Fire Protection System Abnormals

- James Albright (a former G450 driver)

Updated: 2015-07-22

When dealing with the engines or APU, you have fire loops to detect fires and fault detection systems to keep them honest. You have two fire bottles for the engines, one of those also covers the APU. Inside the airplane the trash receptacles have fire bottles and the cabin has a set of fire extinguishers. With the rest of the airplane, bleed air looks for things hotter than 250°F and electrical sensors look for things hotter than 150°F. What follows is a list of the systems and another list of various things that can go wrong.

 

When Things Go Wrong

  • Aft Baggage Flame — Flames detected in the aft baggage compartment, refer to Interior Fire/Smoke/Fumes checklist.

  • Aft Baggage Smoke — Smoke detected in the aft baggage compartment, refer to Interior Fire/Smoke/Fumes checklist.

  • Aft Equipment Hot — Temperature in aft equipment compartment over 250°, refer to Aft Equipment Hot.

  • Aft Floor Hot, L-C-R — Bleed air leak under cabin floor, refer to Aft Floor Hot.

  • Aft Lavatory Flame — Flames detected in the aft lavatory, refer to Interior Fire/Smoke/Fumes checklist.

  • Aft Lavatory Smoke — Smoke detected in the aft lavatory, refer to Interior Fire/Smoke/Fumes checklist.

  • APU Fire — Refer to APU Fire.

  • APU Fire Detector Fail — The APU fire detection system has failed, don't use the APU until it is repaired.

  • Baggage EER Hot — BEER > 150°F, refer to EER Hot.

  • Cabin Fire

  • EER Hot, L-R — LEER or REER > 150°F, refer to EER Hot.

  • Engine Fire, L-R — Refer to Engine Fire in Flight or Engine Fire On Ground.

  • Engine Fire Loop Alert — A fire loop senses a fire but there are no other indications, refer to G450 Fire Loop Alert.

  • Fire Bottle Discharge, L-R — The indicated fire bottle has either been discharged by a switch or its thermal relief plug. Either way you cannot dispatch until both bottles are charged and the message is extinguished.

  • Fire Detection Loop Fault — The system detects a possible fault in the fire loop, you need to do a system test, identify and disable the faulty loop.

  • Forward Floor Area Hot — The area under the forward floor area exceeds 150°F, refer to TRU Hot.

  • Forward Lavatory Flame — Flames detected in the forward lavatory, refer to Interior Fire/Smoke/Fumes checklist.

  • Forward Lavatory Smoke — Smoke detected in the forward lavatory, refer to Interior Fire/Smoke/Fumes checklist.

  • Galley Flame — Flames detected in the galley, refer to Interior Fire/Smoke/Fumes checklist.

  • Galley Smoke — Smoke detected in the galley, refer to Interior Fire/Smoke/Fumes checklist.

  • Pylon Hot, L-R— Pylon temperature is above 250°F, refer to Pylon Hot procedure.

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

Aft Equipment Hot

G450 ECS Synoptic, from Eddie's aircraft.

 

The procedure is fine until step 1, then it goes all wrong. Remember the prime directive when it comes to abnormal procedures in any GV series airplane: CAS, synoptics, QRH — in that order.

Call for the checklist and while the other pilot is looking for it, you can have all this done. Why the rush? In the best case scenario, a blown duct, having all that hot air back there can make things very bad, very quickly. In the worst case scenario, a fire, you are going to need to get the airplane on the ground very quickly. Do do this while the other pilot gets the checklist out:

  1. Pull up the ECS/Pressurization synoptic.

  2. Is the temperature in the duct over 250°F? Yes or no you are going to proceed, but keep the temperature in mind. If it is above 400°F (both bleeds) or 500° (single bleed with wing anti-ice) it is too high and you should be getting a Bleed Air Hot, L-R CAS message.

  3. Scan the engine instruments and pick an engine. Shut the bleeds off on that engine and confirm the pressure drops to zero. If the temperature in the duct drops appreciably you probably got the right engine. If not, swap them and try the other.

  4. That probably took you a minute. Still no relief? Now look at shutting both bleeds and the emergency descent. Your cabin is probably around 6,000 feet and your cabin leak rate should be no higher than 1,000 fpm, so you've got at least three minutes before things get interesting. (They say the maximum leak rate is 1,000 fpm but I haven't found that in writing.)

