5.19 What's the difference between the autopilot VOR and the GPS2.01 auto feature?

From FlightSim

There "can be" a difference between using the autopilot VOR as opposed to the GPS2.01 "auto" feature when tracking to a VOR station. The operative phrase here is "can be." I first discovered the problem on a long cross country flight where I had planned flight segments between VORs in the 100 nm range. I would get lazy and when reaching and crossing one VOR where the next VOR wasn't in range yet I'd just program it into my GPS2.01 module and let it guide me there. In so doing, especially when flying IFR "blind" I didn't pay much attention to the track I was making.

Naturally I had the next VOR radial dialed in on the OBI and was I surprised when the VOR station finally did light light up 50 miles later! Here I was, thinking I was right on "track" when, in reality, I was really a few degrees/miles to the left or right of the VOR radial indicated on the OBI. What happened?

What happened is I had a strong crosswind. And I hadn't paid attention to the difference in tracking techniques between my GPS2.01 module and the autopilot VOR radio. Had I understood this earlier I may have effected a substantial savings in time and gas.

The point is: there is a significant difference in the way the two units track to a destination. (In order to substantiate this I spent a few hours experimenting and will be glad to discuss the particulars of my investigation with anyone interested).

First the VOR. When the plane's VOR is locked into the transmitter on a particular radial (bearing) and the OBI needle centered up with that radial, the autopilot will then drive the airplane in such a way as to keep it aligned on that radial and the bearing will remain constant regardless of the plane's indicated heading. This IS the shortest distance between two points--the quickest way to the VOR transmitter. In the event there is a crosswind, the autopilot will compensate for it by crabbing the plane into the wind the amount needed to keep the plane aligned on the radial. The plane's heading will vary with amount of crab needed to keep the aircraft on track. On the other hand, the bearing to the target will not vary.

Such is not the case with the GPS2.01; it operates differently. Given the same set of circumstances, the GPS uses the VOR (or whichever point it is tracking to) the way a pilot might use the RDF needle to track to an NDB transmitter; it drives the airplane so that the nose of the airplane is pointed directly at the target irrespective of its bearing to the target. It does NOT necessarily keep the plane on a straight line to the target as does the VOR! The stronger the crosswind the more variation there will be in both heading AND bearing.

This is, of course, of little concern in conditions where there is no or very little wind or even when there is a strong head- or tailwind. But when the wind is of substantial force and from the side it will blow the plane to the side even though the GPS will maintain the plane's position so that it is always pointed at the target. Over a stretch of 50 to 100 nm this can amount to a significant variation in bearing from the chosen radial and substantial added distance.

Please take into consideration that the foregoing applies only to MY experience with the GPS2.01 in the Cessna 182. I am not familiar with any other GPS units so I don't know how they operate. Nor do I know how the GPS2.01 operates in heavier aircraft under the conditions I've mentioned.

And don't get me wrong. I'm not picking on David Drouin's GPS2.01 which I find to be an almost indispensable add-on in my flight simming. My purpose in writing this is simply to point out a limitation of the instrument to those who might not be aware that there is a difference in the way it performs as opposed to standard autopilot VOR tracking. I hope understanding this difference might be helpful to some as it was to me in the way I navigate.

Gordo