1900 systems question

[chperplt]

This is for all the brilliant 1900 drivers out there...

During startup, within 2 or 3 seconds after engaging the starter for #1, the T-handle lights up. After troubleshooting a little while later, the fire loop tests ok, but the amp test fails. Maintenance determines that the fire loop is bad. Since the loop test passed, the problem wasn't the integrity of the loop.

My question is.. Other than too much heat, what could make the loop (amp sensing too much heat) fail that soon after engaging the starter? This was the 3rd start of the day on that engine.

Maintenance couldn't answer the question and my curious mind wants to know..

Thanks

 

[flint4xx]

It's been awhile since 1900 school, but it sounds like the amp has an electrical glitch that is caused by the electrical draw from the starter. Does the light go back out as the power drain is reduced?

There are a bunch of circuit boards under the cockpit floor that control that stuff. Spill any cokes lately?

 

[chperplt]

Sorry.. I should have added more..

There are 3 circuit boards that control that system. All 3 boards were changed with no luck. The only way they were able to get the T-handle off was to disconnect the entire loop from the system. They fixed the problem by changing the entire fire loop.

 

[HvyjetFO]

hope this helps

After troubleshooting a little while later, the fire loop tests ok, but the amp test fails.

The control amplifier does several things. It discrimiates between real fires and a false indication caused from a short circuit in the control circuitry. It also has a time delay in it that prevents false warnings due to short "bursts" of resistance signals or from short circuits. When the amplifier fails, false fire warnings could happen because the protection described earlier has been lost.

I know of many false fire indications that occured due to short circuits. Sounds like this is what happened to you. While the 1900 is a great airplane, it does have its systems shortcomings.
Best of luck!

 

[chperplt]

Would a short circuit destroy the fire loop?

Thanks for the info

 

[HvyjetFO]

I'm not sure if a short circuit could destroy a fire loop. From the problems I've seen with short circuits, I have never seen a loop be replaced. Sorry I can't be of more help.

 

[AK737FO]

Refresher course...

It has been a long time since I thought about 1900 systems. I would look for a small puddle of electrons. Find them and you will find your problem.
The BE-99 and BE-1900 seem to be prone to fire loop problems and false alarms. I recall a few dark and stormy nights, flying in heavy rain, with the T-handles looking like a Christmas tree. Doesn't give you the warm fuzzys... I also remember a false alarm that the mechanics think was caused by sunlight? A ray of sunlight getting through the cowl and tripping it somehow.
Overall, it just seems to be a picky system.
Enjoy the Big Beech!

 

[Marko Ramius]

Was it a C-model? I never flew the C, but I was often told by our senior Captains and management types that the T-handles would often light up falsely on the C particularly if it was humid or there was some moisture in there. I think HeavyJetFO was right that it probably started with the control amplifier. The thing I always found funny about the 1900 was that for an aircraft that had a relatively simple electrical system, it sure could develop a lot of weird electrical glitches and they often weren't wired the way the "book" said. Ever turn the battery on, followed by the AC BUS switches out of curiousity? I've found some times that the inverters weren't even wired to the L/R center busses for power backup like they should be!

 

[Birddog]

The only thing that comes to mind is that the cable might have been damaged in some way, perhaps during mx. Perhaps the cable was kinked and the insulating layer was damaged. If you find out what caused it, please let us know.

 

[Timebuilder]

As a lifelong wrenchhead and electronics tech, I can tell you that a great many problem gremlins are caused by ground faults, causing unexpected differences in potential. A loose ground can cause electrolysis (greenish crystalline goo) and a host of maddening symptoms.

The fire loop on the PT-6, from what I have seen, is a very simple part. I'd expect to find the problem in the terminal connections of the loop (possible high/varying resistance), the cable (as mentioned above) or the board connectors.

Sometimes swapping out boards can be an exercise in futility, as merely disturbing the board can clear a problem. Then, when the old board is bench tested, no problem is found.

You coud start with wiggle tests, thermal stressing with hair dryers and aerosol cans, and ground tests.

I'd like to hear about the fix, when it happens.