GENERALThe hydraulic system permits the applicationof substantial force by converting a volume offluid flow into pressure on a hydraulic piston.Hydraulic fluid lines provide the capability totransmit that force wherever it is required inthe aircraft without heavy or complex me-chanical linkages.

In the Mustang, a reservoir stores hydraulicfluid for the pump and receives return flowfrom the system (Figure 13-1). One electri-cally driven pump supplies hydraulic power.

Hydraulic fluid is routed through lines, regu-lated by system valves, and cleaned by filters.The hydraulic system responds automaticallyto the activation of controls for the landinggear and brakes. The engine indicating andcrew alerting system (EICAS) indicates systemstatus. Also, landing gear and brakes each havea pneumatic backup system to provide pressureto their respective systems in the event of hy-draulic system failure (refer to Chapter 14—“Landing Gear and Brakes”).

INTRODUCTION This chapter provides information on the hydraulic system in the Citation Mustang air-craft. Hydraulic fluid driven by a pump and regulated by valves provides pressure forlanding gear and brakes. Operation of these devices is presented in Chapter 14—“LandingGear and Brakes.” This chapter describes the portions of the hydraulic system used byboth subsystems.

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CHAPTER 13HYDRAULIC POWER SYSTEMS

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DESCRIPTION The hydraulic system is classified as a “closedcenter” system. When no subsystems are in use(landing gear or brakes), there is normally noflow in the system, except to maintain accu-mulator charge.

The Mustang uses hydraulic power for re-traction and extension of the landing gear,and operation of the brakes. The system in-cludes a single electrically driven hydraulicpump, which functions to maintain and sup-plement accumulator pressure.

The LANDING GEAR selector and gear-po-sition sensor switches determine which pres-sure switch regulates pump operation andresultant system pressure. When the combi-nation of LANDING GEAR selector, weighton wheels, and gear position sensor switchesindicate the gear is ready for retraction or isin transit, a gear-retraction pressure switch isautomatically selected to regulate the pump tomaintain approximately 1,300 psig minimumsystem pressure. At all other times when thehydraulic system is energized, a normal pres-sure switch is automatically selected to regu-late pressure, maintaining 1,050 psig minimum.(For details about the LANDING GEAR se-lector and gear-position sensor switches, referto Chapter 14—“Landing Gear and Brakes”).Both pressure switches limit maximum pres-sure to approximately 1,550 psig. The systemrelief valve provides additional protection ifsystem pressure exceeds 1,750 psig.

A hydraulic reservoir stores hydraulic fluidfor the pump and receives return flow from thesystem. During system operation, an accu-mulator stores hydraulic fluid under nitro-gen pressure to maintain system pressure.Two filters clean the system: one in the pres-sure flow upstream of the manifold and down-stream of the pump, and one in the returnflow, upstream of the reservoir.

The system uses red MIL-PRF-87257 hy-draulic fluid and is designed for operation inambient temperatures ranging from –65°F to160°F (–54°C to 71°C).

COMPONENTSThe majority of the hydraulic system compo-nents are in the lower left nose compartmentoutside the pressure vessel.

RESERVOIRThe hydraulic reservoir is attached to the leftforward pressure bulkhead (Figures 13-2 and13-3) and stores hydraulic fluid for the pumpand receives return flow from the system. Whenfluid flows, the excess hydraulic fluid returnsto the reservoir. Cabin air pressurizes the reser-voir to reduce foaming and assure positive flowto the pump. A relief valve opens at approxi-mately 10 psi to prevent overpressurization.

There are two sight gauges on the hydraulicreservoir that are visible through the forwardbaggage compartment liner. The reservoir ca-pacity is approximately 2.2 quarts (2 liters) asindicated by the full mark.

PUMPThe hydraulic pump is powered by normal DCpower through the HYD PUMP circuit breaker.The pump operates whenever the pressure dropsbelow the lower set point of the normal gear re-tract pressure switch. Pump operation discon-tinues once pressure is restored to the upper setpoint of the regulating pressure switch.

HYDRAULIC ACCUMULATOR The hydraulic accumulator receives fluid fromthe pump and stores a supply of hydraulic fluidunder pressure. During pump inactivity thispressurized fluid maintains pressure againstnormal internal leakage within the system. It isalso used to supplement pump flow during land-ing gear extension and retraction.

PRESSURE SWITCHES There are a total of three pressure switches inthe control manifold. Two of these switchescontrol pressure to the overall system and gear

retraction cycle; the third pressure switch pro-vides a warning for low system pressure. Theswitches are identified as NORMAL, GEARRETRACTION and LOW PRESSURE. TheNORMAL pressure switch maintains a con-stant pressure on the system under no load. TheGEAR RETRACTION pressure switch pro-vides increased system pressure for the gearretraction cycle. The LOW PRESSURE switchalerts the crew to an overall low system pres-sure through an amber HYD PRESS LO mes-sage in the CAS window.

