Loud High Pitched/Warning Sound

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    I had an issue with pressurization on my 340 where one of the rubber grommets on the nose steering cable had come apart and allowed a leak. I replaced all the hoses up front and the grommet. I can get full pressurization now. However, there is a really loud noise being caused by something. I moved all the controls for the ventilation/heat system and not much change if any. I saw a 421 post having a similar problem and I am looking for some causes or the result on the 421. I might ad that I opened one dump at a time and I will lose pressurization if I start opening either one. I was under the impression that the plane should stay pressurized with one dump valve open. Thanks in advance for your responses.

    quote WWILSON:

    I might ad that I opened one dump at a time and I will lose pressurization if I start opening either one. I was under the impression that the plane should stay pressurized with one dump valve open. Thanks in advance for your responses.

    The amount pressurization from one side only dependent on your power setting. My plane will loose some pressure with one dump valve open, not sure how much, but the cabin does not dump to atmosphere. I get a decent on the cabin VSI but have never left it long enough to see where it settled out at. I have also tested an increased MP and can see the decent level off with one valve open.

    As for the noise, I just chased one down: it was coming from the hose/tube that connects to the valve that diverts the pressurized air to or to bypass the air/air heat exchanger. It was not very noticeable until the cabin reached max diff, then it became annoying. While they had it open, I had all the hoses replaced in that section as I think it was the only remaining original.

    You can test this out by flying unpressurized. It could be something not related to pressurization. I had an inspection cover that was not replaced properly one time and it made a lot of noise.



    I had a similar problem in my 414…a very high-pitched squeal only while airborne. It sounded like it was coming from around the copilots windscreen so I took it to a shop and had them pressure-test the vessel. They found leaks around the copilot’s windscreen from a bad seal job (I think it was original) as well as some cracks at the base so I had them replace it. I was so excited on my first test flight that we had found the issue but as soon as I started the engines…you guessed it…the squeal returned! Now it was even doing it on the ground! So I started fiddling around with anything that had a knob. When I jiggled the adjustment knob on the copilot’s AI, the squealing stopped. Interestingly, I had just replaced this instrument 2 months earlier so I would have never suspected that the seal around the adjustment knob had sprung a leak! My avionics shop had never heard of it happening either. While I doubt this is the problem you are having, my point is to fiddle with EVERYTHING because you never know what might be causing it.

    BTW, I think a fairly well sealed cabin should hold pressure with one engine above 25″ of MP. If you are certain the squeal is caused by pressurization, you could spring for a shop to do a vessel pressure test. It takes about 6 man-hours but they will be able to find a lot of leaks…many of which are not that expensive to fix. For example, many of my leaks were coming from the belly drains.
    Good luck.


    I have a very similar problem. I get a very loud pitch down low. We can’t find it. When I get above 4-5000 it goes away. My plane gets full differential pressurization. We are thinking that it might come from the small vent windows when the pressurization is not pushing the seal tight.


      When I replaced all the ducts in the nose bay, I got a high pitched squeal on the test flight. It turned out to be a small piece of one of the ducts I had cut to length was flapping inside the duct. Once I trimed the piece, the squeal went away. In that you just did some work on the nose wheel extend/retract rod boot, I would look at what was replaced and see if something is not sticking in the duct. I know it is a pain to get back into that spot, but I would think that is where the problem lies.


      I took it to Weststar aviation and they pressurized the aircraft and couldn’t find anything but a loose cap on the port in the front wheel well where you hook up to pressurize the aircraft. They put the cap on tight after the test. The plane pressurizes to full differential but I am still getting a warning horn once I get to about 18,000. The cabin altitude isn’t to 10000, no light on there. I pulled the breakers on the stall warning and gear warning but still had a warning horn of some sort. Any ideas? The only thing left is to take someone up with me and hunt down the warning horn……

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