Forum

  • I saw this article today on the blog section:

    http://wifinetnews.com/archives/2011/01/specious_reasoning_on_in-flight_electronic_interference.html

    I have spent a large part of my career tracking down interference sources in satellite and radio systems. I have seen one tiny little VSAT antenna thrown together by some guys at an ISP take down the entire Internet Backhaul of a country on satellite. One thing that I have learned about interference, is that it is like lightning. There is both an art and a science to it. Just when you think you know all there is to know about lightning and put in all the best grounding and protection systems, a bolt comes down, bypasses everything and zapp??.no more radio.

    Aircraft navigational and flight control systems are becoming more complex with often miles of cabling. That cabling should be grounded and shielded ( weird concept of grounding inside a plane ). However, cracks can and do occur from flight stress ( huge changes in temperature etc ) and from maintenance ( if you ever see maintenance techs tugging and pulling on cable looms at some maintenance facilities, it would make your heart stop ). Interference can "get inside" the tiniest cracks in the protective shields. Personal electronics often contain phase locked loops. If you ever look on a spectrum analyzer at the display produced by a device whose PLL has lost lock, it?s not a pretty sight. ?Junk? frequencies flying all over the place. Harmonics can be produced by faulty devices.

    Even if there has never been a documented case of a plane going down from usage of personal electronics as far as I know, prudence dictates that these regulations stay in place . In our industry, who would ever have thought that a microwave oven could wreak such havoc on a nicely setup Wi-Fi system. How about a LEAKY microwave with a bad seal ?
    We have become so used to technology that we often think nothing bad could happen. I remember when my company was doing contract work for NASA providing satellite backhaul for the shuttle missions, we were informed that they were planning on having one launch a month. I said that was ridiculous. Not only did the weather have to be good at the Cape, but also at the Trans-Atlantic abort sites ( in case one or more of the main engines failed ).

    http://spaceflight.nasa.gov/shuttle/reference/shutref/sts/aborts/tal.html
    http://en.wikipedia.org/wiki/List_of_space_shuttle_landing_runways

    Plus all the technical issues ( leaky hydrogen valves, the foam insulation etc ). It was all pie in the skye. As it turns out, we never came even close to that number ( Vandenburg AFB in California ) was being fitted as a West Coast Site. Shuttle missions were becoming boring to the major TV studios and coverage of launches dropped more and more. If I remember rightly, CNN was the only one covering it. I remember reading a technical paper about the O-ring seals used in the Solid Rocket Boosters at the side of the shuttle. An engineer at Morton Thiokol ( who manufactured the SRBs ) had said that the seals should not be used below a certain temperature. Nobody paid a blind bit of notice to him ( how surprising that administrative types ignore what an engineer has to say?). There was huge pressure to get vehicle up there. The State of The Union address was going to be given with a live videocast, and the Teacher in Space program had been initiated to ?get the public interested again?.
    http://en.wikipedia.org/wiki/Christa_McAuliffe

    People outside of Florida imagine that it?s all sunshine and heat. In the winter at the Cape it can get down to freezing. There were icicles hanging off the gantry when the Challenger was about to lift off. Many people looked on the shuttle as just another plane. Far from it. Every mission was frought with potential hazards, including the O-ring seal issue, which nearly everyone ignored except for one brave engineer at Morton. ?Not likely, could never happen?..?
    I was on duty that day and had just come back from lunch and was watching NASA select TV on the monitor as we prepared for the countdown. We were on ?Critical Standby? which meant that for 24 hours before a launch, we weren?t allowed to touch anything in case turning a switch caused a short which caused etc etc. Our 110 foot diameter parabolic dish was pointed accurately, so there was nothing to do but sit and watch.
    I remember saying to a co-worker as smoke trails flew across the sky ?There?s something seriously wrong here?. He said "Just think it's some extra smoke ". It seemed like an eternity before the Mission Controller?s voice came across with the first statement about all not being right. The SRBs are jettisoned using explosive bolts and then come back down via parachute, but in a controlled sequence and nowhere as soon in time as they did. You could see them flying in an uncontrolled pattern as well as what we later learned were the explosions as the main tank carrying liquid hydrogen and oxygen went up.

    Technology can and does fail.

    With flight systems you just can?t take any chances.

    http://www.youtube.com/watch?v=GYEcimqan1w

    Bye the way, the ?smoke? that you see is not from the burning of the fuel, but rather is part of a sound suppression system. The first STS mission nearly ended in disaster. The sound energy created caused standing waves to be created ( just as we get with VSWR in cables ) which caused the shuttle to come within inches of the launch tower. The water acts as a sound absorber. The noise power in decibels was so high that it liquified the insides of a number of birds in the area. The orbiter could have been literally shaken to pieces. A huge tank drops a massive amount of water down into the flame pit. Note how clean the engines burn.

    http://www.nasa.gov/mission_pages/shuttle/launch/sound-suppression-system.html
    http://sciencestage.com/v/3953/sound-suppression-water-system-test.html
    http://www.space-shuttle.com/challenger2.htm

    Dave

  • Dave,

    I just heard the other day that all of the new planes uses fiber-optic from one end of the plane to the other - obviously not for RF signals or power, but for control signals etc.

    Besides the resistance to noise, the cable is so much lighter compared to copper, that there is a large savings in fuel costs too.

    ----------------------------------------------------------------------------------------------------------------------------------

    I used to have a neighbor who was one of those people "pulling cables" - She and a large number of her friends suffered after retirment with large doses of arthritis in their hands and fingers.

  • Hi Wlanman

    Yes, that's right. A lot of the new "fly by wire" systems use FO which helps counter a lot of the old problems. One of the concerns that many authorities have world-wide relate to the possible effects right in the cockpit itself. In other words interference with the flight display/nav systems etc. There are all sorts of computer systems with all sorts of clocks running at a multitude of frequencies.

    One thing that I have learned in electronics/comms is that when you are told "This could NEVER happen"......well...........

    I've seen people on flights overseas firing up Short Wave radios on planes, small TVs etc. As well as the issue of actual proper grounding/shielding on the aircraft systems, who knows how well or badly grounded/shielded are the actual personal electronics ? Some are cheap clones made overseas with who knows what type of quality control.

    When I was working in the Falkland Islands, we had a 13m satellite antenna operating over the sea. Every day, a group of F-4 Phantoms would come screaming across the water at 150 feet doing about six hundred miles an hour. We would lose comms and all the phone circuits would go down and people would have to re-dial. Data would have to re-train. Lots of complaints. All the experts said "RF mainlobe...blah blah....calculations...blah blah...near field/far field ......there cannot be any problem ...blah blah". Well there was. Difficult to prove, but I manged to get a print out from the old PBX showing calls dropped and then used a videocamera to show the jets going through the beam, and correlated the times on both. RF is weird stuff and sometimes you just never know what is going to do.

    Dave

  • I have a couple friends, who have worked in RF compliance for MANY years.

    I have said to them a couple times that I thought RF was magic.

    They tell me "not really, it's just a pain in the a--"

Page 1 of 1
  • 1