Headset Noise

[dick747]

A significant hum has developed in my headsets.  I have tried different headsets and switched headsets between the left and right plugs but the hum persists.  It is not sensitive to engine rpm.  What it does follow is flap position.  Taking of with 15 degree flap settings there is no hum  When retracting to 0 the hum starts and gets louder when I select -6.  When going to landing flaps the same result, hum goes away when I get to 15.  When parked there is no hum at any flap setting.  Any thoughts?

[Anticept]

Make sure the movement of the mechanism is not rubbing against the wires.

 

Does it occur without the engine running or the alternator off?

[dick747]

Does not happen on the ground with the engine on or off.   In the air, engine rpm has no effect on the hum.   Will look to see if the flap mechanism is rubbing on the wires.

[Anticept]

So it only happens in flight? Pull the flap breaker after setting to -6. See if it still happens.

[dick747]

Does not appear to be any way to pull the flap circuit breaker in my CTLS.  It is a push button on the center console that can only be moved if the breaker trips.  I did check the wiring and found no areas that appear to be rubbing.  Also opened the breaker on the VHF radio and the intercom and hum continued.

[gbigs]

A significant hum has developed in my headsets.  I have tried different headsets and switched headsets between the left and right plugs but the hum persists.  It is not sensitive to engine rpm.  What it does follow is flap position.  Taking of with 15 degree flap settings there is no hum  When retracting to 0 the hum starts and gets louder when I select -6.  When going to landing flaps the same result, hum goes away when I get to 15.  When parked there is no hum at any flap setting.  Any thoughts?

 

Which CT do you have?  In the CTLSi there are two alternators.  When the second one comes up the hum starts.  Some have corrected that by doing the Sport upgrade on the 912i. 

[dick747]

Shut off the alternator today but still had the hum.

[gbigs]

Shut off the alternator today but still had the hum.

 

It could be your radio.  It's gotta be an electronic field interference, or the radio itself.

[Anticept]

So to be clear, this only happens in flight? Are you sure it's not some sort of vibration?

 

Plug a headset into the passenger side, then turn off the intercom. If you still hear the hum and you have a PM3000, then it's probably not electrical. Also, turn off your comm radios to see if that gets rid of it. Radios are super sensitive to power fluctuations and electronic emissions, and this would prove it's not vibration related.

 

As for the flaps breaker: Put in a fuse that you can pull temporarily (carry spares) in series. Or, short the breaker to ground with a 100 ohm resistor (watch the wattage rating, a little 1/4 watt resistor will burn you if you tried that) while it is at -6, then turn on the breaker in flight to see if the hum comes back.

[dick747]

Switched head set to the passenger side, no effect.  Turned off the com radio, no effect.  Turned off the intercom, no effect.  With the intercom and radio shut off I don't understand how the headset can be getting any input. Will tackle the temporary fuse next.  Might just be able to reach into the console in flight and disconnect a lead to the existing breaker.

[Roger Lee]

I would check the back of the radio connections were it plugs into its bracket. All it would take is a loose wire and or bad connection and I have seen this at least a dozen times.

[Tom Baker]

While it is making noise I would turn each breaker off one at a time to see if the problem goes away. I would start with the GPS, and end with the intercom.

[Anticept]

Wait, I thought I posted this earlier!

 

Reprogram the flap board. It might be twitching out thinking it can't reach the flap position. As a sort of test, have someone apply a little bit of pressure on the flaps to resist the motor movement (not a lot!!!), and then try again assisting the motor. See if you can replicate the hum on the ground.

 

Anyways, it's the fact you can't replicate this on the ground that is making this such an extremely weird issue.

[Doug G.]

What kind of headset? ANR by chance? Could the headset be the problem?

[dick747]

I have a Lightspeed ANR and a conventional headset and both pick up the hum.

[Doug G.]

OK, I have used Lightspeed Sierra and now PFX. I have noticed a hum when the batteries were getting low (particularly on the Sierras). You have ruled that out.

[Anticept]

I have a Lightspeed ANR and a conventional headset and both pick up the hum.

 

Can you record the hum? Hearing it would help a lot.

[Doug G.]

I would be looking at those things that have their own power supply. Dynon, GPS, ELT, and (less likely) I think someone said the ELT switch has its own battery. In order to have noise there needs to be some sort of power. (No one is running a pump or generator nearby? ...grasping at straws)

[chanik]

You have a bad regulator in one of the IF stages in the radio itself. Is it some common King KX 165 or whatever that you could get your hands on another unit and simply swap in its tray?

[Anticept]

You have a bad regulator in one of the IF stages in the radio itself. Is it some common King KX 165 or whatever that you could get your hands on another unit and simply swap in its tray?

 

He said it's still present even with the radio off.

 

This is extremely weird because he said it's not reproducible on the ground, only in flight, which is why I went to the possibility of something wrong with the controller generating a lot of noise somehow at 0 and -6, which is not being caused by the alternator as he switched that off too.

[Jim Meade]

I'm waiting for his answer to post # 13.

[Tom Baker]

One other thing could be a split in the wing root tape if you have tape installed. This has been known to cause a hum. It will increase in intensity as speed increases, like when going from 15° flaps to 0 or -6.

[dick747]

Will check the root tape, cycle all of the avionics and run the flap test applying a little pressure on the ground to see if that causes the hum.  Am leaving for a week so won't be able to run theses checks until early next year.  Thanks for all of the suggestions.

[Anticept]

Got off the phone with dick747. He just did a flight with a friend, and when they unplugged the headsets to switch the ports, they still heard the noise. It's a whistle in the hull, and will take it to Roger to try and find the source.

[gbigs]

Got off the phone with dick747. He just did a flight with a friend, and when they unplugged the headsets to switch the ports, they still heard the noise. It's a whistle in the hull, and will take it to Roger to try and find the source.

 

Big difference between a hum in the headset and the hull noises.  Of course its going to be in the wing root, the window vent, thru the firewall or some other goofy place.  these planes are full of nooks where wind noise is always present....wow, how can someone make that kind of mistake?