Narrative:

Approximately twenty minutes into the flight; a white main hydraulic qty lo cas illuminated. We monitored the hydraulic 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 left hydraulic 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 hydraulic press cas. Lowering the gear lever resulted in three green locked gear indications but white hashes for the gear doors. Auxiliary 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.

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Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.