ast 473 citation ii systems pressurization dr. barnhart (abbreviations- c.a. – cabin altitude)

AST 473 Citation II AST 473 Citation II SystemsSystems

PressurizationPressurization

Dr. BarnhartDr. Barnhart

(Abbreviations- C.A. – Cabin (Abbreviations- C.A. – Cabin Altitude)Altitude)

Pressurization- OverviewPressurization- Overview

• System uses pressurized bleed air System uses pressurized bleed air pumped in via the constant flow from pumped in via the constant flow from the air-conditioning system.the air-conditioning system.

• Air is then leaked out at a controlled Air is then leaked out at a controlled rate via the 2 outflow valvesrate via the 2 outflow valves

• 2 elements req. for cabin 2 elements req. for cabin pressurization:pressurization:– Constant source of airConstant source of air– Method of controlling outflowMethod of controlling outflow


• Components:Components:– Pressure controllerPressure controller– 2 outflow valves2 outflow valves– 2 cabin altitude limit valves2 cabin altitude limit valves– Pneumatic relayPneumatic relay– Emergency Dump valveEmergency Dump valve– Regulated vacuum supplyRegulated vacuum supply


• LimitationsLimitations– 8.8 psid8.8 psid– S.L. C. A. up to 23,000 ft.S.L. C. A. up to 23,000 ft.– 8,000 ft. C.A. up to 43,000 ft.8,000 ft. C.A. up to 43,000 ft.

– Ram air pressurizes tailcone- prevents Ram air pressurizes tailcone- prevents moisture from getting in.moisture from getting in.

Pressurization- ControllerPressurization- Controller

• Variable isobaric controller- drives 2 Variable isobaric controller- drives 2 identical outflow valves located on identical outflow valves located on aft pressure bulkheadaft pressure bulkhead

• Valves modulate outflow airValves modulate outflow air• Each valve has a C.A. limiter- Each valve has a C.A. limiter-

overrides any pressurization control overrides any pressurization control system failure which would cause system failure which would cause C.A. to > 13,000 ft. +/- 1,500 ft.C.A. to > 13,000 ft. +/- 1,500 ft.


• 3 solenoid valves- primarily for gnd. 3 solenoid valves- primarily for gnd. Ops.Ops.– Valve A- on controller- normally open Valve A- on controller- normally open

vlave, energized closed when:vlave, energized closed when:•Either or both throttles above 85% N(2) ANDEither or both throttles above 85% N(2) AND

•A/C on gnd.A/C on gnd.

•Valve remains closed on T/O roll allowing rate-Valve remains closed on T/O roll allowing rate-control to be disabled and for the pneumatic control to be disabled and for the pneumatic relay to control pressurization (prevents T/O relay to control pressurization (prevents T/O pressure bump)pressure bump)


• Solenoids B and C- two-way, two-Solenoids B and C- two-way, two-position N.C. (normally closed)- position N.C. (normally closed)- energized open-energized open-– Either throttle below 80% N(2) ANDEither throttle below 80% N(2) AND– A/C on gnd.A/C on gnd.– 3 valves connected via the NORM PRESS 3 valves connected via the NORM PRESS

circuit of left CB panel- gnd. Via left circuit of left CB panel- gnd. Via left squat switch and 2 throttle solenoidssquat switch and 2 throttle solenoids


• Controller- meters control air (vacuum) to Controller- meters control air (vacuum) to outflow valves to regulate position. It outflow valves to regulate position. It consists of:consists of:– 2 chambers separated by movable diaphragm2 chambers separated by movable diaphragm

• 1 chamber- cabin pressure1 chamber- cabin pressure

• 22ndnd chamber- ambient pressure chamber- ambient pressure

• This differential pressure causing diaphragm to move This differential pressure causing diaphragm to move routing control air to pneumatic relayrouting control air to pneumatic relay

• Relay- amplifies signal to vary vacuum delivered to Relay- amplifies signal to vary vacuum delivered to outflow valves- cabin press. Incr. or decr. Until outflow valves- cabin press. Incr. or decr. Until equilibrium is reachedequilibrium is reached


• C.A. is set via the C.A. knob on the press. C.A. is set via the C.A. knob on the press. Cnt’l panel- this applies a spring bias to the Cnt’l panel- this applies a spring bias to the movable diaphragm changing the pressure movable diaphragm changing the pressure btn. the 2 chambers causing pr. Fluctuation.btn. the 2 chambers causing pr. Fluctuation.

• Rate of incr./decr. Controlled by rate knob- Rate of incr./decr. Controlled by rate knob- bleeds air btn. The 2 sealed chambers and bleeds air btn. The 2 sealed chambers and (w. the isobaric bellows) determines rate (w. the isobaric bellows) determines rate spring pressure applied to diaphragm when spring pressure applied to diaphragm when new C.A. selectednew C.A. selected


• Cabin altimeter and rate-of-change Cabin altimeter and rate-of-change indicators- on press. Cnt’l panel- indicators- on press. Cnt’l panel- Cabin altimeter on outer scale and Cabin altimeter on outer scale and diff. pr. On inner scalediff. pr. On inner scale

Pressurization- Outflow Pressurization- Outflow ValvesValves• 2 forces working on the outflow valves:2 forces working on the outflow valves:

