Pneumatics System
- James Albright (a former G450 driver)
Updated: 2018-09-03
The word "pneumatic" simply means air or gas under pressure. It is simply the bleed air between the engines and wherever the bleed air is needed.
The pneumatic air system on the the GII, GIII, and GIV was one of the weaker systems, not always up to the task of cooling the airplane on the ground during a hot day, or keeping the wings warm enough during an icy flight. Starting with the GV, Gulfstream got it right. The system on the G450 is almost identical to the GV/G550 with the primary exception of less bleed air at idle thrust owing to the smaller engines.
Everything here is from the references shown below, with a few comments in an alternate color.
G450 Pneumatics System
[G450 Aircraft Operating Manual, §2A-36-20 ¶1.] Pressurized high temperature air is drawn from the 7th or 12th stage of the engine compressor and modulated by Bleed Air Controllers (BACs) prior to being introduced into the bleed air manifold to supply aircraft systems that operate pneumatically.
The left and right engine BACs communicate with the respective Modular Avionics Units (MAUs) to obtain bleed air requirements, engine speed data and ambient air conditions.
The MAUs also provide pneumatic system oversight, communicating bleed air temperature, pressure and engine valve status furnished by the BACs to the Monitor and Warning System (MWS) and the Central Maintenance Computer (CMC). For ground operations, the APU can furnish unregulated bleed air to the supply manifold for engine starting and air conditioning pack operation. An external air cart can also be connected to the supply manifold for engine starting or air conditioning provided Direct Current (DC) electrical power is available to energize the system switches and valves.
Bleed pressure usually comes from the 7th stage compressor section of each engine, supplemented by the 12th stage when necessary. This air is routed through pylon-mounted precoolers which use engine fan air to cool the bleed air as necessary. The temperature and pressure of the air is computer controlled by Bleed Air Controllers which modulate the amount of fan air to the precoolers. The pressure of the air is primarily controlled by the bleed pressure regulating and shutoff valves. On the ground the bleed air can be provided by the APU or a ground cart, which is not regulated by the aircraft at all. From there it goes to the Air Conditioning System and a few other minor systems.
Bleed Air Sources
Engines
[G450 Aircraft Operating Manual, §2A-36-10] The pneumatic system uses high temperature pressurized air from the compressor section of the left and right engines, Auxiliary Power Unit (APU) bleed air or external air cart. The engine BACs draw air from either the 7th mid-stage or the high temperature 12th stage of the compressor sections of the engines, depending upon the engine power settings and bleed air demand. Air supplied by the APU or a ground cart is not regulated.
[G450 Maintenance Manual, §36-11-00 ¶4.A.] Air is bled from the MS7 or HS12 of the high pressure compressor and supplied through ducts to the aircraft ECS. The MS7 and HS12 offtakes are on the outboard side of the engine. A duct is attached to each offtake and the other ends of these ducts are attached to a Y-duct. From this point, one duct supplies an aft flow to the aircraft pylon interface. An HSRV and an MS7 bleed air check valve are installed in the duct system. The operation of these valves control the MS7 air or HS12 air to the aircraft ECS.
The bleed air sources are called "Mid Stage" and "High Stage" but both are from the compressor section of the engine. (It is easy to mistake bleed air as coming from aft of the burner cans because bleed air is hot. But that is due to a property of thermodynamics which says when you compress a gas, such as air, it gets very hot.) The temperature comes simply from the compression of the air and exceeds 570°F. Air is also tapped from the outer casing to provide cooling air to the pylon precoolers.
Auxiliary Power Unit (APU) Bleed Air
[G450 Aircraft Operating Manual, §2A-36-10 ¶2.D.] The APU is capable of supplying unregulated bleed air for engine starting on the ground. The APU air supply is introduced into the common supply manifold on the right side of the isolation valve. The duct contains a check valve to prevent engine bleed air from entering into the APU supply duct to preclude the interruption of air flow within the APU while it is running. APU air operation is monitored by MAU #1. Control of the APU air is through the pushbutton switch labelled APU on the BLEED AIR Control Panel on the cockpit overhead. Depressing the switch while the APU is operating will open the Load Control Valve (LCV) of the APU, allowing bleed air drawn from the compressor stage of the APU to enter the supply manifold. The ON legend within the switch will illuminate when the LCV is open. The manifold isolation valve will also open, making APU air available for air conditioning pack operation and engine starting. Since there is no BAC for APU air and APU operation is governed by an Electronic Control Unit (ECU) that controls APU rpm to 100% within temperature limits, the amount of bleed air produced by the compressor section will vary with density altitude. The APU must have reached 95% rpm for at least 90 seconds in order to provide bleed air to the manifold.
