737 Fire protection / OVHT DET SWITCH
 

Hello, :)

I have some questions about the fire detection system installed in the 737 (NG but I assume it is probably the same for other variants).

In the FCOM, it is indicated (Chapter 8 - Section 20):
My questions are:
A - What do they mean exactly by a "loop failure" ?
B - Would such a failure be detected automatically during a flight without any pilot action (without using the TEST switch on the Overheat / Fire protection panel) ?
C - If the system is able to 1 - Detect a faulty loop and 2 - Deselect it by itself, why is there an OVHT DET switch to manually select loop A or B instead of the NORMAL position ?

From what I understand, the loops on the 737 are the type filled with gas. The gas expands with the heat, and the pressure triggers a switch that generates the signal. An "under pressure" switch is also fitted in case the loop is leaking, and thus loosing its detection capability. I assume (but maybe I am wrong), that the system continuously monitors the low pressure conditions in the loops. And I assume that this low pressure condition may be the "loop failure" they are referring to in the FCOM. Could someone with a good system knowledge / Access to the AMM confirm if that is the case ? Are other failures monitored by the system ?

Anyone able to help me with this one ?

https://cimg8.ibsrv.net/gimg/pprune....4205491ae.jpeg

My questions are:
A - What do they mean exactly by a "loop failure" ? = Loss of Gas pressure

B - Would such a failure be detected automatically during a flight without any pilot action (without using the TEST switch on the Overheat / Fire protection panel) ? = Yes, detected by the 'Engine and APU Fire Detection Module'

C - If the system is able to 1 - Detect a faulty loop and 2 - Deselect it by itself, why is there an OVHT DET switch to manually select loop A or B instead of the NORMAL position ? = I think its so you can test each individual loop from the flt deck rather than having to go down to the avionics bay prior to Engine start.

Thank you for your contribution ! :)

It makes perfect sense for answers A & B !

I've been thinking about question C. Maybe there is a bit more involved.
It is interesting to see that in the supplementary procedures, whenever one loop is failed, we have to select either loop A or loop B (whichever is operational) at the end of the procedure, and keep it this way.

So I came with the following explanation:
- If a low gas pressure is sensed in a loop, it is automatically detected and the loop is deselected without any pilot input (that would be the answers to questions A & B).
- If any other type of failure is present, it would be detected by the system test performed by the pilot before starting the engines. Such a failure would not be tested automatically, and that's why the pilot is asked to select either loop A or loop B following the identification of a failure.
Otherwise I don't see why the pilot would have to manually select loop A or loop B following the application of the supplementary procedure when one loop is known to be failed (it probably implies that the NORMAL mode of operation cannot monitor all the failures).

What do you think ? If someone has some useful info... :)

Because you could have both loops faulted. Under normal operations the fault light will only illuminate with both loops faulted on the same engine, If you have "A" loop fault on eng. 1 and "B" loop fault on eng. 2 it is not MELable. The way to find this out is to isolate the loops during test, hence the manually selected "A" "B" loop switch.

greg765,

In answer to your questions.....

A: Yes, exactly that. A loop failure could be any number of things e.g. loss/low gas pressure in loop/s, bad/dirty cannon plug connection etc. As a side note, the Wheel Well loop is salt filled, but that's for another day.

B: Yes. Because by design that's the system (and one loop out is covered by MEL). The less pilot distraction the better and when was the last time you flew and tested the fire system every 10mins or so in flight?

C: Simple, if one loop fails u/s (which you will find out on your next fire test and then use the fire detector module in the EE compartment to isolate the fault), lets say loop A fails, then per the MEL we will pull loop A C/B and collar it, then select loop B so the fire detector module is monitoring the serviceable loop (B) only.

Note: With the the Ovht/Det switch in Normal, both loops (A,B) on one engine must have a fault before the fault light comes on. Master caution light does not illuminate with a fault light.

Hope this helps
McHale.

Yes I agree, but my point was that if the system is able to detect a fault automatically, and to isolate automatically a failed loop, you don't need to manually select loop A or loop B because the system does it by itself anyway ! So the only explanation I can come with, is that some failures are not monitored by the system (and thus require a system test performed by the pilot), while some failures are monitored and detected automatically (I assume the loss of gas pressure is one of them).

It would be interesting to know which failures are monitored by the system, and which failures need a system test performed by the pilot. (Not that it is very important, but I like to understand the systems in depth even if it has no "operational" implications. )

Do you have any reference as to which of those failures actually need a system test to be performed by the pilot ? And which of them are monitored by the system without any pilot input ? Because the FCOM doesn't detail anything, they just talk about a "loop failure" but there are actually different kinds of failures !

That's indeed interesting that both types of loops are used on the 737. I used to work on an MD80 and I remember they used the resistor-type loops in the engines as well. It is interesting to see that on the 737 the engines use gas-type loops whereas they use a resistor-type loop in the wheel well. Do you know the reason why they use both ?

Hi greg765,

I think you're getting bogged down in failures. The only test a pilot/engineer has to do is the fire test. This tests the loops for failures WITHIN the loop as I mentioned before, low gas /short circuit etc. There is no individual test for each of these failures. The fire test is it, and it's testing the continuity of the loop. No continuity, for eg, low gas/short circuit, then test fails and fault is recorded in the Fire Detector Module. The Module is then interrogated by engineering and it will point them to the faulty loop for rectification/MEL.

You are correct in that there are different kinds of failures under the "loop failure" umbrella and it is up to the Fire Detector Module to sort it out. FCOM is just decluttering the information for you.

As for Boeings reasoning as to why different detectors were used between the engines and the wheel wells. I was told way way waaaay back and I am at a loss as to why. I should know as it's one of those useless bits of info that you normally keep and forget the important stuff. Will let you know if I can recall.

Rgds McHale.

Yes I totally agree, the only test we can do from the flight deck is the fire test, and it covers several things. :)
I was only trying to gain some more knowledge on the way the system functions as a whole (from an engineering and system design point of view, not from a pilot perspective). I used to work in engineering (not on fire systems though ^^) and I am just curious to know which failures can be detected by the system automatically (without any test performed by the pilot), and which failures actually need a test. Probably some "curiosity" from my former life ;) But with nothing else than the FCOM, there is not too much detailed information available.

Yes if you remember about the reason behind this technology difference between the engine and wheel well loops I would be interested ! Thanks for your help :)