Just joined this forum and read the two threads on the GI 275 display issues. I know it is an older thread issue but still relevant and unresolved. I struggled through the GI 275 non-functionality issues when I redid my panel two years ago and I am revisiting it now that I am having a GFC 500 installed and my installer wants to revisit the whole deal again. I don't think the previous threads got to the heart of the matter so I will see what I can do.
Problem is that the airplane symbol (chevron) is not at the horizon line when the airplane is in level flight. Generally the error is 2.5 to 3 degrees. This is a problem and it is caused by installation calibration error. The GI 275 is a great instrument and not the problem.
To start, read the FAA Instrument Flying Handbook, Chapter 7 Section II Airplane Basic Flight Maneuvers--Using and Electronic Flight Display; Reference FAA-H-8083-15B. The Handbook states para 7-35 "The attitude indicator displayed on the PFD screen is a representation of outside visible clues. Rather than rely on the natural horizon visible during visual flight rules (VFR) flight, the pilot must rely on the artificial horizon of the PFD screen". So the ADI is supposed to work just like the old artificial horizon: In level flight the chevron is at the horizon line, not several degrees above it. The handbook goes on to state on the same page 7-35 "During normal cruise airspeed, the point of the yellow chevron (aircraft symbol) is positioned on the artificial horizon." Para 7-34 states "At slow cruise speeds the level flight attitude is nose-high with indications as in Figure 7-47; at fast cruise speeds, the level flight attitude is nose-low. (figure 7-48) Figure 7-49 shows the indications for the attitude at normal cruise speeds.". To summarize, at normal cruise speeds the chevron is at the horizon line; at fast cruise speeds the chevron drops below the horizon line, but, the distance is virtually imperceptible. In slow flight the indicator rises above the horizon line, but also virtually imperceptible--look at the illustrations. And, for training purposes, load can generally be disregarded in small airplanes. Bottom line, the ADI is supposed to indicate attitude at the horizon line for cruise speed in level flight, and the chevron doesn't move much for faster cruise, slow flight or heavier load. So why are so many of our airplanes with GI275s showing 2 to 3 degrees up in level flight?
The reason probably is something called the angle of incidence (AOI), defined as the angle between chord line of the wing and the aircraft's longitudinal axis. Most airplanes have a positive AOI because it provides benefits, chiefly early lift to help airplanes get off the ground sooner. However, as AOI increases drag also increases so there is a tradeoff. I see references that common AOI is 4 to 6 degrees. Pipers and Mooneys have about 2.5 degrees. AOI is created by the angle at which the wing is attached to the fuselage. Basically our airplanes are flying slightly nose up in level flight, but the aircraft pitch is zero degrees. However, that doesn't mean they are climbing, AOI is part of the designed construction of the airplane and AOI, unlike Angle of Attack, is constant and unchanging. If AOI is incorporated into calibration of the GI 275, it is invisible. If the GI 275 is calibrated without considering AOI, for instance if the GI 275 is calibrated with a ground-leveled airplane, the GI 275 reads the airplane's nose up attitude from the AOI as pitch and shows the result on the face of the GI 275. That is an expensive AOI indicator--you can get the same reading by placing a $10 plumbers graduated spirit level on the passengers seat track. Moreover, this is useless information you don't need to know because the airplane flies just fine with whatever AOI the manufacturer has built into it. Remember, the angle at which the airplane naturally flies doesn't affect the fact that it is in level flight (nether climbing nor descending), with zero pitch, at cruise airspeed. The actual flight postures are shown as described in the Instrument Flying Handbook, above.
AOI can have operational consequences in passenger aircraft because with the aircraft floor at a slant due to AOI, cabin crew in DC 10s had to struggle to push service carts uphill. In aircraft smaller than larger cabin class, which most of us fly, we never even realize the floor is slightly up hill because we are just sitting in our seats.
The GI 275 installation manual at para 4.3.3 states that "the aircraft must be leveled to within 0.25 degrees zero pitch using procedures in the aircraft maintenance manual or AFM/POH. The Piper Cherokee Warrior Service Manual at para 2-14 provides a leveling procedure "for purposes of weighing or rigging" (instrument calibration is not mentioned) which levels the aircraft using leveling points on the airframe. The PA28 airframe from its certification about 1960 to the present is unchanged so this same procedure is probably in every PA28 manual. When the leveling is done, the aircraft is leveled to the surface of the earth, a posture it will never have in flight because of AOI. But, the GI 275 dutifully measures the angle above that calibrated in the "ground-leveling" process. This is the quintessential "garbage in-garbage out formula. Piper in the GI275 STC doesn't pretend to provide procedures specifically applicable to every make and model airplane for which the STC is approved. The STC install instructions are generic, and, like any STC are subject deviation for good cause in particular cases where they are not effective or simply do not make sense. However legions of avionics techs and repair stations do not feel they are responsible for understanding why the GI 275 puts out useless, and unchanging apparent pitch information. AOI should not be conflated on the face of the GI 275 with angle of attack information that is in fact useful and changes with every movement of the elevator.
