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 an Engine Fire Loop Alert CAS message and must complete the Engine Fire Loop Alert procedure.
Everything here is from the references shown below, with a few comments in an alternate color.
Engine Fire, L-R
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.
[G450 AFM, § 4-10-20]
NOTE: When it is necessary to discharge by a fire bottle, ALWAYS use DISCH 1 first. This will discharge extinguishing agent from the RIGHT fire bottle, preserving the LEFT fire bottle for APU protection. Only the LEFT fire bottle can be used for APU protection.
NOTE: When the Main Batteries are selected OFF, the Emergency Power system will activate. If emergency lighting is not required for evacuation, select the Emergency Power Off switch to OFF before selecting the Main Batteries to OFF.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
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