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AF024

How to handle fire loop failures ?

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Hi everybody,

 

I just try the service based failures system (multiplier 10x), and I first experience the FWD CARGO FIRE LOOP B, that I service after landing the aircraft.

Then I had the AFT CARGO FIRE LOOP A, that I service after landing as well.

And now, I have the ENG 2 FIRE LOOP B FAULT.

 

Actually, I'd like to know if I'm doing things properly. Each time I have the CARGO FIRE messages, I arm the corresponding button on the overhead, then I press for 1 second the DISCH button just under (as indicated in the corresponding fire protection checklist. Then I had several failures that appears (Cargo Dump bottle squib, etc...). I service all these failures, then I refill the fire bottle, and I start a new flight.

 

Do I do thing the right way ?

 

And how do I have to handle the ENG 2 FIRE LOOP B FAULT failure ? What does these failures means ? I don't really understand what '"LOOP" means... (sorry I'm french).

 

Thanks in advance.


Alf DAGNON - Flight enthusiast

VA France - AF024

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The fire loops are the detectors that sense whether there is a fire.

 

Unless the fire bell is going and you have FIRE CARGO (etc) displaying in red, you are not actually on fire, so you do not need to discharge the extinguishers.

 

What it sounds like you have is a failure of the fire loops themselves. This means that, depending on the specifics of the failure, your aircraft's capability to detect a fire is now limited or completely gone.

 

This means you now have to make some decisions. Think of it like the battery going flat in your fire alarm at home: the chances of there actually being a fire are small, but if there does happen to be one you may not know about it, or not know about it until it is too late. So it's a weighing of risk: do you continue without fire detection? What other options do you have? Is there any redundancy? What are the chances of there being a fire between the detection failing and the end of the flight? But what are the consequences if there is? Is there a suitable airfield to divert to? Are you over water/remote terrain or about to be, have you just taken off or are you descending toward your destination? What does the QRH (or MEL before dispatch) say?

 

You're the Captain: what would you do? ;)


Simon Kelsey

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Thank a lot for these explanations. And what are the differences between LOOP A and LOOP B ?

And what does the Cargo Dump bottle squib failure means ?


Alf DAGNON - Flight enthusiast

VA France - AF024

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Thank a lot for these explanations. And what are the differences between LOOP A and LOOP B ?

And what does the Cargo Dump bottle squib failure means ?

There are two separate fire loops for redundancy - Loop A and Loop B - specifically to provide backup fire-sensing case one of the two loops fails in flight. If both loops fail the self-test on the ground, it would be a definite "no fly" item until the loops are repaired.

 

The squib is a small explosive cartridge located at the outlet of the pressurized fire extinguisher bottle. A fire extinguisher bottle contains the extinguishing agent (typically halon), pressurized to about 600 psi. There is a sealed metal disk inside the outlet port of the bottle which keeps the contents inside. When the fire extinguisher is activated, 28 volts are applied to the terminals on the squib, which contains an explosive primer similar to that found in a shotgun shell. When the squib explodes, it punches a hole in the sealing disk, which permits the pressurized halon gas rapidly flow out of the extinguisher very rapidly, through a large-diameter pipe, and into the engine cowl, APU enclosure or cargo compartment as the case may be.

 

The electrical continuity of the squibs are constantly monitored by the system. If the continuity on a given squib fails, it means that it likely would not fire when commanded - and would have to be replaced before the aircraft could fly.

 

Another cause of squib electrical continuity test failure would be if it has actually been fired - in which case the related fire extinguisher bottle would be empty. However if a squib is accidentally fired on the ground, usually anyone working in or around the aircraft will immediately know it, as it makes a very loud "bang" when it explodes!


Jim Barrett

Licensed Airframe & Powerplant Mechanic, Avionics, Electrical & Air Data Systems Specialist. Qualified on: Falcon 900, CRJ-200, Dornier 328-100, Hawker 850XP and 1000, Lear 35, 45, 55 and 60, Gulfstream IV and 550, Embraer 135, Beech Premiere and 400A, MD-80.

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Please use full name on posts in the PMDG forums, thank you.

 

Follow the non-normal checklist for cargo fire in the QRH.

 

A fire "loop" is the fire detection circuit, the loop refers to the circuit being a complete path and if the path is broken then it is a loop failure. This might be caused by detector failure, broken wire or loss of fire detection circuit voltage.

 

The server unavailable does not mean your post didn't get posted, it just means AVSIM is busy. AVSIM is always slow to post new topics, get used to it.


Dan Downs KCRP

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Thank you very much for these explanations Jim !  :good: It help me understand the logic of the fire protection.

 

I suppose in real condition, we must always self-test the fire system before any flight to ensure that all the sensors are OK.

 

In my case, I think I'll service the fire protection system, and then continue flying.


Alf DAGNON - Flight enthusiast

VA France - AF024

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You can always MEL all those items too ;) All those can be deferred for 10 days.


Patrick Houghton

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