APU air is controlled and modulated by the load control valve. You can leave the APU bleed air switch on during start without penalty. It is said a weaker APU could actually flame out during the wing anti-ice system check if the load control valve is left open but I've never heard it actually happen to anyone.
Ground Air Connection
[G450 Aircraft Operating Manual, §2A-36-10 ¶2.E.] An external ground air cart may by used for engine starting or to supply the air conditioning packs if the APU is inoperative. A panel on the underside of the aircraft below the aft equipment bay contains the plug in connector that accommodates the hose from the ground cart. The connector is plumbed to the supply manifold on the left side of the isolation valve, so the isolation valve must be opened to route ground air to the right engine or air conditioning pack. Electrical power is required when using the ground air cart in order to actuate the isolation valve and the air conditioning pack control valves.
[G450 Aircraft Operating Manual, §09-01-90 ¶2.] External Air Cart: Used to start engines when APU is not available. Capable of providing bleed air at 112.8 lbs per minute at 48 psi (97 in Hg) within a temperature range of 153-202°C (325-395°F), when corrected for Standard Day conditions.
You need a good air cart and use the right procedures to get the engines started without hurting anyone or the aircraft. Refer to G450 Engine Start for more details. In the cockpit, you have QRH NG-28, External Air Start, but you will more than likely have to use with QRH NG-17, Engine Start - Battery Only.
Components
Bleed Air Controllers (BACs)
[G450 Aircraft Operating Manual, §2A-36-10 ¶2.A.] A BAC for each engine is installed in the forward right hand bulkhead of the baggage compartment. The BACs are microprocessors that provide the control signals for the engine bleed valves, pressure regulator / shutoff valves and precoolers. The BACs open and close the engine bleed and regulator / shutoff valves in response to commands from the pushbutton switches on the BLEED AIR Control Panel located on the cockpit overhead and in response to feedback from pressure and temperature sensors in the supply manifold.
The BACs open the 7th stage bleed valves of the engines to supply up to 40 psi of pressurized air that is temperature modulated to 400°F. Temperature modulation is accomplished by routing the engine bleed air that is normally at a temperature of 500°F through a precooler. The BACs vary bleed air valve openings and the amount of fan stage air to the precooler to obtain the required temperatures measured by sensors at the precooler inlet and outlet.
Pressure modulation is accomplished by the BACs using readings from the pressure sensors in the supply manifold to control the regulator / shutoff valve located upstream of the precooler in the supply manifold. In normal flight conditions, only 7th stage air is required for systems operation. However, if 7th stage pressure falls below 15 psi, the 12th stage bleed valve will open to provide at least fifteen 15 psi.
In the earlier Gulfstreams we said the pylon valves control the temperature and the bleed valves control the pressure. That is still true but only indirectly. The BAC controls it all using these valves and others.
Pressure Regulating Shutoff Valve
[GV Aircraft Operating Manual, §2A-36-20 ¶2.C.] There are two manifold pressure regulator valves, one located on each engine. They are butterfly-type, pneumatically-operated, modulating/shutoff valves. The purpose of the valves is to maintain a bleed manifold pressure of 40.5 psi. The L/R BLEED AIR switches provide 28V DC to the valve solenoid to allow the valve to pneumatically open and begin regulation.
[GV Maintenance Manual, §36-11-00, ¶3.C.] The PRSOV is a spring-loaded closed, butterfly type, pressure regulating and shut-off valve that has a regulation set point of 40.5 psig (279.2 kPA). The PRSOV is controlled by the L and R ENG switches in the BLEED AIR section of the cockpit overhead panel. The switches, when selected on, provide 28 Vdc to energize the solenoid of the respective valve, allowing the PRSOV to open and begin regulating bleed air flow to the aircraft. A relief is incorporated into the PRSOV in order to limit actuation pressure in the event of a reference regulator failure. A position indicator is located on top of the PRSOV housing to indicate valve position.
Of critical importance when diagnosing bleed air problems is to understand that each bleed switch controls its onside pressure regulating shutoff valve which is located on the engine, outboard of the pylon. If you close this valve successfully, the hot bleed air stays outside the fuselage.