Symptoms

   AFT EQUIPMENT HOT   

Analysis

 

Figure: Aft equipment overheat sensors, from Illustrated Parts Catalog, §26-14-00, figure 5.

 

[G450 MM, §26-14, ¶3.A] The aft equipment switches are installed in the tail compartment at FS 660, near the bleed air ducting. The switches close at 250°F ±5°F. After cooling, the switches will open at 235°F ±5°F. When any of the switches close, the circuit is completed to MAUs No. 1 and No. 2. The MAUs will then generate the Aft Equipment Hot (red) message for display on the CAS. The switches are powered with 28 Vdc from the left essential dc bus through the WARN LTS PWR #2 circuit breaker.

The 250°F switches are located fairly high and forward in the aft equipment bay. The tail compartment access door spans from FS 728 to 770, so those switches (at FS 660) are forward of that. The ambient temperature of the aft equipment during flight is said to be around 70°F, just residual heat from the engine bleed air ducts. If you have a AFT EQUIPMENT HOT indication, you either have a leaking bleed air duct, a fire, or a bad sensor indication.

[G450 Quick Reference Handbook, page MA-3] Aft equipment hot: Aft equipment area temperature above 250°F. Possibility exists that high pressure duct has blown or that fire is in progress.

[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 of minus ten degrees Fahrenheit (400°±10 F / 204.4°C).

  • 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 five hundred plus or minus ten degrees Fahrenheit (500±10°F / 260°C).

It should normally be pretty cold back there while in flight. The Aft Equipment Hot CAS message comes on at 250°F so if the message is real, you probably have a blown high pressure duct or there may be a fire. In the case of a blown duct, you need to take the pressure away. In the case of a fire, you need to land immediately.

Procedure

[G450 Airplane Flight Manual, §04-20-30]

  1. APU Air . . . OFF

  2. Affected Engine . . . ATTEMPT TO IDENTIFY

    Note: High Turbine Gas Temperature (TGT) or Fuel Flow (FF) indications, low Engine Pressure Ratio (EPR) indication, or an abnormally low BLEED AIR indication may aid in identifying the engine affected with the Aft Equipment Hot message.

  3. Affected ENG 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.

    This isn't always the case, later G450s have removed the check valve that prevents bleed air coming from outside the engine to reach the cowl anti-ice. This isn't documented (yet).

  4. Affected ECS PACK . . . OFF

  5. ISOLATION Valve . . . CHECK CLOSED

    IF MESSAGE PERSISTS OR RETURNS:

  6. Emergency Descent . . . COMMENCE

    Do you want to subject the airplane to this stress if there is an unknown problem with a hot bleed air duct back there? The cabin leak rate is said to the around 1,000 feet per minute so you should have at least 4 minutes to get down to a suitable altitude. If you are at 45,000 feet, an emergency descent is probably in order. If you are at 15,000 feet? No so much.

  7. Opposite ENG BLEED AIR . . . OFF

    NOTE: This will result in loss of cabin pressurization. Cabin altitude will climb at the cabin leak rate.

  8. Passenger Oxygen Masks . . . DEPLOY (IF REQUIRED)

  9. Airplane . . . PROCEED TO NEAREST SUITABLE AIRPORT AND LAND

 Aft Floor Hot

 
 

The location of the thermal switches make the most probable culprits seemingly obvious:

  • "L" switch is near the left wing anti-ice duct.

  • "C" switch is near the ECS trim air valves.

  • "R" switch is near the right wing anti-ice duct.

The switches are triggered at 250°F so a quick look at the synoptic can help you find the problem. The wing anti-ice ducts come right off the bleed air duct set reliably to 400°F or 500°F. A bleed air leak should reduce the temperature of the air actually getting to the wing. I took the photo of the synoptic above on properly operating system. The temperature of the wings is taken at Gap Band #2, about a third the distance outboard on the wing. If the wing anti-ice controller is unable to get that wing to its 130°F target, a leak on that side could be the issue.

What about the ECS? The trim air valves do have access to that same hot bleed air and should not be discounted as the source of the bleed air leak.