If the tech or repair station realizes that something is wrong with the picture and something needs to be done, the Gi275 install manual provides a subsequent procedure at para 4.3.4 for Manual Pitch/Roll that allows the pitch and roll values to be see manually. One way to fix the problem is to do a test flight to determine the aircraft's Angle of Incidence at cruise power in level flight (zero pitch) with a spirit level and then duplicate that angle on the ground by adjusting the aircraft's attitude by manipulating the nose tire pressure until a clone of the in flight attitude on the spirit level is attained. Using the manual pitch settings, the GI 275 chevron can then be aligned with the horizon line. It takes a couple of tries as there is no linear relationship to the manual values, but probably 10 minutes max. It is also possible that Piper could provide AOI specifications for use in calibration, though it is uncertain that a test posture would be more accurate than an in-flight measurement of a particular aircraft with its current condition and modifications.
My personal view is that having an ADI that doesn't accurately reflect either level flight or pitch is a safety of flight issue. Saying you can add or subtract 2.5 degrees from an erroneous ADI reading is not reasonable. In an upset situation, or with the 2.5 degree descent you get by putting the chevron on the horizon line spells danger, and you can't expect to do the math in the right direction in either a climb or descent--add or subtract?- under stress. But to me, I will have about $45K total in my panel when I get my 180 hp Warrior back with the GFC 500 autopilot and I expect the ADI to function like it is supposed to, as spelled out in the FAA Instrument Flying Handbook.
One caveat I will add is that the above reflects my study and analysis of a subject, aeronautical engineering, that is pretty deep. Maybe I am totally wrong and there is another explanation for the mysterious 2.5 degree error on GI 275 ADIs. If so, I am sure someone wiser than me will point it out. I can say that I have about 170 hours on a properly calibrated (from my view) GI 275 and that it works perfectly. I will also note that none of the Garmin videos or still promotional photos of the GI 275 show the elevated aircraft chevron we are all struggling with. Third party YouTube videos with GI 275 ADIs also do not seem to reflect the elevated chevron indication.
Problem is that the airplane symbol (chevron) is not at the horizon line when the airplane is in level flight. Generally the error is 2.5 to 3 degrees. This is a problem and it is caused by installation calibration error. The GI 275 is a great instrument and not the problem.
To start, read the FAA Instrument Flying Handbook, Chapter 7 Section II Airplane Basic Flight Maneuvers--Using and Electronic Flight Display; Reference FAA-H-8083-15B. The Handbook states para 7-35 "The attitude indicator displayed on the PFD screen is a representation of outside visible clues. Rather than rely on the natural horizon visible during visual flight rules (VFR) flight, the pilot must rely on the artificial horizon of the PFD screen". So the ADI is supposed to work just like the old artificial horizon: In level flight the chevron is at the horizon line, not several degrees above it. The handbook goes on to state on the same page 7-35 "During normal cruise airspeed, the point of the yellow chevron (aircraft symbol) is positioned on the artificial horizon." Para 7-34 states "At slow cruise speeds the level flight attitude is nose-high with indications as in Figure 7-47; at fast cruise speeds, the level flight attitude is nose-low. (figure 7-48) Figure 7-49 shows the indications for the attitude at normal cruise speeds.". To summarize, at normal cruise speeds the chevron is at the horizon line; at fast cruise speeds the chevron drops below the horizon line, but, the distance is virtually imperceptible. In slow flight the indicator rises above the horizon line, but also virtually imperceptible--look at the illustrations. And, for training purposes, load can generally be disregarded in small airplanes. Bottom line, the ADI is supposed to indicate attitude at the horizon line for cruise speed in level flight, and the chevron doesn't move much for faster cruise, slow flight or heavier load. So why are so many of our airplanes with GI275s showing 2 to 3 degrees up in level flight?