The information about these valves in the G450 Maintenance Manual and AOM is sparse. The valves work the same as with the classic GV, so I cited the GV manual here.
[G450 Maintenance Manual, §36-11-00, ¶4.A.(2)] The BLEED AIR control panel L ENG and R ENG switches provide switched signals, via the L and R bleed air control relays, to the annunciator lights dim and test controller for the switch OFF display. The L ENG and R ENG switches also provide signals for the CAS and on side Modular Avionics Unit (MAU) for the L-R BAS Fail message. The on side bleed air controller initiates the on side bleed air system and switched power to the on side manifold pressure regulator valve solenoid that opens the valve to provide bleed air from that side or engine.
Since it takes "switched power" to open the valve, I am surmising the valve fails closed when power is removed, as is true with the GV.
Precoolers
[G450 Aircraft Operating Manual, §2A-36-10 ¶2.A.] The precooler is a heat exchanger that contains cold ambient air extracted from the fan inlet stage of the engine to circulate within the precooler and is then exhausted overboard through louvers in the engine pylon. The BACs vary bleed air valve openings and the amount of fan stage air to the precooler to obtain the required temperatures measured by sensors at the precooler inlet and outlet.
[G450 Maintenance Manual, §36-14-00 ¶3.A.] The fan air valve is a pylon mounted, spring-loaded open, pneumatically actuated, modulating and shut-off type valve. It has a torque motor that is controlled by its on-side bleed air controller. The muscle pressure for the valve control is provided by the servo pressure regulator. The regulators are upstream of the manifold pressure regulator valve. The valve is used to control the flow of engine fan air through the precooler to limit the temperature of the air in the bleed air manifold to no more than 520°F over the entire aircraft operating envelope. Included are minimum miscellaneous bleed flows to the maximum of one pack plus two wings and miscellaneous bleed flows. Temperature at the precooler outlet is controlled to 400 ±10°F in normal operating condition with wing anti-ice off or with wing anti-ice and both engine bleed air on within the engine capacity. Precooler outlet temperature is controlled to 500 ±10°F during single bleed operation with one wing anti-ice on or with one wing anti-ice on and the other wing anti-ice off within engine capacity.
The Bleed Air Controllers manipulate the 7th and 12th stage air valves to provide 570°F pneumatic air to the input side of the precoolers. The BACs then manipulate the fan air torque motor to control the amount of fan air provided to the precooler to produce 400°F bleed air to the bleed manifold, 500°F during single bleed wing anti-ice operations.
Bleed Air Distribution Ducts
[G450 Aircraft Operating Manual, §2A-36-10 ¶2.C.] After engine bleed air has been modulated by the BACs and introduced into the common supply manifold, the air is distributed to meet the requirements of aircraft systems though a system of ducts. Each air conditioning pack has an independent duct plumbed into the supply manifold, with the left and right packs connected on the respective sides of the isolation valve. Supply ducts for the left and right wing anti-ice are also connected on the respective sides of the isolation valve, but the wing anti-ice supply ducts incorporate a crossover manifold downstream of the supply manifold that enables a single anti-ice duct to supply both wings.
Bleed Air Isolation Shut-off Valve
[G450 Maintenance Manual, §36-12-00 ¶3.D.] The bleed air isolation shut-off valve is a normally closed, solenoid actuated open and pneumatically driven open valve. The valve contains a single pole, double throw position switch used to provide discrete logic for the bleed air isolation switch valve open annunciation and for the CAS Isolation Valve Open messages. The valve is installed in the tail compartment to isolate the left and right bleed air manifolds. It is capable of being manually operated open by the cockpit overhead panel BLEED AIR control panel ISOLATION switch and automatically opens with either bleed air / engine start relays and the APU switch (ground only). This valve can be manually wrenched and locked open.
[G450 Operating Manual, §2A-36-20, ¶ 2.B.] The isolation valve is controlled by an electric solenoid, but is powered by pneumatic pressure - i.e. there must be pressure available from a bleed air source to move the valve to the commanded position.
It appears this valve "lies where it dies," if electrical power is removed, it just stays where it was last set.
