# Is it correct to say that the expected behaviour at a Fly-by/Fly-over waypoint derives from when there was no GPS, but rather just VOR/DMEs?

I was re-looking at Fly-by/Fly-over waypoints:

and discussing with a colleague the topic of how these came about. Is it correct that this definitions were put in place so that a human pilot could fly between the given waypoints using VOR/DMEs?

This would kind of require having a VOR/DME station under each waypoint where a turn could be expected, so is it plausible?

The concept of a difference between Fly-By and Fly-Over predates RNAV, GPS, and even VOR/DME.

The most basic navigation happens without any navaids at all, and just a map, compass, airspeed, and stopwatch. Fly to a point defined by some visible feature, reach it, and turn. That's Fly-Over.

When you draw that route on a chart, in order to be precise you have to account for turn radius to determine the ground track you're flying after rolling out of the turn. (When I drew low-level navigation charts flying in the Air Force, we had templates that we used to account for the TAS & associated turn radius.) When the lines on the chart are drawn from point-to-point-to-point, then you have to do something other than flying over the waypoint and then turning to point at the next one... either you lead the turn so that you roll out on that point-to-point straight line course, or else you "S-turn" after flying over the point, to correct back to the that point-to-point line.

When the navigation has limited precision, the difference between the methods doesn't make enough difference to worry about... if my turn radius is a mile, I have 100 miles until my next waypoint, and my wind correction is based on a forecast from a couple hours ago, then the crosstrack error introduced by a 30 or 45 degree turn is probably less than the various other errors that I'll be accounting for. On the other hand, when I can see my crosstrack error to the 0.01 NM on a modern FMC and Navigation Display, and the autopilot is going to zero that out, it's really nice to NOT have it S-turning back to course.

Most waypoints these days are fly-by so that the charts depict all the airways and tracks as point-to-point, and we let the FMC sort out how far we need to lead the turn, given our True Airspeed right now. (And, of course, the FMC doesn't care if there's a VOR station sitting beneath the point or not.)

But there are times, especially when obstacles come into play, that the procedure designers want the aircraft to stay on the given track all the way to the defined point, and don't start an early turn (in order to prevent something undesirable from happening -- probably, in order to assure clearance from some obstacle or terrain feature).

The difference between the two types of waypoints matters more, now with tighter navigation tolerances and highly precise databases, but the fundamental conceptual difference has been around for a long time, probably almost as long as aircraft navigation & Mr Jeppesen's original charts.

For graphical depictions:

All graphics are mine; I'll edit the text into text as time allows.

• Really good information. Your next to last paragraph says "fly-past." Based on the Op's question, do you mean "fly-by?" Also, I think the OP is using "GPS" and actually meaning "RNAV." Not certain it makes a big difference as your answer seems to sort that out ok. When I retired, our B757's did not have GPS, but had IRS/DME/VOR-DME as the nav inputs to the FMC. I imagine that 99 percent of all air carrier type aircraft and corporate jets currently have GPS as one input to their multi-sensor FMC. I'm old.
– user22445
Commented Oct 6, 2023 at 17:50
• @RTO Thanks, I corrected that. Somewhere back in the memory banks, "fly-past" is another term I've seen used, equivalent to "fly-by", but no reason to confuse things here with two interchangeable terms. I agree that RNAV, rather than GPS, is what really raises the distinction between the two types of waypoints, since leading the turn is more a function of an FMS than of just a GPS receiver itself.
– Ralph J
Commented Oct 6, 2023 at 18:11
• I believe the vast majority of RNAV waypoints are fly-by. Some exceptions would be the missed approach waypoint (MAWP) on an RNAV approach and an RNAV waypoint used exclusively for holding and also when obstacle clearance is an issue. There are some symbology issues when the same waypoint is used as a holding waypoint (flyover) and also used as a waypoint as part of the instrument approach (fly-by). Interesting discussion HERE regarding this symbology/useage issue.
– user22445
Commented Oct 6, 2023 at 19:37
• @RTO, that ACM discussion was because there was a misunderstanding. DoD and FAA charted waypoints in the plan view the same way, so there was no conflict. The problem is that the 8260.19 is not the authority on charting. Rather, that would be the IAC 4 specification. The current language is more clear, but the result is the same. If the waypoint is used for holding, it is charted as a fly-by unless it is used in another part of the procedure as a fly-over. Commented Oct 7, 2023 at 0:39
• @Timbo it's good to have someone on this site that understands the charting requirements and issues as well as you do. Thanks
– user22445
Commented Oct 7, 2023 at 14:59