CAS:

Aft Floor Hot, L-C-R

Analysis:

The temperature under the floor near the ECS ducts/valves and wing anti-ice system ducts/valves has exceeded 250°F. There are three areas: Left, Center, and Right.

[G450 AMM, § 26-14-00 ¶3.A.] The left aft floor switches are installed below the cabin floor on the left side at FS 572, FS 580 and FS 596, near the hot air ducting. The center aft floor switches are installed below the cabin floorat FS 556 and FS 572, near the trim air valves. The right aft floor switches are installed below the cabin floor at FS 540, FS 572 and FS 592, near the hot air ducting.

Procedures:

 
 

[G450 AFM, § 4-20-50]

If the Aft Floor Hot, L-R message is displayed, complete Steps 1 through 9. If the Aft Floor Hot, C message is displayed, proceed to Step 10.

If the Aft Floor Hot, L-R message is displayed on CAS:

  1. WING ANTI ICE ... OFF

  2. Affected Engine BLEED AIR ... OFF

  3. Affected ECS PACK ... OFF

  4. ISOLATION Valve ... CHECK CLOSED

    If the message remains after one minute:

  5. Crew Oxygen Masks ... DON

  6. Descent To 15,000 Ft ... COMMENCE

  7. Passenger Oxygen Masks ... MANUALLY DEPLOY

  8. Passenger Oxygen Masks ... VERIFY DEPLOYMENT

  9. Opposite ENG BLEED AIR ... OFF

    If the Aft Floor Hot, C message is displayed on CAS:

  10. WING ANTI ICE ... OFF

  11. ISOLATION Valve .... CLOSED

  12. L Engine BLEED AIR ... OFF

  13. L ECS PACK ... OFF

    If the message remains after one minute:

  14. L Engine BLEED AIR ... ON

  15. L ECS PACK ... ON

  16. R Engine BLEED AIR...OFF

  17. R ECS PACK ... OFF

    If the message remains after an additional minute:

  18. Crew Oxygen Masks ... DON

  19. Descent To 15,000 Ft ... COMMENCE

  20. Passenger Oxygen Masks ... MANUALLY DEPLOY

  21. Passenger Oxygen Masks ... VERIFY DEPLOYMENT

  22. L Engine BLEED AIR ... OFF

 EER Hot

 

Figure: Overheat detection system diagram, from G450 Aircraft Operating Manual, §2A-26-00, figure 6.

 

These overheat switches are typically set at 150°F. Gulfstream is starting to purge the books of these numbers but it still exists for EER Hot and PDB Overheat. Why do I harp on this? Because 150°F isn't that much.

Dropping the cabin temperature could help a lot. Increasing the cabin altitude to 7,900' is a great idea but don't go any higher. (The CABIN PRESS LOW message is usually set a 8,000' and exceeding it could cause an automatic Emergency Descent.)

Symptoms

BAGGAGE EER HOT

EER HOT

PDB OVERHEAT, L-R

 

Figure: Equipment overheat switch, from G450 Maintenance Manual, §26-14-01, figure 405.

[G450 Maintenance Manual, §26-14-00, ¶3.A.]

  • The LEER switches are installed on several of the shelves in the LEER. The switches close at 150°F ±5°F. After cooling, the switches will open at 135°F ±5°F. When any of the switches close, the circuit is completed to MAU No. 1. The MAU will then generate the L EER Hot (amber) message for display on the CAS. The switches are powered with 28 Vdc from the left essential dc bus through the WARN LTS PWR #2 circuit breaker.

  • The REER switches are installed on several of the shelves in the REER. The switches close at 150°F ±5°F. After cooling, the switches will open at 135°F ±5°F. When any of the switches close, the circuit is completed to MAU No. 2. The MAU will then generate the R EER Hot (amber) message for display on the CAS. The switches are powered with 28 Vdc from the right essential dc bus through the WARN LTS PWR #1 circuit breaker.

  • The baggage compartment EER switches are installed in the baggage compartment EER. The switches close at 150°F ±5°F. After cooling, the switches will open at 135° ±5°F. When any of the switches close, the circuit is completed to MAU No. 1. The MAU will then generate the Baggage EER Hot (amber) message for display on the CAS. The switches are powered with 28 Vdc from the right essential dc bus through the WARN LTS PWR #1 circuit breaker.

