If this happens on takeoff before V1, reject the takeoff, and press on with this procedure after you are stopped. If this happens on takeoff after V1, continue the takeoff, get the airplane off the ground, and once above 1,500' press on with this procedure.
The fire detect control unit monitors two fire detection loops and will signal a fire warning if both loops agree there is a fire. The indication goes to the fire handle, fuel control panel, and the MAUs. If the MAUs signal a fire and the fire handle and fuel control switch do not, you could have a fire. If the fire handle and fuel control switch signal a fire, you have a fire no matter what the MAU thinks.
What happens when the two loops don't agree? If one thinks fire and the other doesn't, you will not get a light in the fire handle or the fuel control switch. You will get a Fire Detection Loop Fault CAS message and must complete the Engine Fire Detection Loop Fault procedure.
For more about engine failures, take a look at G450 Engine Out Considerations.
Everything here is from the references shown below, with a few comments in an alternate color.
The CAS may or may not appear, along with any of the following:
Engine fire loop senses a fire. If the lights in the FIRE handle and FUEL CONTROL switch are illuminated, the fire warning is valid. If the lights are not illuminated, you may have a faulty fire detection loop.
[G-450 AFM, §4-10-10]
NOTE: The pilots shall agree on the correct (affected) FUEL CONTROL switch prior to placing it in the OFF position, as well as the correct FIRE HANDLE prior to actuating it.
NOTE: When it is necessary to discharge a fire bottle, ALWAYS use DISCH 1 first.
NOTE: Match FGC to operating engine: FGC 1 = Left Engine; FGC 2 = Right Engine.
NOTE: See Fuel Load Balancing, page EE-23.
NOTE: For approach and landing, operation of landing gear, flaps, spoilers, reverser, and brakes should be accomplished as separate events, not simultaneously.
NOTE: If electing to land at flaps 20°, fly the approach at VREF for flaps 20° +10 knots and select the GPWS GND/SPLR Flap ORIDE to ON. See AFM Section 5, Performance, for landing distance in the flaps 20° configuration.
NOTE: If strong winds are present, add to VREF ½ of the steady state wind plus the full gust to maximum additive of 20 knots.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
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