– Spring- trying to close & pressurizeSpring- trying to close & pressurize– Control Air (Vacuum) regulated by cabin air Control Air (Vacuum) regulated by cabin air

pressure controller & amplified by pneumatic pressure controller & amplified by pneumatic relay- trying to open & de-pressurizerelay- trying to open & de-pressurize

C.A. limit valves- if control vacuum exceeds the C.A. limit valves- if control vacuum exceeds the baro. Ref. in the C.A. limit valves- they open baro. Ref. in the C.A. limit valves- they open & allow cabin air to enter control air line & allow cabin air to enter control air line reducing the vacuum- closing valvereducing the vacuum- closing valve

Pressurization- Outflow Pressurization- Outflow ValvesValves

• Emer. Dump valve- located in Emer. Dump valve- located in vacuum line- routes vacuum directly vacuum line- routes vacuum directly to outflow valve & dumps cabin to outflow valve & dumps cabin press. In an emerg.press. In an emerg.

• Dump lever- guarded switchDump lever- guarded switch

Pressurization- Outflow Pressurization- Outflow ValvesValves• Outflow valves calibrated at 8.7 =/- .1 psiOutflow valves calibrated at 8.7 =/- .1 psi

• During taxi- vacuum through solenoid C During taxi- vacuum through solenoid C routed directly to fully open both outflow routed directly to fully open both outflow valves assuring depressurization on valves assuring depressurization on ground- accomplished by solenoid C ground- accomplished by solenoid C being energized open by left squat switch being energized open by left squat switch AND either or both throttles being below AND either or both throttles being below 80% N (2)80% N (2)

Pressurization- OperationPressurization- Operation

• Pilot selects desired cruise alt. prior Pilot selects desired cruise alt. prior to T/O on controller with dial labeled to T/O on controller with dial labeled ACFT. - C.A. at this cruise alt. is then ACFT. - C.A. at this cruise alt. is then displayed on the adjacent scale displayed on the adjacent scale labeled CABINlabeled CABIN

• Position rate control within the white Position rate control within the white normal arc.normal arc.


• During T/O- advancing throttles to above During T/O- advancing throttles to above 85% causes the solenoid valves to close85% causes the solenoid valves to close– This moves the outflow valves into the This moves the outflow valves into the

controlling range and traps ambient pr. For ref. controlling range and traps ambient pr. For ref. by the pneumatic relayby the pneumatic relay

– This removes vacuum from the outflow valves This removes vacuum from the outflow valves (C closed) permitting the pneumatic relay to (C closed) permitting the pneumatic relay to assume cnt’l. (B closed)assume cnt’l. (B closed)

– Results in outflow valves being partially closed Results in outflow valves being partially closed allowing Cabin to re-press. To approx. 60 ft. allowing Cabin to re-press. To approx. 60 ft. below field pr. Alt.below field pr. Alt.


• This disables rate cont’l fcn. To provide This disables rate cont’l fcn. To provide pneumatic relay with a ref. field alt. press pneumatic relay with a ref. field alt. press during T/Oduring T/O

• Results in outflow valves and control Results in outflow valves and control elements being in the control position at lift-elements being in the control position at lift-off preventing a pressure bump at rotationoff preventing a pressure bump at rotation

• After lift-off all solenoids are de-energized (A After lift-off all solenoids are de-energized (A open, B & C closed) and normal operation open, B & C closed) and normal operation resumes- control returned to controller.resumes- control returned to controller.


• Pre-landing- select 200 ft. above Pre-landing- select 200 ft. above landing field press. Alt on controller landing field press. Alt on controller (ensures landing is de-pressurized) (ensures landing is de-pressurized) and a compatible rate w/ A/C R.O.D.and a compatible rate w/ A/C R.O.D.

• System maintains this 200 ft. until System maintains this 200 ft. until A/C passes below this alt.- valves are A/C passes below this alt.- valves are controlled open as A/C passes controlled open as A/C passes through the 200 lvl.through the 200 lvl.


• At touchdown- throttles less than At touchdown- throttles less than 80% N (2), left. Squat switch opens 80% N (2), left. Squat switch opens solenoid valve C. further ensuring solenoid valve C. further ensuring being depressurized upon landingbeing depressurized upon landing


• Elec. Failure- only affects squat and Elec. Failure- only affects squat and throttle switches thus pressurization. throttle switches thus pressurization. In flight is not affectedIn flight is not affected


• Vacuum system failure- pressure controller Vacuum system failure- pressure controller becomes inop. & outflow valves close and becomes inop. & outflow valves close and 8.8 psid is maintained by diff. press. 8.8 psid is maintained by diff. press. Limiters in ea. Outflow valve.Limiters in ea. Outflow valve.

• Partial depress. in this mode may be Partial depress. in this mode may be achieved by selecting LH or RH on source achieved by selecting LH or RH on source selector and reducing the respective selector and reducing the respective throttle- cuts source air to 6ppmthrottle- cuts source air to 6ppm

• Full depress. Achieved by selecting OFF- Full depress. Achieved by selecting OFF- stops all incoming air and remaining diff. stops all incoming air and remaining diff. leaks out prior to touchdown.leaks out prior to touchdown.

The EndThe End

ast 473 citation ii systems pressurization dr. barnhart (abbreviations- c.a. – cabin altitude)

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