    There is no reference to PDB overheat switches other than in the GV and G550 manuals that speak of the PDB overheat occurring over 175°F.

The electronic equipment racks are normally cooled with conditioned air and dedicated fans, they shouldn't be too much hotter than the cabin temperature. Increasing the flow of cool air by lowering cabin temperature and increasing air flow and the prescribed solutions. Even if it does cure your overheat, you still need to consider the cause of the overheat. You might have more problems ahead of you.

Procedure

[G450 Airplane Flight Manual, §3-20-20]

  1. EER Fan(s) Circuit Breakers . . . CHECK

    NOTE: For Baggage EER Hot message, inspect compartment loading to ensure that airflow to the electronic equipment is not blocked by luggage or other items.

  2. COCKPIT, FWD CABIN and AFT CABIN Temperature Controls . . . SELECT MANUAL, FULL COLD

    If message persists:

  3. Pressurization Control . . . SEMI

  4. Cabin Altitude . . . INCREASE TO 7,900 FT

    Raising cabin altitude increases airflow through the TROV

    For EER Overheat message:

  5. Altitude . . . DESCEND TO 33,000 FEET OR BELOW

    Descending to 33,000 feet or below causes the EER fans to shift to high speed, thereby increasing cooling airflow.

    This also kicks the fans cooling the TRU's into low speed mode. If the issue is high draw from a component, you might consider closely monitoring TRU loads to anticipate a TRU overheat.

  6. EER Cover Panels . . . REMOVE AND CHECK FOR HOT COMPONENTS

  7. For Hot Component(s) . . . PULL COMPONENT POWER CIRCUIT BREAKER(S)

    For PDB Overheat message:

  8. PDB Cover Panel . . . CHECK FOR POPPED CIRCUIT BREAKERS

    CAUTION: DO NOT RESET ANY POPPED CIRCUIT BREAKERS.

    If PDB Overheat message persists:

  9. Nearest Suitable Airport . . . LAND

 Interior Fire/Smoke/Fumes

The good news is Gulfstream put everything in one checklist (EC-8) that leads you to the three other checklists needed as the situation dictates. The bad news is that if you have to read each checklist you might be too late. Statistics show that if you don't put a cabin fire out in eight minutes or less you probably won't and if you don't land the airplane in fifteen minutes or less, you probably won't.

Symptoms:

You might have fire, smoke and fumes; you might also have one of the following CAS messages:

   Aft Baggage Flame   

   Aft Baggage Smoke   

   Aft Lavatory Flame   

   Aft Lavatory Smoke   

   Forward Lavatory Flame   

   Forward Lavatory Smoke   

   Galley Flame   

   Galley Smoke   

QRH:

[G450 QRH, pg. EC-8]

WARNING: DO NOT DELAY DESCENT OR DIVERSION TO FIND THE SMOKE SOURCE. REGARDLESS OF WHETHER A FIRE HAS BEEN EXTINGUISHED OR SMOKE HAS CLEARED, CONSIDERATION SHOULD BE GIVEN TO LANDING THE AIRPLANE WITHOUT DELAY.

WARNING: CONSIDER ALL SMOKE AND/OR FUMES TO BE TOXIC.

  1. Crew Oxygen Masks (100%) and Smoke Goggles . . . DON

    WARNING: EXERCISE EXTREME CAUTION WHEN USING OXYGEN NEAR OPEN FLAME.

  2. NO SMOKE / SEATBELT . . . ON

  3. Passenger Oxygen Mask . . . MANUALLY DEPLOY / VERIFY

    NOTE: Visually confirm that an oxygen mask has deployed for each passenger. Advise passengers to DON oxygen masks.

  4. All Master Switches (Overhead and Pedestal) . . . OFF

  5. Emergency Power . . . ON

    If level flight is not critical:

  6. Rapid Descent . . . BEGIN

    Use of autopilot and autothrottles is recommended. Set altitude preselector to desired safe altitude.

    NOTE: See Emergency Descent Procedure, page EH-3. Proceed to the appropriate section to complete checklist:

    • Fire / Smoke Fumes in Cockpit, Cabin, or Baggage Compartment (page EC-9) . . . Section A

    • Smoke From Air Conditioning Outlets (page EC-10) . . . Section B

    • Electrical Fire / Smoke Source Unknown (page EC-11) . . . Section C

Analysis:

The QRH makes it clear you need to be thinking and acting in two parallel, simultaneous directions: fight the fire and get the airplane on the ground.

