Date: 201508
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: Mixed
Weather elements / visibility: Rain; Thunderstorm
Light: Daylight
Aircraft: Learjet 45
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Descent; Final Approach; Initial Approach; Landing
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ; Tower ZZZ
Aircraft component: Hydraulic Main System
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine
Human factors: Situational Awareness; Troubleshooting; Workload
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory; General Maintenance Action
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

Narrative: Approximately twenty minutes into the flight; a white MAIN HYD QTY LO CAS illuminated. We monitored the HYD schematic for main [and] aux pump status and pressure. Enroute; nothing else abnormal was evident.Enroute; we discussed preparations in the event the CAS resulted from a leak. We reviewed the AFM and QRH for each hydraulic CAS; and prepared for alternate gear extension and landing procedures. We discussed weather in the area (Thunderstorms; heavy rain) and landing at our filed destination or elsewhere if degraded performance necessitated a longer runway. We calculated landing distance for landing with Aux Hydraulic pressure available and with full hydraulic failure. Approximately twenty miles out; the icon for the left main hydraulic pump turned amber and the white L HYD PUMP LOW CAS illuminated. We decided at this point to deploy the landing gear early.When we lowered the gear lever; we lost main hydraulic pressure and gained the amber MAIN HYD PRESS CAS. Lowering the gear lever resulted in three green locked gear indications but white hashes for the gear doors. AUX pump pressure remained stable. We began the Main Hyd System Failure Landing checklist. With three greens we did not extend the freefall lever. We lowered the flaps with the Aux Hyd pump and Hyd Xflow switch; then turned off the Aux pump to preserve aux fluid and normal braking. We noted the gear doors closed during Hyd Xflow operation. We completed the landing checklist and monitored Aux Hyd pressure and pump status to verify brake operation. In landing configuration before accepting the approach; with proper brake operation anticipated; and a thunderstorm currently overhead our destination and another south of our alternate; we elected to proceed with the approach into a nearby airport. We acquired visual contact with the runway about 10 miles out. We anticipated normal braking.We reengaged the Aux Hyd pump on short final and except for partial flaps and no thrust reversers; the landing was normal.After landing; we exited the runway at our usual point - with normal brake pressure available. We continued to monitor aux pump pressure and no difficulties were encountered on the short taxi into parking.Passengers disembarked normally.On postflight; we noted hydraulic fluid up the tail of the aircraft.




Synopsis: Lear 45 flight crew received low hydraulic quantity warning; but observed no obvious anomalies. During descent the crew lowered the landing gear early due to a subsequent left hydraulic pump low warning and at alternate because of thunderstorm.