AST 473 Citation II Systems Pressurization Dr Barnhart

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

Pressurization- Overview • System uses pressurized bleed air pumped in via the constant flow from the airconditioning 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

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

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.

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.

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)

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

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 • 2 nd 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

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 knobbleeds air btn. The 2 sealed chambers and (w. the isobaric bellows) determines rate spring pressure applied to diaphragm when new C. A. selected

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

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

Pressurization- Outflow Valves • Emer. Dump valve- located in vacuum lineroutes vacuum directly to outflow valve & dumps cabin press. In an emerg. • Dump lever- guarded switch

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)

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.

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.

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.

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.

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

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

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

The End