A loose wire

AuntPeggy

AuntPeggy

Final Approach
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As you may remember, my plane has been experiencing annoying problems since this spring.

A warble that we were hearing when we transmitted eventually became audible to tower and finally comm 1 became so unreliable, that we made it a habit to dial in tower in comm 2 whenever we used comm 1 to talk.

ATC would occasionally complain about our transponder and we got into the habit of recycling it from time to time.

Then there was the power dive our autopilot put us into a couple of times which we couldn't cancel with the disconnect button. Only turning off the autopilot master would disconnect.

Finally a mechanic has found a loose connection to the avionics master that is scorched from arcing. We hope it is the source of all the problems. We had 3 hours of flight on Friday and Saturday, and none of the symptoms came back. Keeping fingers crossed.
 
We're heading to Tulsa again on Tuesday or Wednesday, then back to New York on Saturday. Here's hoping all the gremlins stay in Peoria.
 
AuntPeggy said:
Finally a mechanic has found a loose connection to the avionics master that is scorched from arcing. We hope it is the source of all the problems. We had 3 hours of flight on Friday and Saturday, and none of the symptoms came back. Keeping fingers crossed.
Click to expand...


This is one of the big downsides to a single avionics master, it can take out all your radios at once. Personally I like having two and the second one doesn't cost very much.
 
lancefisher said:
This is one of the big downsides to a single avionics master, it can take out all your radios at once. Personally I like having two and the second one doesn't cost very much.
Click to expand...
Doesn't that simply double the liklihood of a screw backing out and a connection coming loose?
 
AuntPeggy said:
Doesn't that simply double the liklihood of a screw backing out and a connection coming loose?
Click to expand...

Nope, not if it's wired correctly. Besides, screws don't come loose if properly tightened, it's the moving parts of the switches that die, often because they aren't even rated to carry the load of all the avionics turning on at once.
 
lancefisher said:
This is one of the big downsides to a single avionics master, it can take out all your radios at once. Personally I like having two and the second one doesn't cost very much.
Click to expand...

How's the setup work Lance? Are both switches wired to all busses or do you split things up? I've always worried some about that single failure point.
 
Now what?

Well, none of the symptoms returned on our trip to/from Tulsa/NY. But, a new one cropped up. Message on the GPS screen popped up giving a suggested heading and

"Com not responding."

We looked at one another and wondered, "What?" After looking around, we discovered that the selected frequency on Comm 1 had changed by itself to 121.5, which might have explained why ATC had become so quiet. We were on an IFR flight plan, in the soup. Switching back to Center's frequency on Comm 1, the push-to-talk had no effect. After a little frantic switching around, we put Center's frequency into Comm 2 and continued as though nothing had happened. We tried Comm 1 several times and it would neither transmit nor receive.

After stopping for fuel, Comm 1 was working just fine again. Now we always dial the current ATC frequency into both comms.
 
I'd be willing to bet that had you turned Comm1 off and back on, it would have started working right away.

-lance

AuntPeggy said:
Now what?

Well, none of the symptoms returned on our trip to/from Tulsa/NY. But, a new one cropped up. Message on the GPS screen popped up giving a suggested heading and

"Com not responding."

We looked at one another and wondered, "What?" After looking around, we discovered that the selected frequency on Comm 1 had changed by itself to 121.5, which might have explained why ATC had become so quiet. We were on an IFR flight plan, in the soup. Switching back to Center's frequency on Comm 1, the push-to-talk had no effect. After a little frantic switching around, we put Center's frequency into Comm 2 and continued as though nothing had happened. We tried Comm 1 several times and it would neither transmit nor receive.

After stopping for fuel, Comm 1 was working just fine again. Now we always dial the current ATC frequency into both comms.
Click to expand...
 
I actually have two backups. First there are two avionics master switches (actually toggle breakers) wired in parallel so if either switch fails I can use the other. Second I have another switch that when on connects the GPS/NAV/COM, MFD, and XM wx receiver directly to the battery bypassing the master contactor and avionics master switch(es). When this switch is off, those radios are powered from the avionics bus. So even if the airplane master must be turned off or fails, I can communicate and navigate. I can also enter a flight plan, check weather, and call clearance on the ground without turning anything else on.

-lance

ejensen said:
How's the setup work Lance? Are both switches wired to all busses or do you split things up? I've always worried some about that single failure point.
Click to expand...
 
Lance what if the failure mode of the av master is not as expected ie a break in the circuit but instead a short to ground? Do the backups add fire risk? (unprotected hot wires off battery)
 
Let'sgoflying! said:
Lance what if the failure mode of the av master is not as expected ie a break in the circuit but instead a short to ground? Do the backups add fire risk? (unprotected hot wires off battery)
Click to expand...

I don't see how they would make it any worse. If there are no inline fuses or breakers between the av master and the master bus the next weakest link is going to load up until it fails with or without his added switches.
 
Let'sgoflying! said:
Lance what if the failure mode of the av master is not as expected ie a break in the circuit but instead a short to ground? Do the backups add fire risk? (unprotected hot wires off battery)
Click to expand...

The wire from the battery feeding the "ground power" switch is protected by it's own CB next to the battery compartment. There's no additional fire risk from the two av. masters either since each one has an appropriately sized wire from the CB to the avionics bus. And a short to ground on the main bus is one of the scenarios where the direct battery feed would be useful. And FWIW the ground power switch is a 3PDT. One pole turns on an indicator light, one powers the two breakers for the GPS/NAV/COM (it has separate feeds for nav and com), and the third handles feeds the CBs for the MFD and XM receiver.
 
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lancefisher said:
I actually have two backups. First there are two avionics master switches (actually toggle breakers) wired in parallel so if either switch fails I can use the other. Second I have another switch that when on connects the GPS/NAV/COM, MFD, and XM wx receiver directly to the battery bypassing the master contactor and avionics master switch(es). When this switch is off, those radios are powered from the avionics bus. So even if the airplane master must be turned off or fails, I can communicate and navigate. I can also enter a flight plan, check weather, and call clearance on the ground without turning anything else on.

-lance
Click to expand...

Thanks, I'm going to talk to my A&P about adding some redundancy.
 
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