Ok, I have a radio issue that has been plaguing me for a few weeks, I have taken every step possible to try and resolve it and nothing has worked. At this point I have asked every LAME and Radio/Avionics outfit at Jandakot for advice, or they have had a go at locating or resolving the issue.
I am out of ideas and so I am looking for suggestions.



Firstly here is the issue.

There is an intermittent white noise that only appears on VHF (ADF is clean) that reduces a received radio transmission to 2's or 3's, however it is extremely directional and only happens if the station making the transmission is on the nose of the aircraft, if whilst receiving the poor signal I request a radio check they get me 5's If I swing the nose of the aircraft 20 degrees either side of the station the signal becomes perfect again. This happens whether the transmitting station is 500 meters or 10 miles away. It only happens when the engine RPMs are above 2200. There is no noise at 2150rpm and then at 2200rpm it is there in full intensity, it does not increase in any way (pitch/strength) as RPMs increase.

The aircraft has 2 separate radio's connected to 2 separate antennas, they both behave the same way.

Sometimes the noise is there and then it is not. It can be gone for a whole day and then return for the first flight of the next day. I can not correlate it with temperature or humidity or rain or time of day or anything, I have tried! The aircraft is working every day for multiple short flights and the problem comes and goes with annoying regularity, you just never know when it will re-appear. It started about 1 month ago, prior to that is was not present. It did not coincide with any maintenance or event.

The Aircraft is a Aero Subaru (Fuji FA-200) Lycoming IO-360B1B with a McCauley B2D34C53 constant speed 2 bladed prop, this is very similar to the powerplant found in a Piper Arrow. The Fuji is a 4 Place all Metal (Aluminium) low wing aircraft with fixed undercarriage.


Here is what has been tried so far:

Idea 1, the radio antenna is on the rear fuselage, perhaps the front of the aircraft is blocking the signal. Even though the SWRs checked out as perfect we replaced the antenna with a new higher gain one, it made the problem worse! Com 2 with the old poorly positioned antenna now gets a weaker white noise interference than com 1 meaning it is 3's when comm1 is 2's

Idea 2, It is poor earthing of the engine or mags. All the aircraft earthing points cleaned and shielding repaired everywhere. No Change

Idea 3, it is bad plugs. All plugs replaced with new plugs. No Change

Idea 4, the alternator. Engine run with alternator field disconnected (so not energised) and master bus disconnected. No Change.

Idea 5, Static build up, all static bonding points and wicks cleaned and tested. All OK. No change.

Idea 6, Noise filters on the alternator and master bus. Both filters replaced with new units. No Change.

Idea 7, A Loose connection. Every connection under the dash tested and tightened. No Change.

Idea 8, Capacitor in Mags breaking down. Mags overhauled accumulator/capacitors replaced. No Change.

Idea 9, Turn and Bank Coordinator causing noise. Fuse pulled whilst engine running. No Change.

Idea 10, Another Nav Aid or System. Everything except Com1 de-energised (including Alternator) so running off battery. No Change.

Idea 11, Prop making static charge, prop cleaned with solvent and re-painted. No Change.

Idea 12, Radio faulty, radio swapped for new unit, No Change.

Idea 13, Magnetised Crankshaft, but this could not come and go, so discounted.



I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio. The noise does not change with pitch, it is not a whine, it does not get stronger or change frequency. It is just white noise.


If anyone has any ideas, please let me know. No-one seems to have any idea what could be causing this.

I have no knowledge of aircraft engine ancillaries. Cars, motorcycles and boats however...

One thought. If it has a centrifugal ignition advance type distributor, is there any chance that 2,200rpm is the speed at which this one is activating at? And if so, is there some odd metal to metal contact creating noise? 2,200 is way too fast for a normal one.

?
 

Take the radio out of the A/C does it still make the noise?

Remove the antenna connection - does it still make the noise ?

Same with headphones ?

Do ground stations hear the noise or just inside the plane ?

I'm sure you have tried a lot of things that you didn't write up in your summary so apologies in advance if I'm suggesting something you have covered here.

It's kind of implied that the radio is still squelching correctly, and that you only hear this noise when a station is actively being received and triggering the squelch? If you disable squelch on the radio is the (offending) noise of a constant amplitude regardless of whether a strong signal is being received? I find the directionality you noted very hard to explain.

