GI275 question

Discussion in 'Avionics and Upgrades' started by TBalch, Feb 16, 2021.

  1. TBalch

    TBalch Pre-Flight

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    In stabilized cruise flight, at standard cruise power settings, my primary, factory attitude indicator shows the airplane on the horizon line. The GI275 ADI page shows the aircraft symbol about 2.5 degrees nose-up. I’m wondering if the GI 275 has to be calibrated when straight and level, like the ADHRS display in ForeFlight does.
     
  2. asicer

    asicer Final Approach

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    Is the factory AI vacuum driven with the knob on the front that adjusts the little airplane? When you adjust it on the ground, do you set it to the horizon line? What is the actual attitude of the plane when you do that? What does the GI275 indicate when you do that?
     
  3. WannFly

    WannFly Final Approach

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    my G5 shows 2.5 degree nose up in the air too with VS = 0, you just cant see it in the old vacuum driven AI unless it has been calibrated like asicer pointed out. even after that its very difficult to notice in air. I am glad that 20 year old AI is now gone
     
  4. Ryan Klems

    Ryan Klems Pre-takeoff checklist

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    This is a common complaint among folks converting to glass instruments. They are used to being able to adjust the "aircraft" on the horizon on a vacuum/pressure AI. The reason this adjustment is present is parallax. Because the horizon and the indicator are in two different planes, adjusting your seat higher/lower, being taller or shorter in the torso, etc, you will see different positions, so there is an adjustment for it. With glass, there is no parallax because the everything is in the same plane. The FAA regs only allow for parallax adjustment which is why there is no zero adjustment on these glass instruments. The calibration steps for these instruments typically call for it to be done with the aircraft leveled per the MM. That level is _not_ level flight level. This will vary based on gross weight, center of gravity, etc. Depending on the aircraft and loading while flying straight and level you may have nose down relative to the horizon, or you may have nose high. This is perfectly normal. Some folks get really bothered by this and go about messing with settings to try to get it to read on the horizon while in some variant of level flight. I personally don't recommend that.

    FAA Reg about this:

    § 23.1303 Flight and navigation instruments.
    (f) When an attitude display is installed, the instrument design must not provide any means, accessible to the flightcrew, of adjusting the relative positions of the attitude reference symbol and the horizon line beyond that necessary for parallax correction.
     
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  5. pnancoz

    pnancoz Pre-Flight

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    I had the same issue on my G5 that I noticed immediately after install. About a week later, I returned to the shop for a few minor squawks and they were able to adjust it. You should return to the shop, or any other Garmin shop and have this corrected. I know I wouldn't want to be inside a cloud and see a discrepancy between my attitude and VS.

    I would assume that the GI 275 would have a similar adjustment.
     
  6. chemgeek

    chemgeek Pattern Altitude

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    If you have a certificated G5 (and I presume a GI275) it is by the installation manual to be adjusted so that it reads zero degrees when the plane is level as per the maintenance manual. And you must disable the horizon adjustment feature on the unit. At cruise, you will likely find that level flight is not zero degrees attitude. On my AA-5, level flight is about +1.5 degrees. A cruise descent is about -0.5 degrees. Approach configuration for level flight at 90 kt is +2.0 degrees. This is not a problem flying IFR. You just need to know the appropriate flight attitudes required for your pitch+power numbers, and of course cross-check level flight with the altimeter and VSI.

    If using non-zero pitch/power settings in cruise is unacceptable, the installer can make an adjustment of the pitch correction to better conform to zero degrees in level cruise flight, but the the horizon adjustment feature must still be disabled for certificated aircraft installation. The electronic gyros are so accurate and easy to read without parallax that you have to learn new pitch/power combinations for various flight regimes anyway. I used to fly the mechanical gyros by "dots" up or down (which was always a gross approximation due to parallax) but the electronic displays can be precisely flown by degrees up or down.
     
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  7. Groundpounder

    Groundpounder Pattern Altitude

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    Zero attitude does not equal level flight.

    Level flight does not equal zero attitude.
     
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  8. Kent Wien

    Kent Wien Pre-takeoff checklist

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    Same applies in an airliner. 2.5 degrees is about level flight in most.
     
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  9. John Myers

    John Myers Pre-takeoff checklist

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    As others have said, zero degrees in cruise is an arbitrary and changing value that depends on weight, etc.
    Calibrating your AI to zero degrees during cruise is sort of like calibrating your angle of attack indicator to zero during cruise. If you had them change this to zero at some arbitrary cruise configuration your AI is now wrong, as far as how the FAA/Garmin/aircraft manufacturer intend it to read. The plane should be leveled to zero degrees pitch per the manufacturer and the AI set. You can then compare your pitch to others in the same type (ex: 10 degrees pitch up during climb, etc.).
     
