AST 473 Citation II Systems Pressurization

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AST 473 Citation II SystemsPressurization

• Dr. Barnhart
• (Abbreviations- C.A. Cabin Altitude)

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Pressurization- Overview

• System uses pressurized bleed air pumped in via
  the constant flow from the air-conditioning
  system.
• Air is then leaked out at a controlled rate via
  the 2 outflow valves
• 2 elements req. for cabin pressurization
• Constant source of air
• Method of controlling outflow

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Pressurization- Overview

• Components
• Pressure controller
• 2 outflow valves
• 2 cabin altitude limit valves
• Pneumatic relay
• Emergency Dump valve
• Regulated vacuum supply

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Pressurization- Overview

• Limitations
• 8.8 psid
• S.L. C. A. up to 23,000 ft.
• 8,000 ft. C.A. up to 43,000 ft.
• Ram air pressurizes tailcone- prevents moisture
  from getting in.

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Pressurization- Controller

• Variable isobaric controller- drives 2 identical
  outflow valves located on aft pressure bulkhead
• Valves modulate outflow air
• Each valve has a C.A. limiter- overrides any
  pressurization control system failure which would
  cause C.A. to gt 13,000 ft. /- 1,500 ft.

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Pressurization- Controller

• 3 solenoid valves- primarily for gnd. Ops.
• Valve A- on controller- normally open vlave,
  energized closed when
• Either or both throttles above 85 N(2) AND
• A/C on gnd.
• Valve remains closed on T/O roll allowing
  rate-control to be disabled and for the pneumatic
  relay to control pressurization (prevents T/O
  pressure bump)

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Pressurization- Controller

• Solenoids B and C- two-way, two-position N.C.
  (normally closed)- energized open-
• Either throttle below 80 N(2) AND
• A/C on gnd.
• 3 valves connected via the NORM PRESS circuit of
  left CB panel- gnd. Via left squat switch and 2
  throttle solenoids

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Pressurization- Controller

• Controller- meters control air (vacuum) to
  outflow valves to regulate position. It consists
  of
• 2 chambers separated by movable diaphragm
• 1 chamber- cabin pressure
• 2nd chamber- ambient pressure
• This differential pressure causing diaphragm to
  move routing control air to pneumatic relay
• Relay- amplifies signal to vary vacuum delivered
  to outflow valves- cabin press. Incr. or decr.
  Until equilibrium is reached

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Pressurization- Controller

• C.A. is set via the C.A. knob on the press. Cntl
  panel- this applies a spring bias to the movable
  diaphragm changing the pressure btn. the 2
  chambers causing pr. Fluctuation.
• Rate of incr./decr. Controlled by rate knob-
  bleeds air btn. The 2 sealed chambers and (w. the
  isobaric bellows) determines rate spring pressure
  applied to diaphragm when new C.A. selected

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Pressurization- Controller

• Cabin altimeter and rate-of-change indicators- on
  press. Cntl panel- Cabin altimeter on outer
  scale and diff. pr. On inner scale

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Pressurization- Outflow Valves

• 2 forces working on the outflow valves
• Spring- trying to close pressurize
• Control Air (Vacuum) regulated by cabin air
  pressure controller amplified by pneumatic
  relay- trying to open de-pressurize
• C.A. limit valves- if control vacuum exceeds the
  baro. Ref. in the C.A. limit valves- they open
  allow cabin air to enter control air line
  reducing the vacuum- closing valve

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Pressurization- Outflow Valves

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

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Pressurization- Outflow Valves

• Outflow valves calibrated at 8.7 /- .1 psi
• During taxi- vacuum through solenoid C routed
  directly to fully open both outflow valves
  assuring depressurization on ground- accomplished
  by solenoid C being energized open by left squat
  switch AND either or both throttles being below
  80 N (2)

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Pressurization- Operation

• Pilot selects desired cruise alt. prior to T/O on
  controller with dial labeled ACFT. - C.A. at this
  cruise alt. is then displayed on the adjacent
  scale labeled CABIN
• Position rate control within the white normal arc.

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Pressurization- Operation

• During T/O- advancing throttles to above 85
  causes the solenoid valves to close
• This moves the outflow valves into the
  controlling range and traps ambient pr. For ref.
  by the pneumatic relay
• This removes vacuum from the outflow valves (C
  closed) permitting the pneumatic relay to assume
  cntl. (B closed)
• Results in outflow valves being partially closed
  allowing Cabin to re-press. To approx. 60 ft.
  below field pr. Alt.

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Pressurization- Operation

• This disables rate contl fcn. To provide
  pneumatic relay with a ref. field alt. press
  during T/O
• Results in outflow valves and control elements
  being in the control position at lift-off
  preventing a pressure bump at rotation
• After lift-off all solenoids are de-energized (A
  open, B C closed) and normal operation resumes-
  control returned to controller.

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Pressurization- Operation

• Pre-landing- select 200 ft. above landing field
  press. Alt on controller (ensures landing is
  de-pressurized) and a compatible rate w/ A/C
  R.O.D.
• System maintains this 200 ft. until A/C passes
  below this alt.- valves are controlled open as
  A/C passes through the 200 lvl.

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Pressurization- Operation

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

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Pressurization- Operation

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

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Pressurization- Operation

• Vacuum system failure- pressure controller
  becomes inop. outflow valves close and 8.8 psid
  is maintained by diff. press. Limiters in ea.
  Outflow valve.
• Partial depress. in this mode may be achieved by
  selecting LH or RH on source selector and
  reducing the respective throttle- cuts source air
  to 6ppm
• Full depress. Achieved by selecting OFF- stops
  all incoming air and remaining diff. leaks out
  prior to touchdown.

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The End