Bad tach...should I go GI 275 EIS?

Discussion in 'Avionics and Upgrades' started by arnoha, Sep 16, 2020.

  1. arnoha

    arnoha Cleared for Takeoff

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    Clearly so, as it's what provides the clock for every steam gauge, IFR-ready 172S.

    But...why? The logic doesn't make sense...why is it OK for Davtron but not OK for other avionics?
     
  2. asicer

    asicer Final Approach

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    The Davtron only displays hours:minutes on the "UT" and "LT" pages but on the "ET" and "FT" pages it only displays minutes:seconds. How is that a "permanent" clock display under FAR91.205(d)(6) which requires hours:minutes:seconds?
     
  3. Salty

    Salty Final Approach PoA Supporter

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    Where does it say permanent?


    *** edit **** Nevermind, you were referring to the letter, not the FAR.
     
  4. Ryan F.

    Ryan F. Cleared for Takeoff

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    In the case of an installed clock such as the Davtron models, you cannot turn the clock "off." Going to timer/other functions does not eliminate the clock functionality. Additionally, these modes meet the requirement for "hours, minutes and seconds with... a digital presentation." The key phrase is "digital presentation." There is no requirement for all of the clock "modes" to be visible at all times, nor is there a requirement for a digital clock to display "seconds" visibly at all times. That is accomplished via the digital presentation. This has long been considered an acceptable method of complying with 91.205's "clock" requirement.

    Since an analog mechanical "clock" cannot change modes in such a fashion, a sweep-second pointer must be present on those models.

    An "installed appliance" in which the clock is but one piece of functionality, and the clock "page" may be replaced with other non time-keeping information or removed altogether, no longer meets the regulatory requirement. It must be a permanent display.

    Hopefully that explains it to your satisfaction, but if you'd like, you could fire off another letter to the Chief Counsel of Regulations, and you'd have it in writing yet another way.
     
  5. asicer

    asicer Final Approach

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    Thanks for the explanation. If I may re-state it more succinctly, it sounds like:
    • A clock is defined as a time-keeping function displaying hh:mm:ss, not necessarily reflecting the time-of-day
    • For permanence, at least one of hh:mm:ss must be displayed at all times but not necessarily all 3 at once.
    Furthermore, these are un-codified generally accepted practices.

    However, this brings up the question of the GT-50. It has OAT, volt and G-meter functions where their display modes do not show one of hh:mm:ss and yet it was approved by the FAA as a replacement clock:
    https://www.aircraftspruce.com/catalog/pdf/gt50approval.pdf

    The ASC5A version of the EI Superclock also has OAT and altitude functions where the display mode does not show one of hh:mm:ss. The standard version also has an engine timer function where one of the display modes shows tenths but not hours.

    Would the FAA object to one of these 3 devices fulfilling FAR91.205? In the case of the GT50 the approval letter would make it appear not.
     
  6. Ryan F.

    Ryan F. Cleared for Takeoff

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    You can re-state it any way you like, but those are your words, not mine.

    You'd have to ask the FAA.
     
  7. asicer

    asicer Final Approach

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    I'm tempted to, but I have a feeling there's a bunch of GT50 and EI owners that would rather I not. :)
     
  8. Ed Haywood

    Ed Haywood Pre-takeoff checklist

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  9. Ryan F.

    Ryan F. Cleared for Takeoff

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    No need to hold back - fire away. Owners of those units are at no risk of a change in policy invalidating their hardware installation on the basis of an airman inquiry for interpretation. But I don't think there's anything new to be gained from the Chief Counsel of Regulations. All of the detail necessary to glean the "how and why" of these units passing muster while the "clock page" of an IFR GPS failing the same test are present in the guidance available to us at this moment. Careful perusal is all that's required for understanding.

    The unit in question, the GT50 (I didn't review the others, but they're likely quite similar in form/function) is a clock, and is in fact considered a "simple replacement clock." Note that in the first interpretation letter you posted, the principal request was determine whether the installation of a replacement clock was considered a minor change in non-transport category aircraft.

    The determination as to whether this unit is still considered a "simple replacement clock" has to do with which additional functions are provided and whether they replace other equipment. In the case of the GT50 the voltmeter and OAT functions cannot "replace previously installed equipment." The accelerometer function is "considered a minor change in this device, since it is incidental to the clock installation and does not interface with other aircraft systems."

    As a result, the GT50 is a clock. And its display is permanent; it cannot be turned off. Even when the accelerometer function is selected, the permanent display remains. Even though it may "appear" to some that this is no different than clock "pages" within other units, it is different. The unit's permanent display is for clock functionality, and the optional data presented there are not replacing pre-existing equipment or interfacing with other aircraft systems. Therefore it is consistent with the later interpretation letter re: 91.205/clock.

    The obvious counter example is an IFR GPS. (I'm not categorically including all IFR GPS' in this discussion, but rather a few common popular GA units -- i.e. the GNS430, GTN650, KLN94, etc.)

    All IFR GPS' will have a clock/timer "page" somewhere within the unit. It may be possible to have this data present on the display some of the time, but in the case of many "Navigators" this will require paging to an Aux / Flight Timers sort of page. It makes the rest of the display unusable for its intended purpose. Clearly, if you were relying on the GNS 430 as your timer for a non-precision approach of some kind, it would be less than desirable in terms of being able to use the other features of the unit during this phase of flight. Additionally an IFR GPS "... interface(s) with other aircraft systems" and is functionally not considered a clock. It is an IFR GPS.

    It's sort of an interesting discussion from an academic standpoint, but there's no big reveal or expose here -- clocks are clocks. There are fancy units and not-so-fancy units. If it's a "simple replacement clock" it may be used to meet the requirements of 14 CFR 91.205. If it's not a clock, it needs a permanent clock display to meet the same requirement.

    Satisfied yet or more discussion needed on this topic? (LOL)
     
  10. arnoha

    arnoha Cleared for Takeoff

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    So, back on topic, I ended up selecting a new stock unit to go back in. As much as I hated to do it, it was the best choice. The failure mode was such that it could not be economically repaired and would have taken far too long...I called around no one had exchange units available. The digital ones were nice and cheaper, but not in total...labor would have been higher and I'd have to get a ferry permit to get the plane to a shop. And the plane would have been down an extra week. EIS is a nice dream, but that's a long and expensive install...not appropriate for right now. That's one I should do when I've already got it planned, hardware in hand, ready to go.

    This isn't the first time that I've been cornered into making a decision on the plane. Leaseback limits options, as you want that plane back on the line as soon as possible. Things that would not make sense in private ownership suddenly do because the opportunity cost is so high. So, this'll be about a $1000 repair. The reality is that a digital version of this wasn't going to be more than a couple hundred cheaper once factoring in extra labor...and more expensive if you count the week extra downtime. Typically, a week on my plane nets about $500. Obviously, that swings all over the place. Generally it would be higher now during the summer, closer to $1000, but the smoke and haze has kept demand down.
     
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  11. Ryan F.

    Ryan F. Cleared for Takeoff

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    Wise choice. The best decision is not always the one which is most satisfying.
     
  12. asicer

    asicer Final Approach

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    Did I tire you out? :)

    Kidding aside, I was wondering from before the thread started about the hh:mm on the Davtron vs the hh:mm:ss in FAR91. I very much appreciate your insight on the matter.
     
  13. Ryan F.

    Ryan F. Cleared for Takeoff

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    It's my pleasure -- I enjoy getting into the technical details. It was a valid question given the circumstances.