Radio/Intercom Squeal

[Red Leader]

Since I have had my 231, I have suffered through a loud radio squeal, both when starting the engine when (after starting) idling with a decent electrical load. After a good flight, it doesn't do it at idle. It only does it after starting when it flashes "Volts High/Low". Adding a little RPM's to the idle usually clears it up immediately, and makes the warning go away. It hasn't been too annoying but I cannot wear my headset during starting and I feel it takes away from the otherwise excellent bird I fly. Ideas?

[Andy95W]

I have always advised against wearing a headset while starting, and when I was actively instructing, would say the same to my students.  When the engine starts at idle, it really isn’t making enough noise to damage your hearing and this is your opportunity to listen for anomalies- which isn’t possible if you’re wearing a headset.

Also, it keeps you from blowing out your microphone when you yell “Clear prop!” and you don’t have to hear the loud squealing in your headset when you start the engine (which is pretty common, BTW).

[A64Pilot]

I don’t turn the Avionics on until well after starting, my theory is the voltage sags and spikes that occur at that time is bad for them, so for all I know what you have may be normal.

I also shut down avionics prior to turning off the engine too

[Fly Boomer]

3 minutes ago, A64Pilot said:

I don’t turn the Avionics on until well after starting, my theory is the voltage sags and spikes that occur at that time is bad for them, so for all I know what you have may be normal.

Me too.  Everyone says it makes no difference with newer radios, but that's the way I was taught, and old habits die hard.  I even do it in my car.

[A64Pilot]

14 minutes ago, Fly Boomer said:

Me too.  Everyone says it makes no difference with newer radios, but that's the way I was taught, and old habits die hard.  I even do it in my car.

Same, I think over the years damage may be cumulative, it just seems logical to not operate them until voltage is stable. 

I cringe every time the power flickers at the house, wondering if I get to buy something.

Many or maybe even most newer cars alternators are controlled by the engine computer and there is a delay before bringing them on line and they “soft start” in that they are brought up to voltage slowly.

All turbines I’ve flown you don’t bring the generators on line until after engine / APU start, then you start bringing avionics on.

[PT20J]

46 minutes ago, Fly Boomer said:

Me too.  Everyone says it makes no difference with newer radios, but that's the way I was taught, and old habits die hard.  I even do it in my car.

I looked into eliminating the avionics master switch when upgrading the panel. According to my Garmin dealer, most Garmin equipment is supposed to be on an avionics master. Some units that display engine instruments are designed to be on the main bus. The difference is brown out protection in the power supplies. Some of them don't like the voltage sag when the starter is running. 

[M20F]

Start it and pull the electronics breakers one at a time and see what happens.  My stormscope makes 123.0 sound terrible for whatever reason.  This was figured out through a complicated dance of pulling and inserting breakers. 

[Red Leader]

More details:

This is my fourth airplane and none of the others did this. One item mentioned previously is that I should wait to turn on my avionics until after startup - they are not on. It must be that the audio panel is wired to be on when the master switch is on. I always turn my avionics on last and off first at shutdown. If it is just because the audio panel is wired to the master and not the avionics master, I can get that changed at the next annual (next month). Is there any reason that I should keep it this way? Any drawbacks to wiring the audio panel to the avionics master?

[Pinecone]

The only minor reason I can see right now is, if you have other warnings routed through the audio panel.  I have my Landing Height System wired through the audio panel.  So, if I had to turn off the Avionics Master (not sure why, since each thing has a CB), I would lose the audio from it.

[PT20J]

What audio panel do you have? Do you have a separate intercom? Do you know whether the noise comes from the intercom or the audio panel?

I would do some trouble shooting to better understand the issue.

1. Pull the audio panel circuit breaker(s) and see if it still does it. This will tell you if the audio panel is wired to the main bus rather than the avionics bus. Do the same with the intercom if you have a separate intercom.

2. Since it only does this when starting and when the alternator is below cut-in rpm and doesn't do it after a flight, it seems voltage related. The battery voltage will be lower during and immediately after a start. After a flight, the battery will have been charging and voltage should be higher. This effect will be exacerbated if the battery is near the end of life, but it still shouldn't cause the avionics to make noise. Try starting with the alternator off (ALT switch off if it is separate from the master switch or pull the Field breaker). t. If that eliminates it, then I would investigate the alternator.

[Red Leader]

My audio panel is original and I have a separate intercom. That alternator idea is a good one - I will try pulling the alt CB at my next start.

[Red Leader]

Okay, I have taken a few flights and can confirm the radio squeals when the alternator breaker is out at startup. I will confer with my A&P at my annual about whether or not to move the audio panel power off the main buss and place it on the avionics switch (which I think is where it was supposed to be). It still doesn't answer the issue with the radio squealing at low RPM after startup when the battery is not yet charging.

[PT20J]

I'm betting that there is something wrong with the power supply in the audio panel that causes the squeal at low voltage. A radio shop should be able to bench test it with a variable power supply. Also, how old is the battery? I would have it capacity tested if it's more than a couple of years old.

[Red Leader]

I probably should've said the squeal is actually coming from the intercom and not the radio (my bad, the radio actually is on the avionics master) and will inquire with my A&P in a couple of weeks about the issue. My battery is about three years old and appears to be working quite well. I believe the power for the intercom should be coming from the avionics master and not from the master switch. Since it is not, should I have it moved?

[PT20J]

Most avionics should be on the avionics bus so that they are not powered during starting. The voltage drop during start can put a strain on power supplies.

I would have the intercom checked to find out why it is squealing. 

A battery will appear to be good until it gets so worn out that it won't run the starter. But, the battery loses capacity long before it gets to that point. If you have older avionics, they will draw quite a bit of power and that means that if the charging system fails you might only have a few minutes of battery power available with a diminished battery. This is something to seriously consider if you fly IFR. The only way to know the condition of the battery is to have a capacity test done. 

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