Pressurization System Overview
[G450 Aircraft Operating Manual, §2A-21-10.]
- The air conditioning system provides pressurized and temperature controlled airflow to maintain a comfortable environment for the occupants of the airplane and provides a source for equipment cooling.
- Hot pressurized air from the compressor sections of the engines or the Auxiliary Power Unit (APU) is cooled through a series of processes by the Environmental Control System (ECS) Air Conditioning Packs (ACPs), remixed with some of the high temperature bleed air to achieve the desired temperature, and then delivered throughout the airplane. Distribution ducts provide air to the cockpit, passenger cabin, and baggage compartment.
- The higher pressure of this airflow allows regulation of the ambient pressure within the airplane to maintain an air density comfortable for breathing even though the airplane may be at the highest operating altitude limit of forty-five thousand (45,000) feet. Air density within the airplane is controlled by regulating how much of the pressurized conditioned air remains within the airplane. The airflow leaving the airplane is regulated by a Thrust Recovery Outflow Valve (TROV) that opens and closes in response to automatic or manual commands to maintain the desired air density level.
It is said that the air in the G450 is completely replaced every two minutes during normal operations.
[G450 Aircraft Operating Manual, §2A-21-30, ¶1.]
- Pressurization is controlled by regulating the escape of conditioned air produced by the Air Conditioning Packs (ACPs). Conditioned air is released from the airplane through the Thrust Recovery Outflow Valve (TROV). The TROV changes the size of the opening available for release of conditioned air, thereby maintaining the desired pressure level within the airplane.
- The TROV is electrically operated in response to commands from the Cabin Pressure Controller (CPC) that has three modes of operation: automatic (AUTO), semi-automatic (SEMI) and MANUAL. The modes of operation are selected with switch buttons on the CABIN PRESSURE CONTROL panel on the cockpit overhead.
- System performance can be monitored on the CABIN PRESSURE SELECTOR PANEL, mounted in the cockpit center console to the right of MCDU #3. The selector panel has digital readouts of airplane altitude, cabin altitude, landing field elevation, barometer setting and cabin altitude rate of change (climb or descent). With the CPC in the AUTO mode, the digital readouts passively monitor system operation. If the CPC mode defaults to SEMI mode, rotary selector knobs corresponding to the digital readouts become active to provide command inputs to control pressurization.
- Because the correct operation of the pressurization system is critical to passenger and crew safety, system components are highly redundant. The TROV may be operated by any of three electrical actuators (motors), two powered by 115V AC current and the other by 28V DC current. The CPC, in addition to the three operational modes, has two control channels available for AUTO and SEMI mode operation. Data for the control channels can be provided by Air Data Systems (ADSs) #1, #2 or #3. In MANUAL mode, cabin altitude data is obtained directly from the separate Cabin Pressure Acquisition Module (CPAM) that has a dedicated independent static port. If all control of pressurization is lost, a Cabin Pressure Relief Valve (CPRV) will open to relieve overpressure to prevent
structural damage to the fuselage, windows and doors. The CPRV also opens to prevent negative pressure within the airplane, both to prevent damage and to facilitate opening normal exit doors or emergency exit windows.
Thrust Recovery Outflow Valve (TROV)
Photo: TROV and PRV, (Eddie's aircraft)
[G450 Aircraft Operating Manual, §2A-21-30, ¶2.A.]
- The aerodynamic shape of the TROV has two opposable shutters mounted vertically within a square frame. The forward shutter pivots outward and the aft shutter pivots inward to provide an opening to exhaust pressurized air. The size of the opening created by shutter movement controls airplane pressurization.
- The shutters are positioned by one of three electrical drive motors (two AC and one DC). Linkages between the shutters ensure symmetrically opposite movement. The operating drive motor rotates a gear mechanism that moves the shutters. The design of the gears is such that positive motor input is required to move the shutters in either direction. This prevents shutter movement and loss of pressurization control in the event of electrical system malfunctions or failure.