NORMAL Pressure SwitchWhenever the GEAR RETRACTION pressureswitch is not active, the NORMAL pressureswitch regulates pump operation. It closes toenergize the pump whenever system pressurefalls below approximately 1,050 psig and opensto deenergize the pump when system pressurerises above approximately 1,550 psig.

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Figure 13-3. Hydraulic Reservoir Sight Gauge

Figure 13-2. Hydraulic Reservoir and Location

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GEAR RETRACTION PressureSwitchWhen the LANDING GEAR selector is se-lected UP and the landing gear is not fully re-t r ac t ed ( a s i nd i ca t ed by up lock s enso rswitches) or when the LANDING GEAR se-lector is selected GEAR DOWN and the air-craft is airborne (as indicated by the squatswitches), the GEAR RETRACTION pres-sure switch regulates pump operation. It closesto energize the pump whenever system pres-sure falls below approximately 1,300 psig andopens to deenergize the pump when systempressure rises above approximately 1,550 psig.

LOW PRESSURE SwitchThe LOW PRESSURE switch (750 psig–1,000psig) signals the HYD PRESS LO message inthe CAS window to display. This message ad-vises the crew that the hydraulic pressure is lowand that the hydraulic pump failed to operateautomatically when signaled by the NORMALor GEAR RETRACTION pressure switches.

SYSTEM RELIEF VALVEA mechanical relief valve in parallel with thelanding gear and brakes maintains the systempressure at a maximum of 1,750 psig.

FILTERSThe system incorporates two filters: one forfiltering fluid leaving the pump and one for fil-tering return fluid prior to entering the reser-voir. Each filter incorporates a bypass valvethat opens at 100 ± 10 psid if the filter elementclogs. As a bypassed filter is a standard main-tenance interval item, there is no cockpit in-dication or filter indication of bypassing.

CONTROLS AND INDICATIONSThe hydraulic system functions automati-cally during normal DC power (supplied bygenerator, battery, or ground power unit).

All inflight indications for the hydraulic sys-tem appear as crew alerting system (CAS) mes-sages on the EICAS as displayed on themultifunction display (MFD) or (in reversion-ary mode) on the primary flight displays (PFDs).

HYD PRESS LO MESSAGEThe low pressure switch in the hydraulic con-trol manifold controls the amber HYD PRESSLO message. As hydraulic system pressuredecreases below 750 psig, the HYD PRESS LOmessage appears, accompanied by MASTERCAUTION lights. As the pump increases sys-tem pressure to greater than 1,000 psig, a cir-cuit opens to extinguish the message and theMASTER CAUTION lights.

NOTE On the ground, MASTER CAUTIONlights will not reset as long as theHYD PRESS LO message remainsvalid. This is to caution against taxi-ing in this condition and to advise thatnormal brakes may not be available.

HYD PUMP ON MESSAGEThe amber HYD PUMP ON message indi-cates that the hydraulic pump has been oper-ating continuously for over 60 seconds. Referto the checklist.

HYDRAULIC RESERVOIRSIGHT GAUGEThe hydraulic reservoir sight gauge indicatescurrent hydraulic fluid quantity. The gauge isintegral to the hydraulic reservoir, and is vis-ible through the aft wall of the nose baggagecompartment (see Figure 13-3).

HYDRAULIC ACCUMULATORPRESSURE GAUGEA pressure gauge on the accumulator (Figure 13-4) indicates current pressure in the accumulator.The gauge is on the aft wall of the nose baggagecompartment, and indicates 0–2,000 psig.

MANUAL ACCUMULATORBLEED VALVEThe manually actuated accumulator bleedvalve is in the left forward baggage com-partment next to the accumulator pressuregauge. It is accessed through a hinged accesspanel (Figure 13-5). On the ground, it enablesthe crew to release pressurized fluid from theaccumulator to the reservoir. This is doneduring preflight inspection if the hydraulicsystem was previously energized to ensureaccurate reservoir fluid level and accumulatorprecharge readings.

OPERATION

PREFLIGHTBleed the accumulator prior to checking theaccumulator precharge and fluid level in thereservoir. Verify that the accumulator isprecharged per the placard and that the hy-draulic fluid level is adequate (no air visiblein lower sight gauge).

IN FLIGHT

Hydraulic SystemThe hydraulic system operates automaticallyto maintain pressure and sends cautionary CASmessages to the crew if there is a fault.

Hydraulic SubsystemsHydraulically powered subsystems includelanding gear and brakes. Application of hy-draulic power to the two subsystems is pre-sented in Chapter 14, “Landing Gear andBrakes.”

EMERGENCY/ABNORMALFor specific information on emergency/abnor-mal procedures, refer to the appropriate ab-breviated checklists or the FAA-approvedAirplane Flight Manual (AFM).

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Figure 13-5. Manual Accumulator Bleed Valve

Figure 13-4. Hydraulic Accumulator Pressure Gauge