Fire Protection System

[G450 AOM, ¶2A-26-10] The fire protection system incorporates sensors to detect the excess heat levels associated with fire in areas surrounding the engines and the APU, elevated levels of heat and/or smoke indicative of conditions that may lead to fire in areas of the aircraft interior, and high temperature ranges detrimental to the function of aircraft equipment.

If a fire is detected, the system sensors alert the flight crew to perform procedures to extinguish the fire by manually directing the discharge of the contents of onboard fire extinguishers. After a fire or overheat condition in the aircraft interior has been eliminated, any remaining smoke can be evacuated overboard through the baggage compartment door seal.

Engine and APU Fire and Overheat Detection and Warning

Each engine has a dual loop fire detector to sense heat levels associated with fire, the bleed air ducting is monitored for leaks by thermal switches, and the APU enclosure is monitored by a single element sensor. The crew is alerted to any excess temperature in these areas or to faults in the detection systems.

Engine Fire Detection and Warning

[G450 AOM, ¶2A-26-20 ¶2.A.] Each engine has dual loop sensors that provide indications of high temperatures associated with an engine fire. Sensors are located in the following positions:

At the engine compressor 7th and 12th stages
At the IDG
At the bleed air duct
Inboard of the engine on the fixed cowling near the front engine mount
The left engine (only) has a sensor positioned at the engine anti-ice duct

The sensors are located on the engine in rails. The sensors are wired together and act as a single dual loop sensor. The sensor loops are designated Loop A and Loop B. Additional redundancy is provided by separating the loop power sources: the left engine Loop A is powered by the left essential DC bus and Loop B powered by the right essential DC bus; right engine Loop A is powered by the right essential DC bus and Loop B by the left essential DC bus.

Each loop is a sheath of stainless steel surrounding a temperature sensitive glass / oxide material. Centered in the glass / oxide material is a coaxial cable wire. The electrical resistance and capacitance between the steel sheath and the center wire are monitored by a Fire Detector Control Unit located in the tail compartment of the aircraft. As a sensor loop is heated, the glass / oxide material loses insulating qualities and allows a current flow between the center wire and the surrounding sheath, signalling a fire. Both loops are located in parallel and at close proximity, so the control unit must receive a simultaneous fire indication from Loop A and Loop B to send a fire annunciation to the cockpit. If only one loop indicates a fire, the indication could result from a breach of the insulating glass / oxide material or other malfunction, and is reported as a loop fault by the control unit.

If both loops indicate a fire, the control unit sends a signal to the Modular Avionics Units #1 and #2 which send red Engine Fire, L-R CAS messages for engine fire and fire loop alerts. Control unit hard wire connections illuminate the engine fire handle and release the solenoid holding the fire handle in the stowed position, illuminate the engine fuel control switch, the master warning light on the cockpit glare shield and both loop A and loop B elements of the fire test switch for that engine.

If only a single loop indicates a fire, the control unit will generate a loop fault signal for that loop, illuminating the appropriate fire detection loop fault indicator and prompting Engine Fire Loop Alert CAS messages for fire loop alert and Engine Fire Detection Loop Fault. The erroneous loop may be selected off and fire detection will be accomplished with the single remaining loop. If a fire is detected when in single loop configuration, all indications and warnings are the same as dual loop detection except there is no loop fault indication for the loop selected off.

Each engine has two fire detection loops, each loop powered by a different DC essential bus. Both loops are monitored for faults and you can take a loop off line if it has a fault. It takes two loops to detect a fire unless you took one off line, then it only takes one. The MAUs send indications to the CAS, hard wire connections to the fire handles and fuel control switches. If you have an Engine Fire, L-R CAS warning, you might have a fire and need to check the fire fault switches. If you have a light in the fire handles and fuel control switches you have a fire.

An Engine Fire Loop Alert CAS message means one loop says fire the other doesn't, one of the loops is broken and you need to test it to find out which. An Engine Fire Detection Loop Fault means the system thinks there is a problem and you need to investigate.

If you get an Engine Fire Loop Alert CAS message, or after successfully putting out an engine fire, you must test the fire loops:

If pressing a fire test switch light results in the normal eight indications — Loop A, Loop B, Engine Fuel Switch, Fire Switch, "Engine Fire Loop Alert" and "Engine Fire" CAS messages, and two Master Warn lights — the fire loops are okay and do not need to be disabled.

If pressing a fire test switch light results in a missing indication, the fire loop is faulty and the indicated loop should be disabled by pressing the applicable fire fault switch. Once the bad loop is disabled the good loop is on its own and the next thing you could see is the fire warning.