- When the CPC is operating in AUTO or SEMI mode either of the 115V AC motors is available to move the TROV shutters. Each motor is associated with one of the two CPC control channels (#1 and #2). To preserve the longevity of the pressurization system components, the active channel switches with every flight (CPC logic requires that the airplane be on the ground with weight on wheels, be fully depressurized for one minute with the TROV fully open before control channels swap - the logic avoids repetitive changes during touch and go landings). If the AC motor normally associated with the active control channel malfunctions, the CPC will automatically switch to the remaining AC drive motor. If a malfunction or failure of the 115V AC power system renders both motors inoperative, the CPC will signal the failure to the flight crew, prompting selection of MANUAL mode. In manual mode the DC motor positions the TROV shutters using switch commands directly from the cabin pressure control panel, bypassing the CPC.
- Failure of a single CPC channel is annunciated on the Crew Alerting System (CAS) display window as a blue CPCS Channel (1 or 2) Fail advisory. Failure of both channels prompts an amber CPCS Fail - Select Manual caution CAS message and illuminates the FAULT section of the FAULT / MANUAL push-button on the cabin pressure control panel. If the TROV malfunctions (independent of the operating control system) a blue Outflow Valve Fault advisory CAS message is displayed.
When the G550 first came out "they" said the TROV vectored the air aft and provided forward thrust, hence its name. They even had wind tunnel tests to prove it. Nobody seems to make that claim any more. I suspect the wind tunnel test improvements came from a reduction in parasite drag caused by a conventional outflow valve's outward vector.
Video: TROV Opening.
Cabin Pressure Controller (CPC)
[G450 Aircraft Operating Manual, §2A-21-30, ¶2.B.]
- The CPC operates in AUTO or SEMI mode using one of the two control channels to position the TROV, regulating cabin pressurization. The active control channel changes with every complete flight cycle, with the active channel shown on the ECS/Bleed 2/3 Synoptic display.
- During normal operation in the AUTO mode, the CPC derives information from the active FMS for airplane ground speed (for pre-pressurization signal) and uses ADM input for airplane pressure altitude and barometric correction. CPC channel #1 defaults to ADM #1 with ADM #3 as primary backup, then ADM #2. Channel #2 defaults to ADM #2 with ADM #3 as primary backup, then ADM #1. Each channel of the CPC unit has an integrated cabin altitude sensor that furnishes data to the active control channel. During CPC AUTO mode, no crew action is required and the cabin pressure selector panel serves only as a display of pressurization information.
- SEMI mode is available in case of FMS data loss or when lower cabin cruising altitude is desired. SEMI mode allows the pilot to manually enter the airplane / cabin altitude, the landing field elevation, the baro correction and the cabin rate of climb / descent.
- In typical flight operations with the CPC in AUTO mode, the pressurization sequence begins with a pre-pressurization phase during taxi for takeoff. With the FLIGHT / LANDING push-button on the cabin pressure control panel selected to LANDING, taxi speed ≥ nine (9) knots, power levers to a position of fifteen degrees (15°) forward or more, and a cabin or baggage door previously opened and closed, the pressurization system will change to FLIGHT. The CPC signals the TROV to a position to pressurize the airplane to five hundred (500) feet below field altitude. This sequence can also be initiated by manually selecting the FLIGHT / LANDING switch to FLIGHT. As the airplane climbs, software in the CPC controls cabin altitude to the highest of the landing field elevation or the climb schedule, ascending the cabin at a software programmed rate (up to 500 fpm) to a maximum cabin altitude of six thousand (6,000) feet. During cruise, a descent greater than 1,000 feet will change the system from FLIGHT to LANDING. When the airplane starts a descent, the CPC uses FMS data for airfield elevation to set up a descent rate (up to 300 fpm) for the cabin such that the cabin altitude will be 250 ft below landing airfield elevation at touchdown. After touchdown, with weight-on-wheels, the TROV opens to depressurize at a rate of 500 fpm for one minute, 2,000 fpm for the second minute, then goes to a full open position. After ninety (90) seconds on the ground (weight-on-wheels), the TROV will open to ensure complete depressurization. The CPC then swaps control channels for the next flight evolution.
