Narrative:

No abnormalities were noted of the aircraft during preflight and the flight earlier that day was uneventful. During the initial climb a short time after selecting 'gear up' the master caution and the 'hydraulic fluid low' lights illuminated. Referencing the checklist; I continued the climb while receiving several vectors from ATC around thunderstorms. At approximately 18;000 ft I heard the landing gear motor make a whining sound and then the landing gear relay circuit breaker tripped. Soon after the red light in the gear handle illuminated signaling an unsafe gear situation. At that point; I notified ATC of the hydraulic failure and landing gear problems and our intention to return to [departure airport] which provided a longer runway and better facilities. I immediately received a clearance to descend and return. During the descent; I asked for and received additional vectors to lose altitude while maintaining an airspeed less than 186 KTS in case the landing gear was partially down. I briefed our two passengers on the situation. I leveled off at 5;000 ft and selected gear down and reset the circuit breaker. I could hear the motor running; but did not receive any green lights after a few seconds. Per the manual gear extension checklist; I again pulled the landing gear relay circuit breaker and performed all the items on the manual gear extension checklist. I received three green lights. I notified ATC and received a clearance for a visual approach. I completed a normal landing without incident or any other abnormalities and taxied to the FBO. A post flight inspection revealed that the belly and lower tail area aft the wing were coated in hydraulic fluid. No other visual indications of problems could be seen. Subsequent maintenance work revealed that an o-ring in a fitting near the hydraulic reservoir had failed causing all the fluid to leak out. The aircraft was repaired; tested and returned to service. Since the failure was internal; little could be done to detect this problem prior to it occurring again.

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Original NASA ASRS Text

Title: A King Air 350 lost hydraulic fluid after takeoff. The Pilot declared an emergency; followed procedures for manual gear extensio and returned to the departure airport.

Narrative: No abnormalities were noted of the aircraft during preflight and the flight earlier that day was uneventful. During the initial climb a short time after selecting 'Gear Up' the master caution and the 'Hydraulic Fluid Low' lights illuminated. Referencing the checklist; I continued the climb while receiving several vectors from ATC around thunderstorms. At approximately 18;000 FT I heard the landing gear motor make a whining sound and then the landing gear relay circuit breaker tripped. Soon after the red light in the gear handle illuminated signaling an unsafe gear situation. At that point; I notified ATC of the hydraulic failure and landing gear problems and our intention to return to [departure airport] which provided a longer runway and better facilities. I immediately received a clearance to descend and return. During the descent; I asked for and received additional vectors to lose altitude while maintaining an airspeed less than 186 KTS in case the landing gear was partially down. I briefed our two passengers on the situation. I leveled off at 5;000 FT and selected gear down and reset the circuit breaker. I could hear the motor running; but did not receive any green lights after a few seconds. Per the manual gear extension checklist; I again pulled the landing gear relay circuit breaker and performed all the items on the manual gear extension checklist. I received three green lights. I notified ATC and received a clearance for a visual approach. I completed a normal landing without incident or any other abnormalities and taxied to the FBO. A post flight inspection revealed that the belly and lower tail area aft the wing were coated in hydraulic fluid. No other visual indications of problems could be seen. Subsequent maintenance work revealed that an o-ring in a fitting near the hydraulic reservoir had failed causing all the fluid to leak out. The aircraft was repaired; tested and returned to service. Since the failure was internal; little could be done to detect this problem prior to it occurring again.

Data retrieved from NASA's ASRS site as of July 2013 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.