Bleed Air Control Panel
[G450 Aircraft Operating Manual, §2A-36-10 ¶2.F.] The pushbutton switches for controlling the bleed air system are located on the BLEED AIR Control Panel on the top right hand corner of the overhead panel. The switches are located within a graphic on the panel depicting the ducting shared by the bleed air system. The graphic extends down to the TEMP CONTROL panel immediately below to illustrate the supply to the air conditioning packs. The engine bleed air switches are normally selected to the depressed or ON position. The OFF legend within a switch will illuminate if the switch is selected off or if there is a malfunction. The APU bleed switch is normally off, but when selected to the depressed or ON position, the ON legend within the switch will illuminate. The isolation valve control switch, labelled ISOLATION is above an indicator that corresponds to the valve position. When the isolation valve is open, the indicator bar lines up with the line graphic depiction of the supply manifold indicating that the valve is open, connecting both sides of the manifold. When the valve is closed the bar indicator below the switch is perpendicular to the graphic of manifold. A line of the graphic also indicates the connection point of APU air to the right of the isolation valve.
Operations
G450 Aircraft Operating Manual, §2A-36-10 ¶2.A.]
Under normal operating conditions, the major demand on the pneumatic system is to supply the air conditioning packs for cabin pressurization and temperature control. The BACs open the 7th stage bleed valves of the engines to supply up to 40 psi of pressurized air that is temperature modulated to 400°±10 F. Temperature modulation is accomplished by routing the engine bleed air that is normally at a temperature of 500°F through a precooler.
Pressure modulation is accomplished by the BACs using readings from the pressure sensors in the supply manifold to control the regulator / shutoff valve located upstream of the precooler in the supply manifold. In normal flight conditions, only 7th stage air is required for systems operation. However, if 7th stage pressure falls below 15 psi, the 12th stage bleed valve will open to provide at least 15 psi. If the engines are operating at low power settings as during descents or when the aircraft is at low speeds when in a holding pattern, the pressure and temperature of the 7th stage bleed air may not be sufficient to satisfy demand. The bleed air controller senses the descent mode by monitoring engine N1, altitude and Static Air Temperature (SAT). Under these conditions the BACs will supplement 7th stage bleed air if the manifold pressure drops below 26 psi with air extracted from the 12th stage of the engine compressor that is hotter and at a higher pressure.
Although both engines and are normally used to provide bleed air to both air conditioning packs for cabin pressurization and temperature control, a single engine can provide sufficient airflow to operate both air conditioning packs provided the engine power setting is high enough that a bleed air pressure of at least 30 psi is maintained. However, if a malfunction results in only one air conditioning pack available for pressurization and temperature control, the BAC for the engine associated with the operating pack changes the setting the regulating / shutoff valve to provide bleed air of at least 35 psi by opening both bleed valves if needed.
The BACs also act as the control authority for operation of the wing anti-ice valves when wing anti-ice is selected on manually or automatically by signals from the ice detectors. With wing anti-ice selected on, the BACs will open both bleed air valves if necessary to increase the temperature of the air supply entering the pre-coolers to 570°F. The BACs monitor the temperature within the wing anti-ice ducts along the leading edge of the wing and modulate the wing anti-ice valves to maintain the leading edge at a temperature of 130°F.
Another adjustment is made by the BACs if wing anti-ice is required with only one engine operating or if a failure results in only one wing anti-ice valve functioning. In either condition an increased amount of high temperature air is required in order for one engine or wing anti-ice valve to supply sufficient heat to both wings to prevent the formation of ice. In this instance the BAC associated with the operating engine or wing anti-ice valve increases the temperature threshold of the air exiting the precooler to 500±10°F.
Example: Bleed Air Off
Example: Engines at Idle
Example: Engines above Idle
Example: Wing Anti-Ice
Example: APU Air
Bleed air is provided by the engines, the APU, or by an external air cart.
The preferred engine source is the HP 7th stage, set to provide at least:
15 psi (normal)
26 psi (during descent)
35 psi (for same side engine during single pack operations)
570°F (with wing anti-ice on) -- With wing anti-ice on, the Bleed Air Controllers manipulate the engine bleed valves to insure the air entering the pre-coolers is at least 570°F. If only one engine is operating or only one wing anti-ice valve is functioning, the BACs provide at least 500°F air exiting the pre-cooler.
If the values fall below the target minimums, the HP 12th stage valve opens to make up the difference.
