Beechjet 400
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 140; Flight Crew Total 3400; Flight Crew Type 170
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Other / Unknown
Flight Crew Landed in Emergency Condition; General Declared Emergency; General Maintenance Action
Procedure; Human Factors; Aircraft
Upon initial descent into the airport; the Low Hydraulic annunciator was illuminated followed by a RH PMP PRESS LO annunciator. Hydraulic pressure was normal in the Green and continued our descent into ZZZ as per the Checklist. Upon initial extension of flaps to 20-degrees; hydraulic pressure dropped into the Yellow arc; accompanied by both LH and RH PMP PRESS LO annunciators illuminating. Flaps only extended to a slight amount past 10-degrees. It was decided to try [Landing] Gear extension. When selected; only the main gears came down and indicated Green. It was decided to abort the approach and level off at 12;000 MSL. An emergency was declared and [we] asked to precede West bound to troubleshoot.While over the airport; hydraulic pressure fluctuated from zero to 700 psi. Flaps set to 20-degrees and the Nose Gear came down to indicate Green. Turned back towards the airport and while in the turn; the gear doors closed and indicated three Green; no Red with flaps set to 20-degrees. Emergency services were dispatched and awaiting our arrival at ZZZ1. Crew planned to stop on the runway after executing a flaps 20 approach and using only emergency brakes. While turning base for Runway 28; the pressure came back to green 1450 psi. Both pilot and copilot brake pedals were pumped to come back to normal pedal feel. The copilot was the pilot flying during the leg and the entire incident. Crew briefed; the copilot would land the aircraft and try normal braking while the Captain was ready to actuate Emergency Braking. Only braking was to be used to slow the aircraft. Aircraft landed and normal braking only was used. No damage or injury occurred as a result of the incident. It was determined the Right Hydraulic Filter housing was torqued too tight during previous maintenance and cut an O-ring and deformed a drop down ring. This allowed the leak. Maintenance was performed at ZZZ1 and aircraft returned to service.
Reporter stated the BeechJet 400 does not have traditional Ailerons.  Instead; spoilerons that function as spoilers and ailerons are used for roll control. Spoilerons and elevators are cable driven; no hydraulic boost.  The Left and Right Hydraulics are not two separate Hydraulic Systems. Instead; both Left and Right Hydraulic pumps share a common hydraulic manifold using only one Hydraulic Reservoir. The aircraft does not have a Standby Hydraulic System and no Stand pipe for standby fluid in the single reservoir. The Left and Right Hydraulic pumps have separate filters mounted next to a single hydraulic reservoir.  The filter housing is located under the Tail section; but access is through the Baggage Compartment and through another panel. The filters are in a very tight area.Reporter stated both hydraulic filters were replaced approximately a week earlier during an A-Check. He believes a Leak Test was performed after the filter changes; but does not know what the Test entails.  The aircraft was grounded after their incident and a Maintenance Team from their Base Station was flown in.  During troubleshooting Maintenance saw hydraulic fluid spraying from the Right Filter housing as they cycled the flaps. He has not heard of other pilots experiencing a similar situation.
A Raytheon Beechjet 400 Captain reports about a Low Hydraulic annunciation followed by a Left and Right LH-RH PMP Press LO annunciations during initial extension of flaps to 20-degrees. Hydraulic pressure dropped from Green to Yellow indication. Emergency declared. Maintenance found Right Hydraulic Filter housing torqued too tight; O-ring cut and drop-down ring deformed causing leak.