We have just been down the road of fault finding this snag only to find that Cirrus know about it already. Thought i’d share.
The more integrated and electrical that our airplanes and our lives get the more chance for electromagnetic interference (EMI) to cause us problems. A personal example of this is the loud hum the clock radio beside my bed makes due to the cordless phone stand that is next to it. In the airplane, we go through many tests to ensure that new electric equipment, or new placement of this equipment and antennas don’t interfere with other equipment in the airplane. It is an unfortunate reality of the modern airplane. One current issue we are having is headset noise caused by the Mode S transponder. The transponder is a fairly powerful transmitting antenna. During the Perspective program, we moved the antenna back from the forward cockpit area to its current location in the tail to reduce the potential for EMI issues. Introduction of the Mode S transponder resulted in a more powerful antenna and transmitter and the start of some complaints about headset noise.
|Original Forward Transponder Location|
|Current Aft Transponder Location|
The noise heard in the headsets is a clicking noise. Some have described it as the sound an electric fence makes. Some people can’t even hear it, and yet for others it drives them nuts. It is more prevalent in the back seats than the front due to the proximity to the antenna. Extensive troubleshooting and testing both at the factory and at an outside service center found the following results. First although caused by the more powerful Mode S transponder antenna, this is mostly an active noise cancelling headset problem. It is worst in the Bose X headset, and basically non existent in the passive headsets. There is something within the ear cup circuitry that is picking up the EMI. We have had discussions with Bose and they said that the A20 should fix that problem.Ground and flight testing have shown that indeed the Bose A20 and Lightspeed’s Zulu’s ANR headsets basically eliminate the problem. Switching headsets is the best short term solution to this problem.
|Bose A20 (Left) Lightspeed Zulu (Right)|
A longer term option would be for us to try to move the antenna aft. This of course comes with it’s own set of issues. The first issue is with the antenna itself. As we move further back, the fuselage narrows very quickly and you run out of places to put the antenna other than through the bondline running down the center. The current antenna connection is designed for thin skin airplanes not thick bonded joints. Therefore an alternate antenna style (ie. a small blade antenna) which meets the required TSO approvals would be required. This confined space brings up two other design criteria. First you have to consider installation access is only through the small access hole on the side of the fuselage. Combining limited access with the oxygen bottle, the shelf that holds it and the various control cables you are limited as to how far back you can move it. It gets crowded in a hurry. Second with the narrow fuselage you reduce the size of the ground plane. This could likely hurt transponder performance. Finally any antenna move like this requires a Major FAA project. There are pretty specific guidelines for determining transponder performance and they require a lot of coordination with the FAA and air traffic control for the various in-flight tests of transponder performance. (that is a whole other interesting blog post) So moving the antenna back a few feet will help but it is not a near term solution.We recommend switching headsets in the short term.