The G5 and Track Steering
18 August 2019
Our club’s C-172 just got a panel update – two Garmin G5’s, replacing the A/H and the D/G. The G5 can function as either one. Here is the G5 acting as Artificial Horizon:
You can see how you get a lot more information than you did on the old A/H. There is an airspeed tape on the left and an Altitude tape on the right. There is a ball (white, bottom centre), and heading (top centre). This display has a new name – Primary Flight Display, or PFD.
Here is the G5 acting as Directional Gyro. Again, there is a lot more information here than there was on the old D/G.
Of course there is Magnetic Heading, just like on the old D/G. But you no longer have to set it to the magnetic compass, like you used to. There is a magnetic flux sensor in the wingtip. The instrument synchronizes automatically. We used to call this a slaved compass. The G5 has the functions of the mechanical Horizontal Situation Indicator, or HSI, that we used on the DC-9, back in the day. It has two bearing pointers (#12 on the illustration). In our installation they are connected only to the Garmin 430W, so they can be set to indicate bearing to the GPS waypoint, or to the #1 VOR.
Probably the most important feature is what we used to call the Track Arrow. This is #10 on the illustration, labelled Course Deviation Indicator (CDI). When the aircraft is on the Desired Track, the CDI nestles into the magenta arrow – pointing to about 043° in the illustration – so that the arrow looks like one long magenta line. Here, however, the aircraft is off course to the left. The Desired Track (DTK) is off to the right.
What about the aircraft (#4)? It is headed away from the DTK (030°). What else can we see from this display? Quite a bit, including Groundspeed (#7) and Distance to Waypoint (#14). But possibly the most important information is a tiny magenta triangle on the compass rose – the Current Ground Track Indicator (#16). (On the NAV 1 page in the Garmin 430W, this is TRK). If you look back up to the PFD display, you can see it under the heading. Here both heading and track are 290°, indicating that there is no drift – and possibly no wind at all.
But in the HSI indicator there is a hurricane-force wind from the southeast. The heading is 030°, and the aircraft track over the ground is 008°. A whopping 22° of drift!
Why do I say hurricane-force wind from the southeast? Because the groundspeed is 202 knots, and unless our airplane is an Aerostar or a jet, we have a substantial tailwind.
Where does this information come from? The GPS. The GPS is computing position about once per second. Then it uses the Calculus, that great mathematical technique invented by Newton and Leibniz, to differentiate the series of positions (dS/dt) and calculate velocity, a vector, which has both speed and direction. These are displayed on the G5 as groundspeed (GS) and track (where magenta triangle is on the compass rose).
I know. This is way too much information. It will take some getting used to. But once the G5 becomes more familiar, the benefits to the pilot are huge.
Remember how, for your Private Pilot Flight Test, you had to measure True Track on the chart, apply wind correction, convert to magnetic, steer the course, and then use the Double Track Error Method when you drifted off?
Now you can flight plan on your tablet, and load the plan into the Garmin 430W. Then you stay on the planned track by flying that little triangle onto the tip of the track arrow.
Don’t believe it? Try it!
And if the airplane does drift off course, you can fly the triangle (TRK) to re-intercept, just like you would if it were a heading. But if the aircraft is off course it won’t be because of wind drift. It was because you let the triangle wander away from the tip of the track arrow.