Pressurization System Abnormals

- James Albright (a former G450 driver)

Updated: 2017-09-15

The old saying about air conditioning and pressurization, "If it is hot or failed turn it off," is generally true with this airplane. Just keep in mind that the air conditioning packs do not need any electrons at all to operate, the pressurization system works down to batteries, and that there are an awful lot of computers involved with just about everything. What follows is a list of various things that can go wrong. If you don't see it here, look under G450 Air Conditioning System Abnormals.

Everything here is from the references shown below, with a few comments in an alternate color.

• Bleed Air Hot, L-R — Bleed Air Hot

• Bleed Pressure Low, L-R — Bleed Pressure Low

• Bleed Pressure High, L-R — Bleed Pressure High

• CABIN DIFFERENTIAL 9.74 — Cabin Pressure High

• CABIN DIFFERENTIAL 9.94 — Cabin Pressure High

• CABIN PRESSURE LOW — Cabin Pressure Low

• CABIN PRESSURE MANUAL — The cabin pressure controller has switched to MANUAL, either because you pressed the switch or the system made the change. If you didn't press the switch, you can do so now to switch it back. (See G450 QRH MB-21 for more.)

• CPAM FAIL — The Cabin Pressure Acquisition Module has failed, you've lost a backup system used by the other systems as a crosscheck. You log it for maintenance action and look at the synoptic to see what else is wrong. You can MEL this. (G450 MMEL 21-8, Item 20)

• CPCS CONTROL PANEL FAIL — Loss of Automatic Pressurization Control

• CPCS FAIL—SELECT MANUAL — Loss of Automatic Pressurization Control

• CPCS LOW AIR FLOW — CPCS Low Air Flow

• Emergency Descent

• PYLON HOT — Pylon Hot

 Bleed Air Hot

As is true with all of these pneumatic abnormals: If it is hot or failed, turn it off.

(This procedure is identical to the Bleed Pressure High scenario.)

The temperature of the bleed air outboard of the pylon should be around 570°F but will certainly not be more than the TGT of the engine itself. You don't want this hot air entering the aircraft fuselage.

The bleed air valve is on the engine itself, outboard of the pylon. If turning it off solves your problem, there shouldn't be an issue operating both packs with the isolation valve open. If the bleed switch fails to close, pulling the circuit breaker should allow it to fail close.

If you cannot get the valve to close, remember that at idle the engine cannot meet the 400°F target and the bleed air hot message should extinguish. If it doesn't, you have either a faulty temperature sensor or a series of failures from either the 7th or 12th stage valves, check valves, bleed pressure and regulating valve, precooler and precooler valves. Turning on the onside wing anti-ice and seeing how its temperature does with the engine at idle can verify hot bleed air or a faulty sensor.

Symptoms

Bleed Air Hot, L-R

The ECS synoptics page should reflect the elevated temperature.

Analysis

[G450 Quick Reference Handbook, page MB-21] The bleed air temperature exceeds limits.

This isn't telling you what you really need to know, for that . . .

[G450 Aircraft Operating Manual, §2A-36-20; 3.B.] In addition to the color changes in bleed system indications on the synoptic and system pages, the MWS will generate CAS messages when the following thresholds are exceeded:

• Precooler inlet temperature exceeds seven hundred thirty-five degrees Fahrenheit plus or minus ten degrees (T > 735°F±10° / 390.5°C±5.6)

• Precooler outlet temperature exceeds five hundred fifty degrees Fahrenheit plus or minus ten degrees (T > 550°F±10° / 288°C±5.6)

Crew Alerting System (CAS) Messages:

• Area Monitored: Precooler Inlet / Outlet Temperature

• CAS Message: Bleed Air Hot, L-R

The issue is the temperature of the air into and out of the precooler.

[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 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 (271.11°C) 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 (204.44 ±5.55°C) 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 (260 ±5.55°C) 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 air coming out of the engine bleeds is often hotter than 500°F and gets cooled to a target of 400°F under most conditions. The precooler uses engine inlet fan air to accomplish this feat so the bleed air hot condition can be caused by a failure of this engine inlet air, 7th stage air, 12 stage air, any of the valves associated with any of these air sources, or the problem could be a faulty indication. You don't want this air entering the fuselage so your normal course of action is to shut if off.

