the center line. I am not sure what caused the low pressure so it is hard to say if anything could have prevented it.
Flight Crew reported loss of hydraulic system quantity resulted in a precautionary emergency landing at the destination airport.