A bleed pressure regulator valve functions as an on/off valve as well as a regulating valve set to provide 40 psi to the pneumatic manifold. The valve requires onside DC ESS and pneumatic pressure to open, the valve fails closed if either is removed. The L / R valves are controlled electrically by the cockpit L / R ENG BLEED AIR switches.
Each engine bleed system has a precooler mounted inside the pylon, which is a heat exchanger that contains cold ambient air extracted from the fan inlet stage of the engine to circulate within the precooler and is then exhausted overboard through louvers in the engine pylon. The BACs vary bleed air valve openings and the amount of fan stage air to the precooler to obtain the required temperatures measured by sensors at the precooler inlet and outlet.
Bleed Air Control
Bleed air controllers worry about maintaining minimum pressures. Air from the 7th stage is usually sufficient, but if it drops below 15 psi in cruise or 26 psi in a descent, 12th stage bleed air is added. The bleed pressure regulator valves worry about maintaining 40 psi.
Bleed air output from the engine, normally about 500°F, is passed through precooler in the pylon. Fan inlet air is passed over the precooler to regulate the bleed air to 400°F. If wing anti-ice is on, bleed air output from the engine is raised to 570°F but output from the precooler stays at 400°F unless operating with only one engine or with one wing anti-ice valve failed, in which case it is raised to 500°F.
LP air is used for engine cooling and precooler air, HP air for everything else.
Wing a/i target: 130°F (Green @ 100°F, Red @ 180°F)
All pneumatic valves fail close except the anti-ice and pack inlet valves.
Isolation valve opens automatically with APU air, start or crank master
Bleed air controllers automatically regulate manifold temp and pressure, monitor precooler inlet/outlet temperature, wing a/i temperature, and establish high stage regulation points (responsible for all temperature and minimum pressures)
Notes
Amber Left Manifold Synoptic Line with APU Air Applied
[From Gulfstream Breakfast Minutes, 12/14/2012] In previous years during cold weather, Technical Operations has received calls from operators regarding an amber Left Manifold Line on the ECS/PRESS synoptic page. This occurs in cold climates following APU start and upon application of APU air. The right manifold line almost immediately goes to green, but the left manifold line may remain amber until left engine start. The following is an explanation of why this occurs and is considered a normal indication.
The precooler outlet sensor is designed to provide the flight crew bleed air manifold temperature in the cockpit when the engines are operating. The precooler outlet sensor is located in the pylon after the air exits the precooler coming into the manifold. In cold weather during APU operations, it is not unusual to see the line on the ECS/PRESS synoptic page for the left manifold to the scroll of the ACM and to the precooler to be amber in color with APU air applied. This line will change to amber when the duct temperature is below 100°F.
The reason the right precooler outlet temp does not fall to 100°F or below during APU operations is due to the proximity of the right precooler outlet sensor to the APU duct entering the right manifold. As viewed in the tail compartment, APU air is ducted directly into the right manifold and then through the right and left pack inlet valves. The left pack inlet valve is located left of the isolation valve and is further from the air source.
Flow equates to increased heat. It is quite hot at the intersection of the APU to right manifold duct when a pack is operating. But, the air is not flowing through the precoolers. It is static, because the engine manifold pressure regulator is closed, both left and right side, so the air stops flowing outward. Flow also equates to temperature rise, so where there is no flow, the air cools off in the pylon during APU operations.
Since the precooler outlet temp on the left is further from the dynamic airflow, the sensor in the left pylon ducting is cooler and will actually reach 100°F or less during cold weather operations, and the synoptic page will display amber as designed. The right pylon ducting is closer to the APU duct, so it never reaches 100°F.
This does not happen on the GV/G500/G550 aircraft, because the precooler outlet sensors are located in the tail compartment duct work, not in the pylon like the G350/G450 aircraft. Therefore, the GV/G500/G550 left precooler sensor is closer to the APU air source.
This comes up just about every winter. Here is an extract from the February 5, 2016 Gulfstream Journal: G350/G450 (ATA 36) Amber Left Manifold Synoptic Line with APU Air Applied.
References:
Gulfstream 350/G450 (ATA 36) Amber Left Manifold Synoptic Line with APU Air Applied, The Gulfstream Journal, February 5, 2016.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013
Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013
Gulfstream GV Aircraft Operating Manual, GAC-AC-GV-OPS-0002, Revision 30, May 13, 2008
Gulfstream GV Maintenance Manual, Revision 25, August 31, 2005