- The TROV will automatically close upon shutdown to prevent the entry of debris or other extraneous matter (birds, insects, etc.) that might interfere with system operation. For the auto-close function to operate, a flight must have been completed and the main entry door must be open prior to the last of the engines or APU being shut down.
- If the cabin altitude exceeds eight thousand (8,000) feet, a red Cabin Pressure Low CAS warning will be displayed. If the airplane is in coupled flight (autopilot engaged) at an altitude of forty thousand (40,000) feet or above when the cabin altitude reaches 8,000 feet, the airplane will enter an emergency descent mode, turning left ninety (90°) degrees and beginning a rapid descent to fifteen thousand (15,000) feet. During descent, the cabin pressure low trip point of 8,000' cabin altitude will be automatically reset in AUTO / SEMI mode when landing at high altitude airports. If the landing airfield elevation is between 7,500 feet and 9,500 feet, the cabin low pressure warning will be reset to trip at a cabin altitude of 10,000 feet. If landing field elevation is 9,500 feet or greater, the trip point will be reset to 14,500 feet. If the airplane is operating in the SEMI pressurization mode, the flight crew should insure that the correct landing field elevation is set in the cabin pressure selector panel in order to reset the cabin pressure low trip point.
- During descent in AUTO mode, if the airplane is leveled at an altitude above 25,000 feet (FL250) for more than three (3) minutes, the CPCS mode will switch from LANDING to FLIGHT and the Cabin Pressure Low trip point will be progressively increased from 8,000 feet to no more than 15,000 feet cabin altitude, based on landing field elevation. The cabin altitude will remain constant until a climb greater than 500 feet or a descent greater than 1,000 feet is initiated.
Cabin Pressure Control Panel
Photo: Cabin Pressure Control Panel, (Eddie's aircraft)
[G450 Aircraft Operating Manual, §2A-21-30, ¶2.C.] The CABIN PRESSURE CONTROL panel on the cockpit overhead provides push-button switches and a rotary (spring-loaded neutral) knob for pressurization mode selection and manual control. An indicator of TROV shutter position is also furnished. The push-button switches are illuminated to indicate the active pressurization mode. The FAULT portion of the FAULT / MANUAL push-button also serves as an indication of AUTO and SEMI mode failure. Selections on the panel enable the flight crew to perform and control the following functions:
- Select the AUTO, SEMI or MANUAL mode of operation.
- Select the FLIGHT or LANDING mode of operation.
- Manually position TROV shutters to open or close, or to any intermediate position.
- Monitor the rate of TROV shutter movement.
[G450 Aircraft Operating Manual §2A-21-30.2.B.] With the FLIGHT / LANDING push-button on the cabin pressure control panel selected to LANDING, taxi speed > nine (9) knots, power levers to a position of fifteen degrees (15°) forward or more, and a cabin or baggage door previously opened and closed, the pressurization system will change to FLIGHT.
If the mode isn't FLIGHT you can simply press the button and make it FLIGHT.
The FLIGHT / LANDING mode is automatically selected to LANDING when you descend more than 1,000 feet but switches back if you level off for more than 3 minutes above 25,000 feet. You can switch it manually.
The MANUAL switch activates the DESCEND/CLIMB rheostat and makes you, the pilot, the cabin pressure controller.
The AUTO / SEMI is how you switch modes. The AUTO mode can do everything without pilot input, the SEMI mode needs to be programmed almost as much as a GIV pressurization system in automatic mode. (The GV/G450 SEMI mode will do things the GIV will not, such as automatically switch to LANDING after a descent of 1,000 feet)
Cabin Pressure Selector Panel
Photo: Cabin Pressure Selector Panel, (Eddie's aircraft)
Click photo for a larger image
[G450 Aircraft Operating Manual, §2A-21-30, ¶2.D.] The CABIN PRESSURE SELECTOR panel is located on the aft right side of the cockpit center pedestal. Panel feature functions are contingent on the mode of operation of the pressurization system:
- With the pressurization system in the AUTO mode, the CABIN PRESSURE SELECTOR panel digital Liquid Crystal Displays
(LCDs) display FMS and ADS data currently in use by the CPC. The rotary selector knobs are disabled.