 Pylon Hot

The engine pylon has air circulating from the leading to the trailing edge. The only source of hot air would occur if you had a bleed air leak in the precooler on the lines leading to and from the precooler. This air is under pressure and is very hot. If you have a valid pylon hot message, you need to get rid of the bleed air. If you can't, you are better off shutting the engine down.

[G450 Airplane Flight Manual, §4-20-20]

  1. APU Air . . . OFF

  2. Affected ENG BLEED AIR . . . OFF

  3. ISOLATION Valve . . . CHECK CLOSED

    If Message Persists or Returns:

  4. Affected Engine . . . SHUT DOWN

 TRU Hot

 

Figure: Equipment cooling fans, from G450 Aircraft Operating Manual, §2A-21-00, figure 5.

 

A Forward Floor or TRU Hot message usually portends future electrical problems that might be fixable by non-electrical means. It doesn't take much to cause a TRU Hot message — 150°F — dropping the cabin temperature could help a lot. Increasing the cabin altitude to 7,900' is a great idea but don't go any higher. (The CABIN PRESSURE LOW message is usually set at 8,000' cabin altitude and exceeding it could cause an automatic Emergency Descent.)

Another thing you can do is to climb above 36,000' if you are below it. That will kick the PSU fans to high speed mode.

What else can you do? You know the normal load on your TRU's rarely tops 33% and if you see one that is well above that there are two possibilities. Something on that bus has a high draw or the TRU itself is going bad. If dropping a fuel pump doesn't help, try pulling the circuit breaker for the TRU, you know the load will automatically go to the AUX TRU so what have you got to lose?

Symptoms

You may see some degradation of the TRU's performance on the DC Synoptic, or it may have dropped off line. Of course you should also get a CAS message:

AUX TRU HOT

ESSENTIAL TRU HOT

MAIN TRU HOT

Forward Floor Area Hot

You may have been sent here via the:

35K Altitude Trip Fail

Analysis

[G450 Airplane Flight Manual, page 03-123] Aux TRU Hot — AUX TRU temperature exceeds maximum allowable temperature.

[G450 Airplane Flight Manual, page 03-131] Essential TRU Hot — Indicated Essential TRU temperature exceeds maximum allowable temperature.

[G450 Airplane Flight Manual, page 03-140] Main TRU Hot — Indicated TRU temperature exceeds maximum allowable temperature.

[G450 Airplane Flight Manual, page 03-134] Forward Floor Area Hot — Forward underfloor area temperature exceeds maximum allowable temperature.

[G450 Airplane Flight Manual, §03-20-10] Cooling air is drawn from the Left and Right Electronic Equipment Rack (LEER) and (REER), aided by the exhaust of the Passenger Service Unit (PSU) fan and routed beneath the forward floor area primarily to cool the five Transformer-Rectifier Units (TRUs): Auxiliary (AUX), Left Essential (L ESS), Right Essential (R ESS), Left Main (L MN) and Right Main (R MN). The amber Forward Floor Area Hot message is displayed on the Crew Advisory System (CAS) when the temperature under the forward cabin floor between the EERs exceeds 150°F. It is likely that an overheat underneath the forward floor area is associated with an overheating Transformer/Rectifier Unit (TRU) and will be accompanied by one of the TRU Hot CAS messages.

The TRU's are located under the forward floor, just inboard of the main entrance door. Any of these messages means one or more of the TRUs may be overheating. The trip point of these switches are not very high, only 150°F. Getting cool, conditioned air over the TRUs may solve what ails you.

[G450 Airplane Flight Manual, page 03-148] 35K Altitude Trip Fail — Relay fails to enable 35,000 feet switchover. EER cooling fans stay in high speed mode while PSU fan stays in low speed mode.

[G450 Maintenance Manual, §21-25-00, ¶3.]