Procedure

[G450 Airplane Flight Manual §03-21-30]

1. Affected Engine BLEED AIR . . . OFF

   CAUTION: ENGINE COWL ANTI-ICE IS NOT AVAILABLE WITH THE ON SIDE ENGINE BLEED AIR SWITCH SELECTED OFF. DEPART ICING CONDITIONS AND AVOID FOR THE REMAINDER OF THE FLIGHT.

   You should see indications of the valve closing on the synoptic: a pressure and temperature drop as well as movement of the valve. If you don't see these indications, your problem isn't going to get better. Pulling the bleed air circuit breaker should cause the valve to fail closed. If this doesn't work, pulling the throttle to idle should do the trick. The air leaving the engines at idle is seldom more than 300°F. Earlier Gulfstreams would have you shut down the engine at this point.

2. Associated ECS PACK . . . OFF

3. ECS / Pressurization Synoptic Page . . . SELECT

   WHEN MESSAGE EXTINGUISHES AND IF AT LEAST 25 PSI IS AVAILABLE FROM OPERATING BLEED AIR SYSTEM, TWO PACK OPERATION MAY BE ESTABLISHED IF DESIRED:

4. Isolation Valve . .. OPEN

5. Two (2) PACK Operation . . . ESTABLISH / MONITOR

   If available bleed air pressure is less than 25 psi, or single pack operation is desired:

6. Affected ECS PACK . . . OFF

7. Isolation Valve . . . CLOSED

 Bleed Pressure Low

The checklist asks you to make two very big determinations but doesn't give you much help on how to make those decisions: do you have a bleed air leak and should you shut down the engine? I would add a third question: do you have a bad pressure sensor?

The AFM procedure doesn't tell you any of this, it just leaves the decisions in your hands. Here is what I would do:

1. Note the bleed pressure on both sides of the isolation valve while slowly retarding the affected side's engine power lever.

   • If the affected side pressure indication does not change, you may have a faulty sensor.

   • If the affected side pressure does change, the sensor is probably working.

2. Close the affected side bleed valve.

   • If the bleed pressure was above zero it should drop to zero. If it doesn't, vary that side's power lever. If the pressure follows power lever movement, your bleed valve has failed to close. Pulling the circuit breaker should allow the valve to fail closed. If you can't get the valve to close you will have to shut down the engine.

   • If the bleed pressure did not drop to zero and does not follow power lever movement, you probably have a bad sensor.

3. If the bleed valve closed successfully, open the isolation valve for a few seconds.

   • If the unaffected side's pressure drops, immediately close the isolation valve; the leak is inboard of the bleed valve. You shouldn't have to shut down the engine as long as the bleed valve remains closed.

   • If the unaffected side's pressure remains steady and the affected side's pressure rises to confirm this, the sensor is good and you either have a leak outboard of the bleed valve or a problem with the bleed air controller. Since you can't troubleshoot the BAC you should suspect a leak and should shut down the engine.

   • If the unaffected side's pressure remains steady and the affected side's pressure does not change, you have a bad sensor. You shouldn't have to shut down the engine.

Symptoms

Bleed Pressure Low, L-R

The ECS synoptics page should reflect the decreased pressure.

Analysis

[G450 Quick Reference Handbook, page MB-21] The bleed pressure is below 5 psi.

[G450 Aircraft Operating Manual 2A-36-20, 2.A.]

• The BACs open the seventh (7th) stage bleed valves of the engines to supply up to forty plus or minus three point five (40±3.5) psi of pressurized air that is temperature modulated to four hundred plus or minus ten degrees Fahrenheit (400°±10 F / 204.4°C).

• 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 fifteen (15) psi, the (12th) stage bleed valve will open to provide at least fifteen (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 seventh (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 seventh (7th) stage bleed air if the manifold pressure drops below twenty-six (26) psi with air extracted from the twelfth (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 thirty (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 thirty-five (35) psi by opening both bleed valves if needed.

So how low is too low? You should never see less than:

• 15 psi during low power conditions

• 26 psi in a descent mode

• 30 psi when single-engine

• 35 psi when only a single pack is available
  
  

The AFM procedure provides a procedure "if a bleed air leak is suspected" but doesn't offer any trouble shooting hints to determine if there is a leak. The AFM also recommends you shut down the engine "if necessary."

We can better analyze the cause of the low pressure and the need to shut down the engine if we understand where the pressure is measured. The bleed air manifold is divided in three places: two bleed pressure regulating and shutoff valves and one isolation valve. Each bleed pressure regulating valve is located on its engine.