Pylon Overheat Detection and Warning

Figure: G450 Pylon access panels, (G450 Maintenance Manual, §26-12-00, figure 501)

[G450 MM, §26-12-20 ¶3.A.] 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 Pylon Hot, L-R message on the CAS.

Each pylon has three thermal switches which alert you with Pylon Hot, L-R CAS messages when air in the pylon itself exceeds 250°F.

APU Fire Detection

[G450 AOM, ¶2A-26-20 ¶2.C.] The APU fire detector is a continuous element routed around critical areas within the APU container. The element consists of a seven foot long tube filled with helium gas and a stabilizing chemical, sealed at both ends. Two sensors are installed in the end of the tube: one sensing high pressure and the other sensing low pressure.

If the gas within the tube is heated, a pressure increase above a preset threshold indicates a temperature of approximately 1,000°F over a small section of the sensor tube or by a temperature level of 450°F over the length of the sensor. When sensor pressure exceeds the threshold, a fire signal is sent to MAUs #1 and #3 for initiation of APU Fire CAS visual and aural fire warnings. Hard wire signals are generated to illuminate the FIRE legend on the APU overhead panel, the red master warning light on the cockpit glare shield and if the aircraft is on the ground (weight-on-wheels), to the APU fire warning horn in the nose wheel well. The APU ECU automatically shuts off fuel to the APU if a fire is detected.

The second tube sensor monitors low gas pressure in the fire detector. If an APU malfunction or other failure causes a rupture in the tube structure allowing the escape of the gas within, the sensor will detect the resulting loss of pressure and signal a failure of the APU fire detector to MAUs #1 and #2 to initiate APU Fire Detector Fail CAS annunciations.

The APU fire detection system is a simple helium tube inside the APU enclosure. If the pressure in the tube increases you probably have a fire; if it decreases you probably lost the detection system. An APU Fire CAS message means the MAU thinks you have a fire, a light in the APU fire light means you do.

Engine and APU Fire Extinguishers

Each engine has a fire handle that cuts fuel and hydraulic pressure to the engine, disconnects the IDG, and can be used to activate one or both fire bottles. There are two fire bottles dedicated to extinguishing engine and APU fires. Each engine can accept extinguishing agent from either or both bottles. The APU can accept extinguishing agent from only the left bottle.

[G450 AOM, §2A-26-30 ¶1] Engine fires are extinguished by pulling engine fire handles out and injecting the contents of one or both fire extinguisher bottles into the engine. Pulling the fire handle out shuts off fuel at the engine fuel control, shuts off hydraulic pressure and de-energizes the engine IDG. These actions remove combustible fluids and possible sources of ignition. After the fire handle has been extended, the handle is rotated outboard and, if necessary, inboard to inject a fire extinguishing agent into the engine.

Fire Extinguishing Bottles

[G450 AMM, §26-21-00 ¶3.C] The two engine fire bottles are located in the tail compartment. The bottles are interchangeable. There are three fire bottle cartridges installed on each fire bottle. Only two cartridges are used for engine fire extinguishing. The cartridges are installed in bonnets on the bottle. Each bonnet has a color coded ring around it. The red band identifies the outlet for the left engine, the blue band identifies the outlet for the right engine and the yellow band identifies the outlet for the APU. On the right fire bottle, the yellow banded bonnet is capped off. Only the left fire bottle is hooked up to the APU fire extinguishing system.

Each bottle contains CF 3 Br (Halon 1301) and is pressurized with nitrogen to approximately 600 psi at 70°F. A temperature compensated low-pressure switch is mounted on each bottle. The switch will send a signal to modular avionics units No. 1 and No. 2 when bottle pressure has dropped below 200 psi. If pressure drops below 200 psi due to leakage or due to the bottle being discharged, the applicable Fire Bottle Discharge message will be displayed on the CAS.

The fire bottles are discharged by pulling and rotating the engine fire pull handles. The right fire bottle is shot 1 and the left fire bottle is shot 2. When one of the cartridges is set off, a scored metal disc sealing the bottle is broken and the bottle will release its entire contents into the selected engines fire extinguishing system plumbing through the bonnet. A metal screen in the bonnet prevents pieces of the metal disc from entering the lines.

You only have two fire bottles for three potential users: each engine and the APU. Don't bother looking for a gauge on the bottle — the AOM claims the gauge exists but this is a cut and paste error from the GIV manual — the CAS will alert you if the pressure is low.