  • The fans in the Left Electronic Equipment Rack (LEER) and Right Electronic Equipment Rack (REER) are located on the bottom shelf of each rack and pull air down through the racks. The LEER fan exhausts to the Transformer Rectifier Units (TRUs) below the REER, then overboard through the thrust recovery outflow valve. The REER Fan exhausts under the floor and across the aircraft to the outflow valve. The fans come on when power is applied to the L MAIN 28 Vdc bus, if the circuit breakers in the LEER circuit breaker panel labeled L/R EER FAN are closed.

  • The Personal Services Unit (PSU) fan, located under the entrance way, pulls air from the PSU and exhausts it under the floor, aft of the FS 169 bulkhead. This air cools the center TRUs and exhausts overboard through the thrust recovery outflow valve.

[G450 Maintenance Manual, §21-25-00, ¶4.]

  • The LEER and REER fans have two speeds which are controlled by applying a ground to the logic pins. Each fan changes from a fast speed to a slower speed at 35,000 feet altitude. The air conditioning units energize two relays to ground the required pins to change the speed of the fans. These relays are energized by the air conditioning control units. Only one air conditioning control unit has to be operating for the fans to change speed.

  • The PSU fan is a two speed fan. It runs at a low speed until either of the environmental control system relays are energized at 35,000 feet and then shifts into high speed. The PSU fan is powered by the left main 28 Vdc bus and comes on when power is applied and when the circuit breaker labeled L PSU FAN, located in the LEER circuit breaker panel, is depressed. This fan also has a Low Speed Warning Detector (LSWD) attached. The LSWD inputs a discrete GND (ground) to the CAS when the fan speed drops below a predetermined speed. The CAS then displays the L-R EER Fan Fail (blue) or L PSU Fan Fail (blue) message.

The LEER and REER fans need to be in high speed mode at low altitude while the PSU fans need the higher speed at high altitudes. If either of these fans fail to go into high speed when called for, you might next see a TRU overheat.

Procedure

[G450 Airplane Flight Manual, § 03-20-10]

If a "Forward Floor Area Hot" or one of the TRU Hot messages is displayed while on the ground:

  1. FAULT / MANUAL Switch ... MANUAL

  2. MAN HOLD Knob ... FULL OPEN

  3. All Zone Temperatures ... MANUAL / FULL COLD

If message(s) does not clear within two minutes and TRU loads are normal, shut down airplane and investigate.

If a "Forward Floor Area Hot" or one of the TRU Hot messages is displayed while in flight:

  1. L PSU FAN Circuit Breaker ... CHECK IN

  2. FWD CABIN and AFT CABIN Temperature Zones ... MANUAL / FULL COLD

  3. Pressurization Control Mode ... SEMI / INCREASE CABIN ALTITUDE TO 7,900 FT

    Raising cabin altitude increases airflow through the TROV.

  4. Pressurization Control Mode ... AUTO

  5. DC Power Synoptic Page ... SELECT

  6. TRU Load(s) ... CHECK

    If any TRU load is excessive:

    What is excessive? We usually see our TRU's in the high twenties, sometimes low thirties. I've never seen a G450 TRU above 38%.

  7. Load (On Bus That TRU Supplies) ... REDUCE

    • If a main TRU is high, consider turning off corresponding alternate fuel boost pump.

    • If an essential TRU load is high, consider turning off corresponding main fuel boost pump.

    • If the AUX TRU load is high, consider turning off the fuel boost pump associated with the Main or Essential DC Bus that the TRU is powering.

    CAUTION: ENSURE THE CROSSFLOW VALVE IS OPEN BEFORE TURNING OFF BOTH BOOST PUMPS ON ONE SIDE TO PROVIDE A SOURCE OF PRESSURIZED FUEL TO THE ENGINE.

    If message(s) persists:

  8. Circuit Breaker (For Overheating TRU) ... PULL

    If electrical system integrity must be preserved or if Forward Floor Area Hot message persists:

  9. Pressurization Control Mode ... SEMI / INCREASE CABIN ALTITUDE TO 7,900 FT

You should not set the cabin pressure above 8,000 feet since that will trip the CABIN PRESSURE LOW warning and could cause an automatic emergency descent.

 References

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G450 Aircraft Operating Manual (Historical), Revision 24, September 18, 2009.

Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013

Gulfstream G450 Illustrated Parts Catalog, Revision 17, October 31, 2012

Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013