Straight-Line Distance

[Jeff]

To paraphrase the definition of a Cross-Country Flight in Part 61, a cross-country flight is a flight from one point to another and is a straight-line distance of at least [so many] miles.

 

Can anyone provide a definition of "straight-line distance"?

 

I cannot find an FAA definition of it, and there is not much on the internet pertaining to the subject, that I can find. I did, once, however, find something in a Army FM that says that a "straight-line distance" is determined by measuring the distance between two points on a map and comparing that distance to the scale on the map.

 

~Jeff

[Mikemv]

Jeff, this is done to make sure that point A and point B are apart the distance stated in the regs. Think of it as a non-curved, non deviated flight path. A straight line is the shortest distance between 2 points. Mike

[Gomer Pylot]

Technically, it's a great circle over the earth, since the earth isn't flat. Practically, just measure the distance on a map, or use a GPS to calculate it. What it really means is that you can't use two points a couple of miles apart, and fly around making S turns to increase the distance. It's not that difficult.

[Jeff]

I've had time to do a little more research. I found an article on Wikipedia that discusses great circle distances and basically says that great circle distances are not straight lines. A straight line distance is the distance between two points along a line that crosses each meridian at the same angle. Army Field Manual FM 3-25.26 describes how to determine the straight-line distance between two points.

FM3-25.26 Chptr 5 SCALE AND DISTANCE

 

So, a straight-line distance is measured on a map (aeronautical chart), and a great circle distance is calculated based on the lattitude and longitude of the two points. The math is rather extensive: Great-circle distance - Wikipedia, the free encyclopedia. But there are computer programs that will help. Airnav.com has a distance calculator. Or just put the departure and destination points into a GPS and it will give you the great circle distance. I'm sure straight line distance can be calculated mathematically, but the "map" method seems to be the best method.

 

14 CFR § 61.1 defines a cross-country, for the purposes of pilot certification, as a flight, "that includes a point of landing that was at least a straight-line distance of more than [50/25] nautical miles from the original point of departure." (There are other factors in the definition, but this is the part that is pertinent to this discussion.) It is possible to use your plotter to measure the distance between two airports and for the distance to be more than 50 nm; but, buy using AirNav.Com or another distance calculator, the distance would be less than 50 nm. The difference is that the measurement you make on an aeronautical chart is a straight-line distance and the calculation on AirNav.Com is a great circle distance.

 

Here is a good example. Determine the distance between KMFD and KAKR, (both on the Detroit Sectional). On AirNav.Com, I get 49.3 nm. Measuring with a plotter on a sectional chart, being careful to measure from the Airport Reference Points (ARP) at each airport, as closely as I can determine based on the scale of the chart, I get just over 50.0 nm.

 

So the question is, does a flight between KAKR and KMFD satisfy the cross-country flight requirements for certification purposes in an airplane (50 nm)? Prior to the advent of GPS it did! We measured it on a chart, and there was no dispute. Nobody at the time was very keen on the mathematical solution for a great circle distance. Of course we never tried to determine the great circle distance because the regs said it had to be a straight-line distance. Now, with the advent of GPS, which uses great circle distances, many have become complacent in the terminology. How is it, then, that the regs specifically say straight-line distance, but people substitute great circle distance which is different? Did the FAA change the regs but forget to write it down?

 

Someone said that the shortest distance between two points is a straight line. I agree that is true. However, due to the fact that the earth is a sphere (approximately), we cannot navigate in straight lines--especially in airplanes--for the simple reason that we cannot easily bore a hole though the ground to make a straight line to our destination. Since we navigate over the surface of Earth, and considering the principles of spherical geometry, we have to measure our straight lines on a map--a flat representation of a spherical object.

 

~Jeff

[Gomer Pylot]

IMO, the airports qualify. You don't depart from the ARP, you depart from somewhere else, and I think the distance is close enough. I don't really think anyone is going to split hairs that fine. You have to be realistic about these things.

[captkirkyota]

Since not everyone has a GPS, some planes still only have one VOR and will fly IMC with that! AND since we and the FAA still use charts, AND since the regs do not state a distance measured by means of GPS, get out you ruler and measure on the map X distance straight away from your point of take off and if it is at the min dist or greater, you are golden.

Heck, one time with the assistant chief instructor, he had me come out from the foggles, do a pinnacle, and told me to log the flight as cross country time since we were straight line away from point of take off. Heli's are great.

[Jeff]

IMO, the airports qualify. You don't depart from the ARP, you depart from somewhere else, and I think the distance is close enough. I don't really think anyone is going to split hairs that fine. You have to be realistic about these things.

 

Agreed!

 

I brought this up because an airplane flight school that I used to work for at KAKR would send students on solo cross countries to KMFD to build cross-country time without needing to schedule the airplane for more than a 2-hour block. Eventually, once the pilots got to Florida to do their CFI training (I think at Delta academy?), the pilots were forced to do more XC time because when they punched the airports into their computer, they got a great circle distance of slightly less than 50 nm. I don't know if the school did this or the FAA.

 

It doesn't affect me directly, but I think it is worth discussing in a forum such as this.

 

~Jeff

[Gomer Pylot]

Sounds like a good excuse for the school to charge extra money. For some schools, any excuse is better than none.

[rotormandan]

Sounds like a good excuse for the school to charge extra money. For some schools, any excuse is better than none.

 

I agree with Gomer. $$$. Why would someone need more cross country time for cfi training any way? There's no cross country requriements for a cfi rating that I know of. A chart is what is used. The way I look at it a gps is a luxury. If the gps fails your going to fall back on your chart. (I hope you brought one and the little pink line isn't your only position indicator)

Once again it sounds like it's all about the $$$.

Edited July 17, 2008 by rotormandan