Engine Fire Handles

[G450 AOM, §2A-26-30 ¶2] The left and right engine fire handles are normally locked in the stowed position by an electrical solenoid. When a fire signal is provided by the engine fire detection system, the solenoid opens, allowing the associated fire handle to be pulled out. If the solenoid malfunctions, a manual override button underneath each fire handle allows handle activation.

Each fire handle can be rotated to either of two positions after it is pulled out to inject fire extinguishing agent into the engine. Rotating the fire handle outboard to the DISCH 1 position sends an electrical signal to an explosive squib on the right fire bottle, rupturing the seal separating the pressurized bottle contents from the plumbing connected to the engine nacelle. The DISCH 1 position reserves the left fire bottle for use in an APU fire, since only the left fire bottle is plumbed to the APU container.

If an engine fire persists, however, the fire handle may be rotated to the inboard DISCH 2 position to inject the agent in the left fire bottle into the engine.

Power for extinguisher activation is from the left essential DC bus for the right extinguisher bottle (DISCH 1) to either engine, from right essential DC for the left extinguisher bottle (DISCH 2) to either engine and left essential DC for the left extinguisher bottle to the APU enclosure. Fire extinguisher activation is therefore available whenever the aircraft main batteries are selected ON and the essential DC buses are powered.

Some manuals refer to the two fire bottles as "Bottle 1" and "Bottle 2" — you need to banish this from your lexicon. They are the left bottle and the right bottle. When discharging a bottle into an engine, you rotate the fire switch outboard in the direction of the engine and this is Shot 1 and it always comes from the Right Bottle. If you need to discharge more extinguishing agent into the same engine, you rotate the handle inboard and discharge Shot 2, this will always be from the Left Bottle. For an APU fire, pressing the APU FIRE EXT switch always discharges from the Left Bottle. Always.

APU Fire Extinguishing:

[G450 AOM, §2A-26-30 ¶2.C.] Only the left fire extinguisher bottle is plumbed into the APU compartment. If a fire is detected within the APU enclosure, fuel is automatically shut off at the APU fuel control. The flight crew may discharge the contents of the left bottle into the APU by pushing in the guarded FIRE EXT DISCHD pushbutton above the APU FIRE indicator light. When the left bottle is discharged, the bottle low pressure sensor will signal the activation of the L Fire Bottle Discharge CAS message and the illumination of the amber FIRE EXT DISCHD legend in the pushbutton.

Aircraft Interior Overheat and Smoke Detection, Smoke Evacuation, Portable and Fixed Fire Extinguishers

Various areas of the aircraft are monitored by sensors for undesirable levels of heat in areas containing electronic equipment, in the underfloor areas near hot air manifold ducting of the air conditioning system, and in the tail compartment where the APU and hydraulic reservoirs are located. The baggage compartment is monitored for both overheating in electronic equipment racks and smoke as an indication of combustion. Smoke in the baggage compartment or anywhere in the cockpit or cabin may be evacuated from the aircraft after the cause of the smoke is eliminated. Portable and fixed fire extinguishers located in aircraft interior can be used to prevent smaller scale combustion from becoming a hazard to the aircraft.

A cabin fire is one of the most dangerous things that can happen to an airplane.

More about this: Cabin Fire.

Overheat Detection

[G450 AOM, ¶2A-26-40 ¶ 2.A.] Twenty-two heat detector switches are installed in the aircraft interior to monitor temperature levels, set at two trip points, depending upon location and type of equipment monitored. Ten high temperature switches set at 250°F are placed in the following locations:

Two switches in the aircraft tail compartment
Three switches beneath the right aft cabin floor near hot air ducting
Two switches beneath the center aft cabin floor near the hot air manifold
Three switches beneath the left aft cabin floor near hot air ducting

Twelve lower temperature switches set at 150°F are placed to monitor electronic equipment temperature levels in the following locations:

Two switches in the Aft (baggage) Electronic Equipment Rack (AEER)
Five switches beneath the cabin floor near the main entrance door to monitor electronic equipment installations
Three switches in the Left Electronics Equipment Rack (LEER)
Two switches in the Right Electronics Equipment Rack (REER)

You will get a red CAS message for exceeding one of the 250° switches, these are associated with bleed air. You will get an amber CAS message for exceeding one of the 150° switches associated with an equipment rack or TRU.

Smoke Detection

[G450 AOM, ¶2A-26-40 ¶ 2.B.] A smoke detector is installed in the ceiling of the baggage compartment to warn of possible combustion within the aircraft. The detector is a light emitting source and a photoelectric cell placed on opposite sides of an enclosure with a white interior. Vents in the enclosure allow compartment air to circulate through the interior. In normal conditions the photoelectric cell receives a constant level of illumination from the light source and produces a steady voltage. If less illumination is received by the photoelectric cell the detector communicates with MAU I/O modules to initiate a CAS message and aural alerts.

Airflow in this airplane is generally from aft to forward, everything heads for the TROV. It is possible to have smoke in the galley that escapes notice of the smoke detector until it might be too late. Remember, if you don't put a cabin fire out in eight minutes or less, you probably won't. For this reason and others, if you don't have a crewmember in the cabin, you are best served by keeping the cabin door open.

See: G450 Cabin Fire.

Smoke Evacuation

[G450 AOM, ¶2A-26-40 ¶ 2.C.] If smoke is present in the baggage compartment or anywhere in the aircraft interior, it may be evacuated from the aircraft by deflating the seal around the baggage compartment exterior door and allowing cabin differential pressure to eject the smoke. A panel installed in the passenger compartment side of the bulkhead above the interior baggage compartment door contains controls for smoke evacuation.

Rotating the handle to the VENT / SMOKE position deflates the pressurized seal around the exterior door of the baggage compartment, allowing cabin differential pressure to escape to the atmosphere porting any smoke in the cabin interior overboard. At the completion of smoke evacuation, normal pressurization can be restored. Rotating the handle to the NORMAL OPS position allows the bleed air system to re-inflate the baggage door seal.

The interior baggage door on the G450 is not a pressure bulkhead, like it is in the GV/G550. You can open the smoke evacuation valve without having to open the interior door. But should you? This is why you get paid the big bucks: to make these kinds of judgement calls. If the source of the smoke is in the baggage compartment, you risk fanning the source and making things worse. If the source is in the cabin and is understood — a burnt roast in the oven, perhaps — you are probably well served by this valve.

Portable and Fixed Fire Extinguishers

[G450 AOM, ¶2A-26-40 ¶ 2.D.] The aircraft is equipped with two types of portable fire extinguishers to control fires in interior areas accessible to the crew. Each extinguisher is used to control fires from different sources:

A Halon extinguisher with a capacity of 8.2 pounds is stored on a quick release bracket in the cockpit on the pilot side. This type of extinguisher is most effective against oil or grease fueled fires and electrical fires
An extinguisher containing approximately seven pounds of a water and antifreeze mixture pressurized with nitrogen is mounted on the right side of the forward cabin bulkhead. This type of extinguisher is most effective against fires originating in paper or cloth materials

Photo: Galley trash bin fire extinguisher, (Eddie's aircraft)

Each cabin lavatory is equipped with an integrated fire detector and extinguisher unit mounted over lavatory trash bins. The unit consists of a container holding 9 cubic inches of a bromotrifluoromethane extinguishing agent and discharge tubes capped with a fusible alloy. The discharge tubes are positioned over the trash container. If the temperature level below the tube caps exceeds 170°F, the discharge tube caps will melt and the contents of the extinguisher will be directed into the trash bin in three to fifteen seconds.

Photo: Aft lavatory trash bin fire extinguisher, (Eddie's aircraft)

Our trash bins are exactly as stated in the AOM. I've heard not all G450/550 are outfitted this way, however.

Engine Tailpipe Fires

There is a final category of engine fire for which we don't have any detection system at all other than our eyes and depending on how we shut the engine down, no protection system. It was a more common problem on the GII and GIII and I'm not sure it has happened on a G450, but it is worth considering. (We have a tailpipe fire procedure in the QRH, so perhaps Gulfstream thinks it is a possibility.) There is a chance raw fuel can get into the tail pipe after engine shutdown and ignite. In older style engines with thermocouples in the tail pipe, if you had the presence of mind to look, you might note the increased TGT. With our newer engines the IGT or TGT is actually taken well forward of that point. (In the middle of the LP turbine section in the G450.) If you have a tail pipe fire, your fire detection system will never notice and the fire extinguishing agent doesn't reach inside the tail pipe. No detection or protection?

There are a few techniques that can help you. Some may argue these are too much trouble for something that has a very low likelihood of ever happening. Maybe so. But these are expensive engines, so I recommend:

Use the tail camera to watch the engine during shutdown. If you see flames from the tail during shutdown, motor the engine by ensuring you have APU available to the engine and selecting the CRANK MASTER to ON and the affected ENG START switch to ON. This is covered in the QRH, EC-6.
Do not use the Quiet Shutdown Checklist because that leaves you without the APU to blow out a tailpipe fire.