The change of antenna to a high gain on COM1 causing the noise to become louder indicates strongly that it's RFI coupled via the antenna rather than via power cabling/chassis etc. Does selction of different radio frequency affect the noise amplitude in any way? Noise generated from an engine source is likely to be relatively low frequency and coupling into the last stages of the radio electronics so it's likely unaffected by tuning but affected by audio controls such as volume. Does no antenna at all cause the noise not to be heard?

The fact that the frequency of the noise is completely unaffected by RPM really makes it not sound like classic alternator/magneto noise sources. I know you described the noise as "white noise", but is it really a pure random sounding hiss of wide audio frequency range or does it have some type of structure? What could possibly form a simple "off/on" switch for RFI based on RPM? Position of the governor?...that's a completely mechanical solution surely. And if the noise is frequency independent of the engine RPM then what is the frequency source for the noise?

Can you hear it on a handheld radio? If so you might have some luck sniffing for a source, but that's logistically formidable beyond sweeping the in cabin avionics.

Sorry, scratching around, just trying to give you new ideas.

I'll reply to you all at once to save some time!

The prop is Aluminium a McCauley B2D34C53

The electric fuel pump can be turned on and off with no change, could the engine driven mechanical pump create RF?

The Mags were removed, disassembled and overhauled, they check out fine.

The noise in only on recieve, only VHF and only the aircraft hears it.

Having the squelch off makes no difference. 2 Radio's fitted to the aircraft do the same thing. One radio was totally replaced with a new unit that did exactly the same thing.

I have tried taking a hand held on a flight and that never picks up the noise, so when pointing at a station the built in radio will get 2's and the hand held will get 5's I can have the antenna of the handheld quite close to the aircraft antenna and with the same line of sight. This seems to suggest it is something that is leaking into the aircraft fuselage from somewhere.

I have not tried no antenna yet, I will try today.

Thanks for the suggestions.

Do you have access to a TDR? (Time domain reflectometer).

It's used to scope coax cables, and check the continuity of the signal wire, as well as the dielectric. If there's any breakdown it will show up.

Does the radio pick it up if you disconnect the aerial input to the T/R?

How about if you disconnect from the aerial, but leave the T/R connected?

Maybe there's something switching on at 2200 on that engine type - a governer perhaps? What's max RPM on it?

Be interesting to get a spectrum analyser on the output of the T/R whilst it's doing this, and see what the frequency band was...

OK, your mention of the fuel pump got me thinking...low wing right? So there's going to be an auxillary electrical fuel pump also...that's going to always run at a constant speed regardless of RPM...question is when is it on? I'd have thought it would be a manual switch only but I'm at the edge of my knowledge base here.

The fact the handheld doesn't hear the noise eliminates something, the noise is not "inband" in the VHF range. Such a typical noise source would be modern and microelectronic/digital in nature..anyhow that possibility is eliminated in my mind now. The handheld most likely doesn't hear the noise source because the internal design of the radio is completely different and so it's potentially sensitive to different frequencies of noise.
(Typical radio designs transform the signal several times to different
frequency ranges before it becomes an audio signal, and each stage generally has a filter that only passes on a certain frequency range to the next radio stage. These frequencies are typically referred to as IF - intermediate frequencies and are generally much lower than the RF signal you are listening to)
I take it the replacement radio's you tried were the exact same design/model?
I am still bothered by the directionality you report....the direction of the noise source relative to the radio/antenna position should not be changing regardless of the position of the station you are receiving.....so why do you only hear the noise when the station is in a certain orientation?...there is only a single antenna per radio, right?

I think you must have a bonding issue combined with some antenna shadowing.

Worse when the engine/prop cowls are between the transmitting station and your antennas.
When at right angles to the other station the antenna is probably not being shadowed as much by the fuselage, gear, prop etc allowing a bit more of the other stations audio through.Remember most bonding problems produce fairly weak signals but create localised havoc.
I have had several very tricky bonding problems particularly on VHF comms systems where HF ADF are unaffected.

If it only happens with engine revs it must have something to do with the engine/prop (or relative airspeed with revs at those levels).I also include what engine vibe may be doing to airframe bonding points.