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  10. Aviator305

    Aviator305 Pre-takeoff checklist

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    I have learned to fly as prescribed in Positive Flying, and it has worked really well for me. I consider these instruments to be tools to be used as we each see fit, and I wish there was some flexibility afforded to us by the manufacturers. If I could not adjust the “zero” to level cruise attitude, I would truly miss that capability afforded by my old attitude gyro.
     
  11. chemgeek

    chemgeek Pattern Altitude

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    Having transitioned from vacuum gyros to dual G5s, it's just not an issue for IFR flying. You simply have to learn new "numbers" for various flight regimes. Then you are done. The G5s actually make the "numbers" more reproducible because they are so precise, always report absolute attitude relative to a constant reference, and do not have parallax issues.
     
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  12. asicer

    asicer Final Approach

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    Max Trescott, who wrote a few G1000 books, says that the proper way to set your old school AI is to boot up the glass AI first then adjust the legacy AI to match the G1000 AI.
     
  13. Aviator305

    Aviator305 Pre-takeoff checklist

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    Right. So, I would have to learn new numbers just because there is no provision to make a small adjustment.

    There’s nothing wrong with flying the way you say, but there’s nothing wrong with the way that I grown accustomed to flying either. I like having mine attitude reading zero when I am flying straight and level so that I can then instantaneously eyeball if I am one or two needle widths of the horizon (which corresponds to predictable performance for a couple of power settings).

    Different strokes for different folks.
     
  14. asicer

    asicer Final Approach

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    Nah. Just install a GFC500/GFC600 and turn on the FD bars.
     
  15. WannFly

    WannFly Final Approach

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    Are you saying no matter what the weight is or the power setting is, your attitude reading is zero? That doesn’t sound doable
     
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  16. Aviator305

    Aviator305 Pre-takeoff checklist

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    I nearly always cruise at the same power setting with similar-enough weight. It works very well! Sure, it can be off slightly, but never an entire needle width or even really anything very perceptible, at least not in my experience in my plane. If I start using different power settings, of course everything gets thrown out the window.
     
  17. John Myers

    John Myers Pre-takeoff checklist

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    I’d agree that for most small GA aircraft it’s not a big deal if the reference is something other than what the manufacturer intended, as you say for the old style AIs it wasn’t really possible to do that anyways. I can see both sides of it, but I don’t think it’s fair criticism to say that, now that we do have instruments that can be set precisely, that they should also be able to be set what is technically incorrectly so folks don’t have to learn a different (and now standard) pitch reference.
     
  18. Aviator305

    Aviator305 Pre-takeoff checklist

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    It’s not incorrect. It’s simply a matter of preference. Do you zero at level attitude at the ground or at level flight? It should be up to the pilot. I find that saying one is correct and the other incorrect somewhat of an authoritarian attitude.
     
  19. Kent Wien

    Kent Wien Pre-takeoff checklist

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    Well, Boeing and Airbus don’t give you a feature to ‘zero’ out the pitch attitude. The zero point is constantly changing with flap setting and speed.

    But, what you are really asking for is a Flight Path Vector indicator, which you can display on the Boeing and Airbus. I’m not sure if Garmin offers that. It’s slick. You put the little vector symbol (a small circle with tiny stick wings and a rudder) on the horizon and you are level regardless of airspeed or configuration.

    Edit: I just looked it up. Garmin has included the Flight Path Vector in the GI-275! Problem solved.
     
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  20. John Myers

    John Myers Pre-takeoff checklist

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    It’s not correct per the aircraft maintenance manual, the FARs, and Garmin’s installation manual. This one isn’t up to preference, technically speaking. It’s impossible to set it to zero in level flight because your pitch will always vary depending on weight, power, etc.
     
  21. Aviator305

    Aviator305 Pre-takeoff checklist

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    What FAR says that I can’t level set my attitude indicator to whichever way I find most useful? If this is true, I would find it surprising given fact that a well respected book was written flying with this technique.
     
  22. Aviator305

    Aviator305 Pre-takeoff checklist

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    Exactly. I just ran into a video showing the flight path marker on the GI-275 just as I read your post. Very nice. I would not miss my manual leveling capability with this feature activated.
     
  23. Ryan Klems

    Ryan Klems Pre-takeoff checklist

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    See my post above:

    § 23.1303 Flight and navigation instruments.
    (f) When an attitude display is installed, the instrument design must not provide any means, accessible to the flightcrew, of adjusting the relative positions of the attitude reference symbol and the horizon line beyond that necessary for parallax correction.
     