The pressure sensors are located inside the aft equipment compartment, well inboard of the engine itself. The pressure on the synoptic page is inboard of the engine bleed valve but outboard of the isolation valve.

Procedure

[G450 Airplane Flight Manual, §3-21-50]

If a bleed air leak is suspected:

1. Affected Engine / ECS Readings . . . OBSERVE

2. Affected Engine BLEED AIR . . . OFF

   CAUTION: ENGINE COWL ANTI-ICE IS NOT AVAILABLE WITH THE ON SIDE ENGINE BLEED AIR SWITCH SELECTED OFF. DEPART ICING CONDITIONS AND AVOID FOR THE REMAINDER OF THE FLIGHT.

3. Associated ECS PACK . . . OFF

4. Isolation Valve . . CLOSED

5. Affected Engine Instruments . . . MONITOR

   If necessary:

6. Affected Engine . . SHUT DOWN

   If a bleed air leak is NOT suspected:

7. Affected Power Lever . . ADVANCE

8. ECS / Pressurization Synoptic Page . . . SELECT

Bleed Pressure High

As is true with all of these pneumatic abnormals: If it is hot or failed, turn it off.

(This procedure is identical to the Bleed Air Hot scenario.)

The AFM procedure is fine except that it assumes the bleed air switch will shut off the high pressure air flow. What if it doesn't?

The pressure should be around 40 psi. You don't want pressure higher than this entering the aircraft fuselage.

The bleed air switch is on the engine itself, outboard of the pylon. If turning it off solves your problem, there shouldn't be a problem operating both packs with the isolation valve open. If the bleed switch fails to close, pulling the circuit breaker should allow it to fail close.

If you cannot get the valve to close, remember that at idle the engine does not normally meet the 40 psi target and the bleed pressure high message should extinguish. If it doesn't, you have either a faulty pressure sensor or a series of failures from either the 7th or 12th stage valves, check valves, bleed pressure and regulating valve, precooler and precooler valves.

Symptoms

Bleed Pressure High, L-R

The ECS synoptics page should reflect the elevated pressure.

Analysis

[G450 Quick Reference Handbook, page MB-21] The bleed pressure exceeds 75 psi.

[G450 Aircraft Operating Manual 2A-36-20, ¶2.A.]

• The BACs open the seventh (7th) stage bleed valves of the engines to supply up to forty plus or minus three point five (40±3.5) psi of pressurized air that is temperature modulated to four hundred plus or minus ten degrees Fahrenheit (400°±10 F / 204.4°C).

• 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 fifteen (15) psi, the (12th) stage bleed valve will open to provide at least fifteen (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 seventh (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 seventh (7th) stage bleed air if the manifold pressure drops below twenty-six (26) psi with air extracted from the twelfth (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 thirty (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 thirty-five (35) psi by opening both bleed valves if needed.

  So how high is too high? You should never see more than 40 psi. There are two pressure switches right next to each other separated by the isolation valve. If the pressure indicates high and the bleed switch does not work, you might try this:

  1. Bring the associated side's power lever to idle, the engine rarely achieves 40 psi at idle. If the pressure still reads above 40 psi you either have a bad pressure sensor or a series of failed valves.

  2. If the pressure did not change after you brought the power lever to idle, you should suspect the sensor. To verify the sensor is bad you can momentarily open the isolation valve to see if the opposite pressure sensor agrees. If the opposite sensor does not indicate high pressure, try bringing the retarded power lever back to its original setting and evaluate the pressure performance.

  3. If the pressure did change with power lever movement, you might indeed have high bleed pressure and you should consider shutting the engine down.

  Procedure

  [G450 Airplane Flight Manual, §03-21-40]

  1. Affected Engine BLEED AIR . . . OFF

     CAUTION: ENGINE COWL ANTI-ICE IS NOT AVAILABLE WITH THE ON SIDE ENGINE BLEED AIR SWITCH SELECTED OFF. DEPART ICING CONDITIONS AND AVOID FOR THE REMAINDER OF THE FLIGHT.

     Verify the valve closed and the pressure has dropped. If it doesn't, see the notes above.