- With the pressurization system in the SEMI mode, the rotary selector knobs are active and are used in conjunction with the LCD displays to enter airplane / cabin altitude, barometric correction, landing field elevation and cabin rate of climb / descent.
In AUTO mode, this panel simply reports to you what the system is doing, based on inputs it gets from the FMS. In SEMI mode, this panel enters the inputs shown, removing the FMS inputs.
When would you use this? Let's say you have an ill passenger who wants to keep the pressuizaation as low as possible for a short flight. You basically start by switching to SEMI and programming in the desired cabin altitude. The window will show you the maximum available airplane altitude on the left as you rotate the right figure to show your desired cabin altitude. You also need to put in the landing field elevation and you current altimeter. For taxi out I believe the system will go from LANDING to FLIGHT automatically as wel as to LANDING during descent. I’ve done this a few times, but it has been a while. Generally speaking, you can get up into the low twenties and maintain a sea level cabin.
Cabin Pressure Relief Valve (PRV)
Figure: Pressure Relief Valve Block Diagram, from G450 Aircraft Operating Manual, §2A-21-00, Figure 7)
[G450 Aircraft Operating Manual, §2A-21-30, ¶2.H.] The CPRV is located under the lower shelf of the REER, adjacent to the TROV. It prevents excessive positive or negative pressures from damaging the airplane fuselage, doors and window and associated seals. The CPRV provides:
- Positive differential pressure relief at 9.74 to 10.15 psi
- Negative differential pressure relief at -0.25 psi
- Additional outflow capability during ground operations (in AUTO mode only)
During ground operations when the TROV is fully open, the cabin pressure relief valve also opens to ensure a minimal differential cabin pressure with high ECS output.
The books don't spend a lot of time on the PRV, but you can put together the following:
- The PRV is connected to outside pressure via the two aft-most static ports on the right of the aircraft.
- The PRV is connected to the cabin via a filtered port located inside the REER.
- The PRV is connected to high pressure air (20 psig) used to drive itself full open when on the ground.
- The only electrical connection is through a single circuit breaker (REER E-10) to connect the PRV to the cabin pressure controller, which commands the PRV to dump pressure when on the ground in AUTO mode.
Cabin Pressure Relief Valve Operation
Note: The AOM calls it a Cabin Pressure Relief Valve (CPRV), the MM call it a Pressure Relief Valve (PRV).
1. Positive Differential Pressure Relief
[G450 Maintenance Manual, §21-33-00, ¶4.A.] The PRV contains two fully independent positive pressure relief metering sections, which sense differential pressure between the cabin and atmosphere. When the differential pressure exceeds a calibrated set point, the metering sections open to modulate the control chamber pressure. This serves to modulate the poppet valve open enough to regulate the cabin-to-atmosphere differential pressure below the maximum value. The primary metering section is set to limit cabin pressure to 9.74 - 9.94 psid. The secondary metering section is set at 9.95 - 10.15 psid.
2. Negative Differential Pressure Relief
[G450 Maintenance Manual, §21-33-00, ¶3.A.] When the atmospheric pressure exceeds the cabin pressure, the atmospheric pressure acts on the valve poppets balance diaphragm which causes the valve poppet to raise off of the poppet seat. This enables the higher pressure ambient air to enter the cabin. This function limits the negative differential pressure to a maximum of -0.25 psid.
3. Additional Outflow Area in Ground Mode
[G450 Maintenance Manual, §21-33-00, ¶4.A.] The PRV is actuated open for certain ground operations in AUTO mode to provide additional air outflow area. The PRV contains a jet ejector pump and an air shutoff solenoid to actuate the valve open. This occurs on the ground when high pressure air is allowed to enter the jet pump and create a secondary vacuum flow at the air shutoff solenoid. When the air shutoff solenoid is commanded open, the secondary vacuum flow draws air from the control chamber, opening the poppet.