fault light on the b737ng

[airborneforever]

Hi,
one more for you guys.
If the APU fault detection system is faulty, you get a apu det inop light along with a master caution and the system annunciator light. NNC says donot run the apu and to switch it off if it is running. if the same fault is there for dispatch, the mel says that the apu is not to used at any time on the ground or in the air.
now, my question is that if there is a fault in the engine overheat detection system, the fault light will come on, but without an accompanying master caution, why?
doesnt it seem a bit strange that the apu det inop gets a master caution but not the engine fault light?
and if the same fault exists on the ground for both loops on one engine, you cant be dispatched, the aircraft is simply not airworthy. so what do you when the fault light comes on again in the air and you have no way of finding out which loop has perished now?
the checklist at no point of time asks for a landing asap or ansa.
regards,
a

P.S. If i am the captain and i get the fault light in the air, i am putting down ANSA.
you have no way of knowing whether one or both loops have gone. and if both have gone, then its a pretty serious situation.
but then again, i am not a captain, so i'd rather ask all of you and learn. :E

[Checkboard]

... because shutting down an engine gives a MUCH bigger problem than shutting down an APU, which is only a minor inconvenience. :hmm: The chance of a fire occurring on the same flight in which a detector fails is so small, it is well within safe limits to continue to destination.

It would be so for the APU as well, however as the APU isn't necessary there is a tiny improvement in safety by shutting it down.

If you are my FO, you can watch the nearest suitable sail by as we continue to destination ... :ok:

[airborneforever]

alright, fair enough, we shall let the nearest suitable sail past.:}
but why isnt there a master caution for it?

[airborneforever]

hey can i get a few more points of view here?
thanks you guys.

[rudderrudderrat]

Hi airborneforever

Checkboard is correct. The APU is wired for "OR" logic. The Engine is wired for "AND" logic. You don't want to shut an engine down unnecessarily just because one Fire wire develops a fault. If one fire wire develops a fault (fault light initially) and then sometime later the second fire wire senses a fire - then you get the full fire warning.

The APU is wired for belt and braces so it can be left unattended whilst running.

[CHfour]

Airbornforever,
I'm converting to the NG from the classic so have both manuals and it does seem that neither A/C gives a master caution for an engine fire detector fault, even if both have failed. I agree with you that a MW would be helpful as, on a sunny day, you could easily miss the annunciator.

[Port Strobe]

but why isnt there a master caution for it?

Could it be because the engines are visible from the cabin whereas the apu is not? Just a punt.

[Kirks gusset]

Could it be that the APU has auto shutdown for certain conditions and certainly when in the air we don't want the APU cooking or overspeeding with no fault indications, this would be a hull loss for sure. The Engines are both visible, have two fire bottles available, have numerous other indications of malfunctions and seizures. The dual loop system will still operate with a fault indicated whereas the APU system will not. And of course, you can identify which loop is inop on the engines ,Just a thought for the melting pot.

[plain-plane]

Master Caution Lights
Two MASTER CAUTION lights illuminate when any caution occurs outside the normal field of vision of the flight crew. The lights remain illuminated as long as the caution condition exists, or until the crew resets the system.

FCOM 15.20.2
red by me...
this might have something to do with it

[Kirks gusset]

It also states:

A single fault in certain redundant systems, or some simple faults, do not
illuminate the MASTER CAUTION or system annunciator lights.

[Sagey]

Fault light isn't in view. All the QRH says is that one or both loops are faulty. Logical that it is designed so that people don't decide to shut down a fully functional engine, due to a detector fault. The probability of both failing must be extremely slim anyway.

Edited to add, it is normally in the Normal Position and therefore requires both loops to fail for the fault light to illuminate. You can check if one has failed by selecting A or B.

S

[rjay259]

The 'stab out of trim light' is in full view but still would like the master caution light to pop up and tell me its gone, especially in sunny conditions.

Does the APU not have only one fire det. loop anyway?? Therefore as you have no fire detection you would get the MC light.

[Sagey]

APU does only have one loop, and a fault produces the light to illuminate and Master Caution etc. This doesn't happen with the engine overheat/fire detection loops, whether one fails and you have selected the corresponding loop or whether both have failed and you are in Norm. Only indication of a failure is a FAULT light. As Checkboard has mentioned you are not going to shut down a functioning engine in flight due to no overheat/fire detectors on one engine, you can still monitor the engine parameters and you would suspect you would be told PDQ if flames or smoke was seen in an engine.


S

[Kirks gusset]

The fault light QRH:

ENGINE FIRE/OVERHEAT
DETECTOR FAULT
Condition: Engine fire and overheat detection is
inoperative.

The fire detection system in one or both engines is
inoperative.

As you say, on the ground, you can check system A or B and select the operative loop or Normal if they are both good.

The APU is a single loop an thus warrants the MC as a fire could go undetected.. V bad:\

[Sagey]

You also test both on first flight of day/first flight of that crew. If the fault light or the APU Det Fault light don't illuminate there is a problem with the system.

[ampclamp]

Any electrical cct / system is only as serviceable as the last time it was used or tested. Testing is c/o on FFOD & on preflts etc.So could be tested many times daily.Probably enough without continuous monitoring.

As stated a fault will not necessarily preclude the loop giving a fire warning.

Unless being used because of a DDG item the apu generally used on or close to the ground.
Been working 737's for many years with very few APU loop failures.I've not personally seen or been involved with a 737 apu fire.
Ideally everything would be full time monitored but it aint a perfect world and neither is full time monitoring.