  2. Associated ECS PACK . . . OFF

  3. ECS / Pressurization Synoptic Page . . . SELECT

     If message extinguishes and at least 25 psi is available from operating bleed air system, two pack operation may be established (if desired) as follows:

  4. Isolation Valve . .. OPEN

  5. Two (2) PACK Operation . . . ESTABLISH / MONITOR

     If available bleed air pressure is less than 25 psi, or single pack operation is desired:

  6. Affected ECS PACK . . . OFF

  7. Isolation Valve . . . CLOSED

 Cabin Pressure High

There are two risks here.

First is having the CPRV all of a sudden let go and ending up with an Emergency Descent. If the pressure is high and still climbing, going to SEMI may instantly make things better. If that doesn't work, turning the packs off might give you some time. If that doesn't work, then raise the cabin altitude on the Cabin Pressure Selector Panel. If that doesn't work, consider MANUAL mode and opening the TROV a bit, but that is likely to give you another set of problems. Unless you are oceanic, your next move might be an immediate, but controlled descent. If you are oceanic you can try MANUAL, but remember to be gentle with the TROV.

Second is having actual structural damage to the airplane. It has happened. This is one of those things that will require you to think quickly. Turning the packs off shouldn't result in an instant loss of pressurization and may give you time to think. Here's how things can go from bad to worse: case_study_giv.

Symptoms

CABIN DIFFERENTIAL 9.94

CABIN DIFFERENTIAL 9.74

You should be able to verify the pressure on the cabin pressure indicator panel:

Analysis

[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)

[G450 Airplane Flight Manual §1-21-10] Maximum cabin pressure differential permitted is 9.94 psi.

You may have a problem with a cabin pressure controller, the TROV, the CPRV, or a combination of any of those. You need to reset or take the problem out of the equation and end up with as much of a normal system as possible.

Procedure

CABIN DIFFERENTIAL 9.94

[G450 Quick Reference Handbook, page MA-4]

1. Select SEMI-AUTO mode

2. Raise cabin altitude

3. Increase cabin rate of climb if required

CABIN DIFFERENTIAL 9.74

[G450 Quick Reference Handbook, page MB-21]

1. In AUTO pressurization control mode, maintain level flight or decrease climb rate.

   Or:

2. Switch to Semi-Automatic (SEMI) or MANUAL pressurization control mode, if required, raise cabin altitude or increase cabin rate of climb.

 Cabin Pressure Low

We practice the Emergency Descent every six months but that is in a contrived atmosphere looking for a set outcome. I've had this happen to me once, at 35,000', and it was nothing like any sim scenario. Some thoughts:

• Get on oxygen but don't make a big production out of it — just getting the mask to your face puts you ahead of the game so you can start to deal with the rest of the problem.

• Let the Emergency Descent Mode of the autopilot handle flying the airplane, you can bring the throttles to idle once the nose is pointed down to help things along.

• Get the pax on oxygen.

• Let everyone below you know that's where you are headed.

• RAM Air? That's going to shut off the air conditioning packs and could make things worse. If you've got low cabin pressure chances are air flow will be the least of your problems.

Symptoms

CABIN PRESSURE LOW

You may have the classic Rapid Depressurization scenario with a loud pop, a rush of cold air, and the air fogging instantly. Or you might just have the CAS message which you should be able to verify the pressure on the cabin pressure indicator panel.

Analysis

Remember that pressurization occurs because the engines pump air into the vessel and the TROV lets it out in a metered, controlled way. If you lost pressurization there is either not enough air coming in or too much is getting out.

You may have lost a structural component of the fuselage, a window, or a seal. Or it might just be a problem with a cabin pressure controller, the TROV, the CPRV, or a combination of any of those.

Procedures

[G450 Quick Reference Handbook, page EH-3]

If cabin pressurization is lost, proceed as follows:

1. Crew Oxygen Masks . . . DON

2. Emergency Descent Procedure / Checklist . . . COMMENCE

   See Emergency Descent Procedure, page EH-3

3. Passenger Oxygen Masks . . . MANUALLY DEPLOY

4. Passenger Oxygen Masks . . . VERIFY DEPLOYMENT

5. Pressurization . . . RESTORE

   If unable to restore cabin pressurization:

6. RAM Air . . . SELECT ON TO PROVIDE CABIN AIRFLOW

   NOTE: See Loss of Automatic Pressurization Control, page EH-5.

CPCS Low Air Flow

This is an odd procedure and there isn't much documentation about what actually triggers the CPCS Low Airflow CAS message. The best I could do was the wiring diagram which refers to a flow switch on the pack inlet valve (highlighted in red above) which wants at least 25 ppm. The QRH gives you three possible reasons but the procedures don't make any sense given those reasons.

By all means pull out the QRH but this is a problem you need to think through, following the QRH blindly can make things worse.

Symptoms

CPCS Low Airflow

You probably won't notice this until you get the CAS, then on the synoptic you may notice problems with one or both air conditioning packs. The cabin altitude may be climbing.

Analysis

[G450 Quick Reference Handbook, page MB-24] Causes:

1. The Cabin Pressure Control System (CPCS) less airflow than required, or

2. The outflow valve is fully closed, or

3. One or both pack inlet valves have failed.

Procedure

[G450 Quick Reference Handbook, page MB-24]

1. Increase thrust, if able.

2. Select L-R Wing Anti-ice to ON.

3. Select ECS/Pressure synoptic page.

4. Monitor pack outlets and temperatures (34 - 36°F).

5. If unable to maintain or control cabin altitude, execute an Emergency Descent procedure.

Technique

It appears the low air flow is reported at the pack inlet valve. But even if this isn't true and you take the three suggest causes listed in the QRH (and the G450 Airplane Flight Manual, page 3A-17), a better course of action may be as follows:

1. Select ECS/Pressure synoptic page.

2. If the outflow valve is fully closed or if cabin altitude is lower than scheduled on the chart shown, switch the pressurization system to SEMI and select the scheduled cabin altitude as a way of increasing air flow.

3. If there is no output from a single air conditioning pack and it appears its pack inlet valve is closed even though the switch is on, try turning that pack switch off. The logic of the system should compensate the operating pack inlet bleed pressure to 35 psi.

4. If there is no output from both air conditioning packs and it appears the pack inlet valves are closed even though the switches are on, try pulling one or both pack inlet valve circuit breakers (marked as "L PACK CONT" D-10 on the LEER and "R PACK CONT" D-9 on the REER), these valves fail open without electrical power.

If all that fails you can always go back to the QRH procedure.

Loss of Automatic Pressurization Control

This procedure assumes either your cabin pressure control panel is broken or both cabin pressure controller channels have failed and you are in danger of over pressurizing the fuselage. If you can't get control using MANUAL, the book wants you to turn both packs off and execute an emergency descent.

This is one of those things that will require you to think quickly. Turning the packs off shouldn't result in an instant loss of pressurization and may give you time to think. Here's how things can go from bad to worse: Gulfstream GIV N450KK.

This might be your best course of action, no doubt about it. But what if you are half way across the pond on top of a bunch of tracks and still have a long way to go? You might be taking a controllable situation and creating an uncontrollable mess. There is another way, but it is just my technique. You might consider adding it to your bag of tricks.

Symptoms

CPCS Control Panel Fail

CPCS Fail-Select Manual

If you are in level flight the only symptom may be on the CAS. If climbing or descending, the pressurization system might behave erratically.

Analysis

[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 (see Figure 8). 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.

If the CAS says the CPCS control panel has failed you still use the panel to select manual. If the CAS says CPCS Fail-Select Manual, it is saying both channels have failed and again you select manual. Either case leads you to a checklist that tries to get control back using SEMI.

Procedure

CPCS Control Panel Fail

[G450 Quick Reference Handbook, page MB-23]

• Cause: CABIN PRESSURE CONTROL panel has failed.

• Corrective Action: Select FAULT/MANUAL switch to MANUAL, Control cabin pressurization with manual (MAN HOLD) knob, go to page EH-5

CPCS Fail-Select Manual

[G450 Quick Reference Handbook, page MB-23]

• Cause: Both CPCS channels are failed.

• Corrective Action: Select FAULT/MANUAL switch to MANUAL, Control cabin pressurization with manual (MAN HOLD) knob, go to page EH-5

[G450 Quick Reference Handbook, page EH-5]

1. FAULT / MANUAL . . . MANUAL

2. MAN HOLD . . . CONTROL TO ±500 FPM RATE OF CHANGE

3. FAULT / MANUAL Switch . . . DESELECT MANUAL

4. AUTO / SEMI . . . SEMI

5. Cabin Pressure Selector Panel . . . SET VALUES AS REQUIRED

   Remember you are resetting a computer and it could very well be the simple act of deselecting/re-selecting could get the system back.

