AST 473 Citation II Systems Pressurization

1. AST 473 Citation II SystemsPressurization Dr. Barnhart (Abbreviations- C.A. – Cabin Altitude)

2. 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

3. Pressurization- Overview • Components: • Pressure controller • 2 outflow valves • 2 cabin altitude limit valves • Pneumatic relay • Emergency Dump valve • Regulated vacuum supply

4. 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.

5. 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 > 13,000 ft. +/- 1,500 ft.

6. 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)

7. 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

8. 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

9. Pressurization- Controller • C.A. is set via the C.A. knob on the press. Cnt’l 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

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

11. 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

12. 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

13. 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)

14. 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.

15. 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 cnt’l. (B closed) • Results in outflow valves being partially closed allowing Cabin to re-press. To approx. 60 ft. below field pr. Alt.

16. Pressurization- Operation • This disables rate cont’l 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.

17. 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.

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

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

20. 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.

21. The End