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  24. WannFly

    WannFly Final Approach

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    I think what John is saying is the way you have it set up, it shows 0 attitude only under one very specific situation. so what happens when you are in IMC, ATC asks you to slow down, you pull power, the nose goes up but you are level with VS = 0. i am guessing you train yourself to ignore the attitude indicator then and rely on VS and altimeter to judge if you are climbing or not. vacuum gyro and EFIS are two different things, no point trying to make EFIS work like gyro. but hey, if it works for ya, have at it. the installer will install based on A/C install and mx manual and follow direction as provided by manufacturer.

    i havent read the book you reference, but if its written keeping vacuum gyro in mind, it might not apply to EFIS directly
     
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  25. Aviator305

    Aviator305 Pre-takeoff checklist

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    This does not exclude the operation of the attitude indicator in the manner in which I and others that I know use it. This FAR does not define the zero level.

    At any rate, I really do not wish to debate this point. I know that others use the technique I use, because I am not the originator of this idea. I picked up on it because someone that I respect recommended the above book, and the philosophy works very well for me. Everything can work under ideal circumstances, but in the soup performing a go-around, minimizing variables is a desirable goal and I believe that the technique prescribed in said book is optimal. The FD bars that I just learned on the GI-275 look like a superior substitute to flying with this technique which is indeed aimed towards analog attitude gyros. If I were to replace my analog instruments, the GI-275 would be at the top of my list now that I see this working.
     
  26. John Myers

    John Myers Pre-takeoff checklist

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    That’s in reference to an analog AI with a movable bar. You’re talking about asking a Garmin dealer to make modifications to an AI counter to the install instructions. I’m not bashing you for doing it, but on the point of if it’s technically correct, it simply isn’t.

    I don’t care enough to find the relevant FARa beyond that one (I think it’s in part 23), but clearly this would be dangerous in some situations if a pilot flew by pitch reference and the plane did things it shouldn’t do at that pitch. Again, for small GA aircraft, or one only you fly, I don’t think setting it up counter to the way it was intended presents any serious issues.
     
  27. Ryan F.

    Ryan F. Cleared for Takeoff

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    Many of us who flew behind mechanical attitude indicators over a very long period of time (raises hand) became accustomed to adjusting the "attitude reference symbol" to be superimposed on the horizon line in cruise flight.

    But, that wasn't really what it was for. The purpose of this adjustment is just for parallax (thanks for the part 23 reference, Ryan) w/r/t seating position, pilot height, etc.

    The attitude indicator indicates the attitude of the aircraft, not its flight path. Most commonly, airplanes will have a slight nose up attitude even in cruise flight. A properly calibrated attitude indicator reveals this. The Bombardier and Gulfstream products I fly (all/mostly glass flight decks) do in fact show a slight nose up attitude at cruise on the PFD. Obviously, the higher the speed, the lower the delta between the horizon line and the reference symbol.

    If the ADI is properly calibrated, and showing you a nose up attitude, that is the attitude of the aircraft and you should not want to adjust it just based on a comfort factor. There's a reason it's not to be adjusted on electronic attitude indicators. It is, after all, there to indicate your actual attitude. You don't want it to indicate something else.
     
  28. Aviator305

    Aviator305 Pre-takeoff checklist

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    Those aircraft have flight directors?
     
  29. GRG55

    GRG55 Final Approach

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    You must have a lot of trouble with other aspects of our heavily regulated aviation activities then. :rolleyes:
     
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  30. Ryan Klems

    Ryan Klems Pre-takeoff checklist

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    FYI, FD (flight director) bars are going to come from your autopilot, so you'd need to have an AP capable of providing them to have them displayed on the GI-275. The flight path vector (separate indication) requires the synthetic vision unlock (and a 3D GPS position fix) for it to display.
     
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  31. chemgeek

    chemgeek Pattern Altitude

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    Exactly. Attitude in level flight at 70% power won't be the same as level flight at 65% power won't be the same as level flight at approach speed. Putting the AI at zero degrees at cruise doesn't guarantee level flight in every flight regime. You are in level flight when the altimeter is constant and the VSI reads zero. What the AI says at that time is the appropriate attitude. Flying by the numbers means knowing what those numbers are. When you can adjust the AI in flight, the attitude setting loses absolute meaning. So if level flight at 65% power is zero degrees at 2500 rpm, then level flight at approach speed CAN'T also be zero degrees at 2200 rpm. And then your missed approach climbout will be...what? If it is 10 degrees nose up at full power, is that 10 degrees based on level cruise flight being zero degrees, or 10 degrees based on zero degrees for level flight at approach speed, or 10 degrees based on zero degrees sitting on the runway? Having an absolute reference point adds precision to one's power/pitch settings, and is a step way ahead of the mechanical AI gyro instrument which was difficult to display precise attitude in the first place due to significant parallax errors. New instruments, new procedures. I now better understand why when I trained 30 years ago why the AI always needed "adjustment" for level flight when I slowed to approach speed, or leveled off at cruise after initial climbout.
     
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