I note no maint action was done to cause it and it is intermitant.This leads me further to the bonding conclusion.Maybe caused by eng vibe that only occurs above 2200.Just enough to break done a poor bonding wire or point.
Silly question, have you ground run the engine in the dark over 2200 to see if you have any flashover anywhere? Hard I know.
Does it only happen at altitude, reproducable on the ground, ever?

Please keep us informed.

Ion Berkley. Ion is right but I think the only way you’ll pin down the source of the problem is to use a radio as a mobile detector. If you can get hold of a connector to the back of one of your radios then wire up only the power lines, the phone output (terminated in a phone socket) and the antenna with six feet of good quality 50 Ohm coax. Plug in a decent antenna to the coax. Tie the antenna onto a length of broomstick.

Power the radio with a battery and take it aboard. In flight, have a passenger listen in to the headset while moving the antenna around the cabin, windows, and into the foot-wells.

If this doesn’t exactly pin anything down it may give you an area for the source of interference.

The flashover I refer to may occur in hi tension ccts and can be visible in very low light. Having looked under a few hoods / bonnets sometimes you can see where the electricity is leaking out.Picked this up from old WWII heavy piston guys years ago.
I dont think this is the problem and this kind of testing is not routine.

But I do think it maybe a combo of bonding and perhaps some shadowing.
I'm not betting my gonads on it but am very interested in the fix to further my knowledge.Could dig me out of the poo sometime!
I am avio and have reasonably wide experience also a ham operator.

Steady on guys!!

While the attempts to resolve the problem are admirable you can't just start drilling holes or installing unapproved equipment in an a/c.

In my opinion it's almost certainly the co-ax. VSWR can still show within limits for a degraded co-axial cable. Try slaving in a nice freshly made and checked cable and do the checks again. while you're at it check the bonding of the racks and the antenna too.

Mono, I am all for stripping it back to basics with a frustrating problem like this and it's definately worth conclusively eliminating the coax...but bear in mind that he reported an identical problem for both COM1 and COM2...both coax's bad the same way?

Someone also suggested headset...hmm, it's common audio for both radio's...got to be worth eliminating the possibility that it's a fault related to the audio panel too ...though again I think the effect that changing the COM1 antenna had says that the radiator is getting in via the antenna.

Not the headset....
 

Re-reading the original post, the problem occurs when:

- Engine RPM is 2200RPM or greater
- Tuned station is directly off the nose
- Is a receive only effect
- Noise does not change in pitch or volume (i.e. it is on or off)
- Noise sometimes is not present, other times it is
- Only on VHF

So, the headset has no idea about any of these things, so let's eliminate that.

So what's possible?

1) Above 2200RPM the received signal strength is significantly reduced?
2) Above 2200RPM something is generating a wideband VHF whitenoise signal? But the problem with this as a theory is what does that have to do with the direction of the source (wanted station) - nothing, so this does not make sense.

If the coax was bad (for both radios!) then how does the orientation of the aircraft w.r.t. to the source station matter? Nope don't think that is it.

What does the hand-held tell us? The fact the hand-held was OK, makes me think this is not generated RF (per [2] above), but to me reinforces the idea that the problem is reduced signal strength when the station is directly off the nose, and related to the aircraft antenna installation w.r.t, the engine/prop orientation.

Unfortunately I really can't come up with a believable theory for this right now. :hmm:

- GY

GarageYears, I agree with your reasoning. I brought up the headset/intercom because it's not entirely clear to me here if we have a problem with an unusual radiator (RFI source) or a shielding problem (RFI sink). I do like your received signal strength thoughts.

What happens if we rewrite your 2) to be: Above 2200RPM something is generating an RF whitenoise signal? (intermittently)

Then we also introduce your concept of an antenna null towards the front of the aircraft (The OP does refer to the antenna installation location as "poorly positioned")

I wish we knew if its the the noise amplitude or the station amplitude that varies intermittently.

GarageYears.

That's why I proposed the distributor theory. Advance happens at a fixed speed. It's in the nose.

And I was wrong about the speeds. 2,200 is right in the band for centrifugal advance changes.