The reason probably is something called the angle of incidence (AOI), defined as the angle between chord line of the wing and the aircraft's longitudinal axis. Most airplanes have a positive AOI because it provides benefits, chiefly early lift to help airplanes get off the ground sooner. However, as AOI increases drag also increases so there is a tradeoff. I see references that common AOI is 4 to 6 degrees. Pipers and Mooneys have about 2.5 degrees. AOI is created by the angle at which the wing is attached to the fuselage. Basically our airplanes are flying slightly nose up in level flight, but the aircraft pitch is zero degrees. However, that doesn't mean they are climbing, AOI is part of the designed construction of the airplane and AOI, unlike Angle of Attack, is constant and unchanging. If AOI is incorporated into calibration of the GI 275, it is invisible. If the GI 275 is calibrated without considering AOI, for instance if the GI 275 is calibrated with a ground-leveled airplane, the GI 275 reads the airplane's nose up attitude from the AOI as pitch and shows the result on the face of the GI 275. That is an expensive AOI indicator--you can get the same reading by placing a $10 plumbers graduated spirit level on the passengers seat track. Moreover, this is useless information you don't need to know because the airplane flies just fine with whatever AOI the manufacturer has built into it. Remember, the angle at which the airplane naturally flies doesn't affect the fact that it is in level flight (nether climbing nor descending), with zero pitch, at cruise airspeed. The actual flight postures are shown as described in the Instrument Flying Handbook, above.
AOI can have operational consequences in passenger aircraft because with the aircraft floor at a slant due to AOI, cabin crew in DC 10s had to struggle to push service carts uphill. In aircraft smaller than larger cabin class, which most of us fly, we never even realize the floor is slightly up hill because we are just sitting in our seats.
The GI 275 installation manual at para 4.3.3 states that "the aircraft must be leveled to within 0.25 degrees zero pitch using procedures in the aircraft maintenance manual or AFM/POH. The Piper Cherokee Warrior Service Manual at para 2-14 provides a leveling procedure "for purposes of weighing or rigging" (instrument calibration is not mentioned) which levels the aircraft using leveling points on the airframe. The PA28 airframe from its certification about 1960 to the present is unchanged so this same procedure is probably in every PA28 manual. When the leveling is done, the aircraft is leveled to the surface of the earth, a posture it will never have in flight because of AOI. But, the GI 275 dutifully measures the angle above that calibrated in the "ground-leveling" process. This is the quintessential "garbage in-garbage out formula. Piper in the GI275 STC doesn't pretend to provide procedures specifically applicable to every make and model airplane for which the STC is approved. The STC install instructions are generic, and, like any STC are subject deviation for good cause in particular cases where they are not effective or simply do not make sense. However legions of avionics techs and repair stations do not feel they are responsible for understanding why the GI 275 puts out useless, and unchanging apparent pitch information. AOI should not be conflated on the face of the GI 275 with angle of attack information that is in fact useful and changes with every movement of the elevator.
If the tech or repair station realizes that something is wrong with the picture and something needs to be done, the Gi275 install manual provides a subsequent procedure at para 4.3.4 for Manual Pitch/Roll that allows the pitch and roll values to be see manually. One way to fix the problem is to do a test flight to determine the aircraft's Angle of Incidence at cruise power in level flight (zero pitch) with a spirit level and then duplicate that angle on the ground by adjusting the aircraft's attitude by manipulating the nose tire pressure until a clone of the in flight attitude on the spirit level is attained. Using the manual pitch settings, the GI 275 chevron can then be aligned with the horizon line. It takes a couple of tries as there is no linear relationship to the manual values, but probably 10 minutes max. It is also possible that Piper could provide AOI specifications for use in calibration, though it is uncertain that a test posture would be more accurate than an in-flight measurement of a particular aircraft with its current condition and modifications.
My personal view is that having an ADI that doesn't accurately reflect either level flight or pitch is a safety of flight issue. Saying you can add or subtract 2.5 degrees from an erroneous ADI reading is not reasonable. In an upset situation, or with the 2.5 degree descent you get by putting the chevron on the horizon line spells danger, and you can't expect to do the math in the right direction in either a climb or descent--add or subtract?- under stress. But to me, I will have about $45K total in my panel when I get my 180 hp Warrior back with the GFC 500 autopilot and I expect the ADI to function like it is supposed to, as spelled out in the FAA Instrument Flying Handbook.
One caveat I will add is that the above reflects my study and analysis of a subject, aeronautical engineering, that is pretty deep. Maybe I am totally wrong and there is another explanation for the mysterious 2.5 degree error on GI 275 ADIs. If so, I am sure someone wiser than me will point it out. I can say that I have about 170 hours on a properly calibrated (from my view) GI 275 and that it works perfectly. I will also note that none of the Garmin videos or still promotional photos of the GI 275 show the elevated aircraft chevron we are all struggling with. Third party YouTube videos with GI 275 ADIs also do not seem to reflect the elevated chevron indication.
