So, I like the GI 275 system in my PA 28 151 (180). However, the airplane indicator on the ADI horizon is always about 2.5 degrees up in level and stabilized flight. The instrument is so precise, this is pretty unsettling. Probably the horizon bar on our old Artificial Horizons was 2.5 degrees thick by itself but this indicator is so exact the discrepancy bothers me. My shop says they have a similar issue with a G3X in a C 172. So this may well be the sort of issue that was always present but we didn't have good enough instruments to show it.
I asked Garmin and received a reply quoting 14 CFR 23.13.1303(F) that limits flight crew adjustment for an attitude display to parallax adjustment. Since this is a flat panel display there is no parallax, so, no adjustment. Basically they said a couple of degrees is not a big deal and things may vary with loading and CG anyway. My airplane is consistently nose up however, in the same degree, over a range of loading options and speeds.
Apparently, per Garmin the GI 275 is calibrated with the airplane in leveled status using the two leveling screws on the left side of the fuselage below the pilots window. I think the result of that is if the airplane does not fly perfectly level the ADI will not line up with the horizon line while in level flight, ie not climbing or descending. I confirmed this by putting a level on the copilots seat track and in level flight and it showed half a bubble up.
Per the Warrior maintenance manual, the leveling process with the two screws is not an end to itself, but simply establishes a base condition "for purposes of weighing or rigging". Looking at the stabilator and stabiltor tab rigging instructions, these seem to be focused on the amount of travel up and down. There is an interesting figure that shows the stabilator chord line, from which the up and down travel is measured, as the stabilator being in line with the front seat tracks. To me that sounds useful as it would appear to be an effort to align the stabilator and its trim tab with the airplane longitudinal structure, which may be more helpful than focusing on "level"--unless of course the two things happened to be the same. An additional reference that actually relates to trim being an issue is the section that discusses the gyro horizon. The troubleshooting table says that if the instrument does not show level flight, the instrument is not level in the panel or the aircraft is out of trim. This sounds familiar to the question presented.
Question posed to the group is whether rigging of the stabilator or the stabilator trim tab has had an impact on the nose up or down pitch of your Cherokee in cruise. I would ask if Cherokee pilots see level flight on their AI, but, unless you have an ADAHRS unit of some sort, you probably, (as I am sure I did) simply adjust the horizon bar to show level flight if it was not doing so. Put another way, have any of you had a longitudinal trim issue on your Cherokee that was fixed by re-rigging? If so, who did the work, just your A&P or are there some shops that specialize in this?
I would not be disappointed to find that my Cherokee is mushing along in an inefficient aerodynamic posture as it entered the fleet in 1974 and there are no indications in the logs of any re-rigging. Say with the stabilator pushing the nose up and the trim tab pushing the nose down to maintain level flight. Fixing such an issue would probably pay some dividends in airspeed as well as well as economy. The airplane flies fine and has good numbers with 180hp but this issue posed by a new state of the art instrument may indicate that tweaking is called for.
I asked Garmin and received a reply quoting 14 CFR 23.13.1303(F) that limits flight crew adjustment for an attitude display to parallax adjustment. Since this is a flat panel display there is no parallax, so, no adjustment. Basically they said a couple of degrees is not a big deal and things may vary with loading and CG anyway. My airplane is consistently nose up however, in the same degree, over a range of loading options and speeds.
Apparently, per Garmin the GI 275 is calibrated with the airplane in leveled status using the two leveling screws on the left side of the fuselage below the pilots window. I think the result of that is if the airplane does not fly perfectly level the ADI will not line up with the horizon line while in level flight, ie not climbing or descending. I confirmed this by putting a level on the copilots seat track and in level flight and it showed half a bubble up.
Per the Warrior maintenance manual, the leveling process with the two screws is not an end to itself, but simply establishes a base condition "for purposes of weighing or rigging". Looking at the stabilator and stabiltor tab rigging instructions, these seem to be focused on the amount of travel up and down. There is an interesting figure that shows the stabilator chord line, from which the up and down travel is measured, as the stabilator being in line with the front seat tracks. To me that sounds useful as it would appear to be an effort to align the stabilator and its trim tab with the airplane longitudinal structure, which may be more helpful than focusing on "level"--unless of course the two things happened to be the same. An additional reference that actually relates to trim being an issue is the section that discusses the gyro horizon. The troubleshooting table says that if the instrument does not show level flight, the instrument is not level in the panel or the aircraft is out of trim. This sounds familiar to the question presented.
Question posed to the group is whether rigging of the stabilator or the stabilator trim tab has had an impact on the nose up or down pitch of your Cherokee in cruise. I would ask if Cherokee pilots see level flight on their AI, but, unless you have an ADAHRS unit of some sort, you probably, (as I am sure I did) simply adjust the horizon bar to show level flight if it was not doing so. Put another way, have any of you had a longitudinal trim issue on your Cherokee that was fixed by re-rigging? If so, who did the work, just your A&P or are there some shops that specialize in this?
I would not be disappointed to find that my Cherokee is mushing along in an inefficient aerodynamic posture as it entered the fleet in 1974 and there are no indications in the logs of any re-rigging. Say with the stabilator pushing the nose up and the trim tab pushing the nose down to maintain level flight. Fixing such an issue would probably pay some dividends in airspeed as well as well as economy. The airplane flies fine and has good numbers with 180hp but this issue posed by a new state of the art instrument may indicate that tweaking is called for.
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