Currently I only have diesel engined cars here, so I can't take a gander, but my memory tells me that the stops for the weights are generally hammered pimples on the backplate.

garage years , boguing
 

Not sure if you have read my previous posts but you seem to agree it could be a shadowing of the antennas causing reduced received signal strength.But....
There is the issue that it has not always been a problem and apparently not related to recent maintenance , so in addition, I think there maybe a bonding problem causing the white noise that is only a problem when the received signal is low.
I have had an aircraft within 800 metres of the tower, line of sight and no signal. Rotated the aircraft a few degrees and I'd get 5s. The antenna was shadowed almost perfectly by the fuselage and other antennas.We could just hear the tower but they could not get squeak out of us.The aircraft was pointing directly at the tower antennasTrue story.

But given that this issue just cropped up gives reason to believe we have 2 conditions causing the defect.
If the handheld is not getting anything it maybe because it is not grounded to the fuselage which forms part of the bonding circuit.
If parts of the fuselage/ engine / cowls are not bonded there maybe circulating currents / potential differences causing the white noise.

Boguing if what you say is right the dizzy maybe a problem

Sorry to say that the ignition timing is fixed on almost all piston-engined aircraft. The chain is: two gear-driven magnetos timed with each other (typically 20 or 25 degrees btdc on a Lycoming) with points & condensor in each, a rotor/distributor cap assembly, HT leads, then spark plugs. Some magnetos have retard points for easy starting but there's no facility for automatically advancing the ignition with rpm increase.
Rather Neolithic in comparison to automotive engines but simplicity is the key, even to the slight detriment of efficiency.

Oh booger.

Here was I congratulating myself on being an aviation Holmes.

All I can say is 'how quaint'! Fred drift, I know, but do Fadec types embrace the last century?

I've read this thread several times and the problem is very puzzling. There are no obvious answers - only a large number of improbable single events or combinations which might be the cause. Component substitution - where possible - is one way of isolating the issue, but you've already gone some distance down this track.

On balance it seems highly unlikely that the noise should be coming in through the RF side of the system - i.e. antenna / coax combination. A check on the VSWR for each installation would be worth trying. It's accepted that this checks primarily the transmitting side of any given radio installation, but such a check, if satisfactory, should further eliminate the chance of the problem noise coming in through the antenna. This seems especially unlikely anyway given the expected signal strengths in the regime under test. These should be very strong signals, so much so that their being overloaded by an interference source seems highly improbable. Unless of course it's a large arcing issue somewhere which is more likely to produce a cracking sound rather than a (squelch open) white noise.

It could be useful to connect the hand held antenna input to one of the aircraft antennas if that hasn't been tried (and can be). If the noise is still there and on the HH, then the antenna installation is suspect in some way or the noise is coming in through the antenna system.

If this were to be eliminated, it leaves some other problem common to both radios. Is some sort of critical system voltage level (upper limit) being reached at the 2200 RPM level? Is there any other system indicator which betrays something special about that RPM level.

If it were possible, (and safe naturally) I would like to try the radio/ intercom on a DC supply system independent of the A/C DC rail. Come to think of it, is the main A/C DC system nominal 12 or 24 volt? I'm presuming that the radios and com system are on 12 volt. Can other significant electrical loads like lights etc be switched on (or off) when the noise is present? Does this change things - even in minute levels?

Clues may lie in a critical appraisal of the noise itself. Is it exactly the same intensity / pitch / character as what is heard when the squelch is opened manually?

I'm clutching at straws like everyone else. But these are things which I would follow up if the issue were mine. I hasten to add that I'm not in the aircraft maintenance area. My background in this area comes from being an amateur radio operator with a long experience in mobile as well as other fixed installations. I've occasionally had the need to look into 'simple' A/C radio installations. There are a lot of variables in the way these things are set up, rarely are there any circuit diagrams, or explanations at to why things have been set up as they are, and almost every installation is 'different'.

Congratulations on your perseverence up to this point. There has to be solution somewhere, but for the moment anyway I'm as puzzled as everyone else. I do hope you can get to the bottom of it.

Sorry I can't offer more for the moment. It really is a puzzle.

regards
FOR

No experiense of planes,but had a similar "funny noise" on marine radio.

Turned out to be a cable with the screen 'earthed' at both ends. Any screening should only be grounded at one end,preferably at the 'sending' end.:ouch: