Loud high pitch wistle sound

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  • #83684

    I flew my new 421B yesterday for the first time. Great flying plane, except it had a very loud high pitch whistle sound. We could not figure out where it was coming from. We had perfect pressurization with full differential. The noise was extremely loud. Any suggestions are welcome.

    #95518
    quote SGERBER:

    I flew my new 421B yesterday for the first time. Great flying plane, except it had a very loud high pitch whistle sound. We could not figure out where it was coming from. We had perfect pressurization with full differential. The noise was extremely loud. Any suggestions are welcome.

    Pressurization leak somewhere is my guess. The 421 can put a lot of air into the cabin so even decent sized leaks can be compensated for with the turbo air.

    Try moving the cabin heat levers. I’ve got a leak in my defrost air duct – I hear a whistle if I have it open, but when I push it back in it goes away.

    Robert

    #95520

    Sounds like a pressurization leak.

    Right after buying our 340 we found the storm window was whistling. If we adjusted the latch tighter it would go away. Ended up being the o-ring/seal in the bolt that holds the latch.

    #95525

    agree sounds like pressurization. Have you tried flying without? I have heard you can partially pressurize on the ground with an electric leaf blower but have not tried. The easiest way would probably some sort of wood frame mount to the one for the storm windows with a mount for a hose.

    For some high frequency sounds a ~ foot long tube held up to the air can help localize. First spots are heating system, door seals, DV window seals, wire pass thru’s, etc

    #95527

    Last spring I was climbing through 14k and suddenly a loud whistle started. I could tell it was in the cockpit on the copilot side somewhere but I couldn’t isolate exactly where it was coming from. I tried everything to get it to stop but all I could accomplish was to get the pitch of the whistle to change slightly by changing the pressure differential in the cabin. Even though the cabin pressure was holding just fine, I assumed the problem had to be in the pressurization system. When I got home I had the cabin pressure checked and several leaks were found. The most significant leak was around the copilot windshield. The plan was to just re-seal the windshield but when the trim was pulled off, cracks were found and it had to be replaced. This had to be what was causing the whistle, right? Wrong. The very next flight the squeal was still there…only now it actually squealed on the ground – before the cabin even had pressure. What the ____? So I went up with an instructor, and while I flew the airplane he did a thorough “snoop” to find the problem. It turned out to be the the attitude indicator on the copilot side (that I had just replaced a month earlier) was the problem. The center knob that adjusts the horizon was leaking vacuum and causing the squeal. I could jam a folded matchbook behind it and it would stop. Who da thunk the new AI could cause such a whistle?

    I have never found anyone else who had this problem…even the avionics shop had never seen a case like this. It is unlikely that this is your problem but it is easy enough to check. You should wiggle any knobs on vacuum instruments. What you think is a pressurization leak might actually be a vacuum leak. Unlike my situation, I hope you find your problem before spending $$$$ chasing the wrong problem.
    Good luck.
    Pete

    #95550
    rwelsh
    Participant
      quote :

      First spots are heating system, door seals, DV window seals, wire pass thru’s, etc

      There are also 5 or 6 little rubber seals in the cabin floor that close with cabin pressure and open when on the ground with no cabin pressure. They are supposed to drain water from the floor. They get old and brittle and need replacing after 20 or so years. Most A&Ps do not know they exist but they can be a source of a leak and or squeal.

      #95551
      quote RWELSH:

      quote :

      First spots are heating system, door seals, DV window seals, wire pass thru’s, etc

      There are also 5 or 6 little rubber seals in the cabin floor that close with cabin pressure and open when on the ground with no cabin pressure. They are supposed to drain water from the floor. They get old and brittle and need replacing after 20 or so years. Most A&Ps do not know they exist but they can be a source of a leak and or squeal.

      Yes – good reminder! I also heard that sometimes people poke them out thinking they are simple drain holes. Also, doesn’t the AC system have a special drain for condensation?

      #101703

      I was flying this weekend from KBOI to KSLC for quick business trip. I climbed to 17K on the way to SLC no issue.. On my decent to thru 15K I heard a loud whistle that seemed to be coming from the pilot window near the compass. As I was trying to isolate it thru the decent it went away at about 12K and never heard it again.

      On the way home from SLC I climbed to 10K hold, 12K hold 14K hold 16K cruise and never heard it again.. However, on my decent it started the whistle again at 15K-12K this time.. then I leveled at 10K and it went away, I then messed with pushing the cabin pressure down with the pressure gauge and I was able to reproduce it off and on but nothing consistent.

      Any ideas what to check?

      Kelly

      #101706
      rpinkowski1
      Participant

        I have a similar problem in my 340, but it is airspeed related. Above about 150KIAS I begin to hear a whistle that becomes annoyingly loud by 170KIAS. Not pressurization related as it will occur at 0DP. Seems to be coming from pilot side. I have no trouble maintaining max DP while pressurized and pressurization has no impact on the whistle. I’m stumped.

        #101707

        There is a technique to use a directional microphone sensitive to high frequencies and use it to scan for the source of the sound. The ear hears the whole spectrum making it appear to come from everywhere.

        This would probably be possible using an iphone mic (wired headset) and a sound spectrum app. you would sweep the microphone around until the signal was maximized. A small bottle of soapy water would also help. You could also use some of the invisible UV dyes and spray suspected areas (and examine the external airframe afterwards with a UV light)

        To pressurize on the ground with a leaf blower (electric only) I think you would have to jack the plane up, power it with a GPU, seal off the sonic venturis at the engines and supply a source of vacuum to the pressure controller.
        To get the air in the airframe, you could either create a frame to pump it into the pilot or co-pilot DV window, the upper part of the door, or perhaps via the engine sonic inlet.

        speculation only, have not done it nor seen it done.

        #101729

        Is there any concern of something like a window blowing out if the pilot window had a small crack causing the leak? I have the older plexi-glass style window not the Glass style. I’m trying to determine if the pressure sounding leak is a flight safety issue or if I’m ok to fly and try and isolate it in flight?

        #101731
        Gerald T. Alves
        Participant

          I had the same problem in my 421C. It was a door seal leak and very small.

          #101733
          quote khouseholder1:

          Is there any concern of something like a window blowing out if the pilot window had a small crack causing the leak? I have the older plexi-glass style window not the Glass style. I’m trying to determine if the pressure sounding leak is a flight safety issue or if I’m ok to fly and try and isolate it in flight?

          Kelly, I suspect you have a leak at a seal or decaying line. I don’t think you would have a sound from an edge crack around a bolt in the window edge.

          #101743

          I once had my defrost hose come disconnected from the heater manifold. The pressurized air rushing through the defrost vent makes a real loud whistle. Would suggest checking your un-pressurized ducting between your forward pressurized bulkhead and heater.

          Jon

          #104060

          today on my way back home from portland the whistle showed up at 12K and held until cruise altitude of FL190 where it continued to be annoying for an hour. However the silver lining in this is that I was able to locate the leak by placing my down coat in different areas of the windows until I found how to make the noise go away.

          Found that the bottom right side of the co-pilot window is where its coming from. On the decent about 13K it went away again.

          Flew my flight over at 16K and never heard it so it’s a bit off and on.. suspect it’s the seal.

          Can this be fixed without removal and re-installing the entire window?

          Kelly

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