Power switches for electric instruments?

Discussion in 'Maintenance Bay' started by Pedals2Paddles, Dec 13, 2014.

  1. Pedals2Paddles

    Pedals2Paddles Cleared for Takeoff

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    I've seen this before, but am wondering if it is legal. Can your electrically powered instruments have their own power switches? For example, an electric gyro turn coordinator. Could you have a switch to turn it on and off. Day VFR, it isn't really needed. Why wear it out. Seen it before but not sure if it was passable. It was logical though.


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  2. Norman

    Norman En-Route Gone West

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    If it's required equipment it must be operable.
     
  3. GeorgeC

    GeorgeC Administrator Management Council Member PoA Supporter

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    Pull the CB.
     
  4. poadeleted20

    poadeleted20 Deleted

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    I have seen older planes with a separate switch for an electric turn and bank instrument, so I guess it's possible. But given the fact that it takes thousands of hours of use to wear out a turn coordinator, I don't see it as something I'd want to do -- all I see that as doing is creating the risk of that switch failing open, shutting down my TC in the goo some day without providing any significant return.

    I might point out that my Tiger has an avionics master switch which turns on/off power to the entire avionics stack. I have a second avionics master switch in parallel with the main switch just to eliminate the as chance of an open failure of the primary avionics master switch shutting down my whole avionics stack at a critical moment. So, if you choose to go ahead with this plan, I'd advise you to install two switches in parallel, not just one -- just in case.
     
  5. Pedals2Paddles

    Pedals2Paddles Cleared for Takeoff

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    That completely evades the question and has nothing to do with it.

    Not helpful unless every electrical device has it's own CB. Which usually is not the case. Hell our 172 doesn't have any CBs for the instruments. Just radio 1, 2, & 3. Not sure what circuit the TC is on to even try.

    I definitely said something along the lines of "I hope that switch doesn't break..." The person who had used the plane for patrol work. He put enough hours on it to actually burn them out regularly.
     
    Last edited: Dec 13, 2014
  6. poadeleted20

    poadeleted20 Deleted

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    Many light planes have c/b's which cannot be pulled, and Cessna recommends against using a pullable c/b as an on/off switch. In addition, as noted above, the c/b which feeds the TC may also feed other items you don't want to turn off.
     
  7. Stewartb

    Stewartb Final Approach

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    Toggle breakers are common. My plane is full of them for switched loads while I use pull breakers for common on loads, most of which are on the avionics bus. Your instruments will likely die from lack of use rather than over use. Use 'em, break 'em, replace 'em. There's no free lunch.
     
    Last edited: Dec 13, 2014
  8. Pilawt

    Pilawt Final Approach

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    Here's the panel of a 1951 Cessna 170A, photographed in 1966. The first "piano key" switch to the left of the throttle is labeled "Turn & Bank - Down On."

    [​IMG]
     
  9. Pedals2Paddles

    Pedals2Paddles Cleared for Takeoff

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    The one time I saw this, the guy had a very legitimate reason for it. He was overhauling the TC every year due to the number of hours he flew. A $5 switch solved the problem.

    I guess the only reason I just thought of it again is it would be nice to turn the thing off on the ground. When you need the power on to tinker with things, there is no sense in having that thing screaming away. It is annoying noise and a waste of battery. Hardly anything I would go crazy trying to remedy. But it made start wondering if it was even legal to switch on and off.
     
  10. W. Stewart

    W. Stewart Pre-takeoff checklist

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    I sincerely doubt that his problem was due to "the number of hours he flew". Something was wrong with his instrument, or he had a crummy overhaul shop or something.

    I think that the TC is pretty robust as far as gyro instruments go, but I have heard that the other gyro instruments are less susceptible to shock when spinning (due to rigidity provided by the mass/momentum) than when docile (or especially when slowly moving).

    Two other reasons for not rendering a TC inoperative during flight (more important for an instrument rated pilot than a "blue sky burger run" pilot) would be the habit of including it in your scan, even subconsciously, and. . . doing away with the "oh, crap--i forgot to turn it on" possibility).

    Wells
     
  11. Pedals2Paddles

    Pedals2Paddles Cleared for Takeoff

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    Well a few months ago, his 172 had 28,000 hours on it. 26,000 by him over the course of 34 years. So ya I think it was his number of hours.


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  12. weirdjim

    weirdjim Ejection Handle Pulled

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    So you were laying on your tummy in the back yard when you sent this?

    :goofy:

    Jim


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  13. Pedals2Paddles

    Pedals2Paddles Cleared for Takeoff

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    I knew it was you hiding behind that shrub.


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  14. Tarheelpilot

    Tarheelpilot En-Route

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    We have 2014 production aircraft in our fleet and the TC has a dedicated power switch. I would say it is legal to have a switch on it if you wanted one.
     
  15. Henning

    Henning Ejection Handle Pulled

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    Sitting is more destructive to most moving objects than moving as designed. Since most gyros already sit doing nothing 95% of their life, having it spin when you go fly is a good thing.
     
  16. DJTorrente

    DJTorrente En-Route

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    I am oh so slightly disappointed that this thread did not pertain to -musical- instruments. I was just thinking OK, my keyboard has a power switch, but not an electric guitar... why would you want one on a guitar? :D
     
  17. weirdjim

    weirdjim Ejection Handle Pulled

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    Good Lord ... how about a second ELT in case the first one fails, or a second flap switch in case the first one fails ...

    Jim
     
  18. poadeleted20

    poadeleted20 Deleted

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    I fly in the weather a lot, and in that situation, loss of all avionics would be life-threatening. OTOH, my life is not immediately threatened if either the ELT or the flaps fail, so I don't feel the need for them to have backups.
     
  19. bnt83

    bnt83 Final Approach PoA Supporter

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    Pretty easy on Cessnas that have an electric clock circuit, bypassing the battery solenoid and everything.
     
    Last edited: Dec 19, 2014
  20. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    It's not the switch but the relay. I've got a second breaker (normally left open) than jumps the avionics master relay. I've never had a relay fail but the Avionics guys around here love putting those in.