   If semi-automatic pressurization control is not regained:

6. FAULT / MANUAL . . . MANUAL

   CAUTION: WHEN OPERATING IN MANUAL MODE, MONITOR CABIN ALTITUDE CLOSELY, ESPECIALLY IN DESCENT, TO ENSURE THAT CABIN ALTITUDE DOES NOT DECREASE TO LESS THAN LANDING FIELD ELEVATION. CARE MUST BE TAKEN TO LAND UNPRESSURIZED WITH THE OUTFLOW VALVE IN FULL OPEN POSITION.

7. MAN HOLD . . . CONTROL TO ±500 FPM RATE OF CHANGE

   CAUTION: CLOSELY MONITOR CABIN DIFFERENTIAL PRESSURE. DO NOT ALLOW IT TO EXCEED 9.74 PSID.

   NOTE: When operating in the MANUAL mode, the rate of change should be monitored to maintain a maximum of 500 feet per minute UP or 500 feet per minute DOWN. Care should also be taken to land unpressurized with the outflow valve indicator in full OPEN position.

   If manual pressurization control is not successful, and if cabin differential pressure will exceed 9.74 PSID:

8. RAM AIR Switch . . . RAM

   NOTE: Selecting RAM will shut off bleed air for pressurization and the airplane will depressurize.

   Selecting RAM has the effect of turning both packs off. You might consider this instead:

   1. Turn one pack off.

   2. Decrease thrust on the engine driving the operating pack to control pressure within limits.

9. Crew Oxygen Masks . . . DON

10. Altitude . . . DESCEND TO SAFE ALTITUDE (25,000 FT OR BELOW)

11. Passenger Oxygen Masks . . . MANUALLY DEPLOY

    Upon reaching a safe altitude, attempt to regain control of the pressurization system as follows:

12. RAM AIR Switch . . . OFF

13. AUTO or MANUAL Control . . . ATTEMPT TO RESTORE

Pylon Hot

A pylon hot warning probably means you have a bleed air leak that can only mean very bad news if not treated. The idea here is to get rid of the air pressure and that might mean shutting the engine down. If you understand how the switches work and where the valves are, you can improve your odds of keeping the engine running while fixing the problem. Don't follow the checklist procedure without thinking about what you are doing and why.

Symptoms

Pylon Hot, L-R

You might see bleed temperatures go up as a result.

Analysis

[G450 Maintenance Manual, §26-12-00, ¶3.A.]

• The pylon thermal switches are mounted on the rib structure of the left and right pylons at FS 556, FS 580 and FS 651. There are three switches per pylon. The switches are normally open. When the ambient temperature near any of the switches rises above 250°F ±5°F, the affected switch closes. The left and right pylon thermal switches are electrically connected to the Modular Avionics Units (MAU).

• The left switches are powered with 28 Vdc from the left essential dc bus routed from the WARN LTS PWR #2 circuit breaker and through the equipment area overheat test relay #1. The right switches are powered with 28 Vdc from the right essential dc bus routed from the WARN LTS PWR #1 circuit breaker and through the equipment area overheat test relay #2.

• When any of the switches close, the circuit is completed to the MAUs. The MAUs then generate the applicable L or R Pylon Hot (red) message on the CAS.

Procedure

[G450 Airplane Flight Manual, §04-20-20]

1. APU Air . . . OFF

   The bleed air valve is located on the engine so if you are dealing with a right engine, the APU bleed will enter the pylon if it is running regardless of bleed air valve position. If the isolation valve is open the same will hold true with the left engine.

   CAUTION: ENGINE COWL ANTI-ICE IS NOT AVAILABLE WITH THE ON SIDE ENGINE BLEED AIR SWITCH SELECTED OFF. DEPART ICING CONDITIONS AND AVOID FOR THE REMAINDER OF THE FLIGHT.

2. Affected ENG BLEED AIR . . . OFF

   Look at the bleed air pressure on the ECS synoptic as you are doing this to confirm the bleed air valve actually closed. What if the valve doesn't close? Try pulling the bleed shut off circuit breaker, it fails closed.

   (See: G450 Pneumatics / PRSOV.)

3. Affected ECS PACK . . . OFF

4. ISOLATION Valve . . . CHECK CLOSED

   IF MESSAGE PERSISTS OR RETURNS:

5. Affected Engine . . . SHUT DOWN

 References:

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