Planning rulers
 

Having managed to break my standard 18" flight planning ruler by shutting it in the car boot, I was just looking online for a replacement...

... when it occurred to me how useful it would be to have an extra-long one, or better still a 1:500,000 nautical miles tape measure.

Does anybody know if any such thing exists? I've had a good look around the usual websites and can't seem to find anything.

Anybody?

G

Haha, found it.

I could have just deleted the original post, but thought somebody else might want one as well.

G

Take a piece of string. Any string will do - sewing thread, shoelaces, guitar strings - it doesn't matter as long as it's not elastic.

Measure distance with piece of string, then hold string against the left or right side of the chart (note: not the top or bottom). One degree of latitude = 60 miles. One minute of latitude = 1 nm (or near enough for practical purposes). Bonus points for marking the piece of string in 1 nm intervals.

Unless you've got a single leg that stretches from a bottom corner of the chart to the opposite top corner, the scale on the left/right side will always be long enough.

Technically speaking this method is even more accurate than using a 1:500.000 marked ruler, since your chart isn't a uniform 1:500.000 throughout. However in actual practice the difference is so small as to be negligable. (The winds aloft forecast is less accurate, for instance.)

Yep i had that issue around question 15 in my Nav exam when i realised my fuel planing for the trip was quite wrong. lots of re measuring and erasing.. and quiet swearing. My ruler wasn't long enough.

Thats the first time thats happened!! :oh:

When I was a Boy Scout we used to use bootlaces or compass strings in just such a way. However I seem nowadays to regularly try and plan long VFR routes across several charts where a very long scale rule is very useful. Anyhow I've ordered the tape measure and will see how it works.

G

First, for practical purposes, I wouldn't disagree with anything said so far!

Taking the 1:500,000 chart for Southern England, which is Conformal Conical Lambert, and knowing the scale varies between the standard parallels, I wondered by how much, so I measured 1 degree distances between 50-51, 51-52, and 52-53 degrees of latitude.

There was essentially no difference, it was 59.7 nm in each case. But this was "the wrong answer". Not a big deal in real life, but it could matter in an exam.

I suspected my ruler. So I measured my 18" nav ruler with a long ordinary ruler, and 90nm on the nav was 333mm on the ordinary, as best as I could measure. Now 90 x 1852 / 500,000 = .33336m = 333.36mm, so my ruler was OK.

I measured the scale on the chart, and that was also 59.7 nm. So the whole chart was "wrong". I imagine it has shrunk since I bought it, or perhaps it was always too small.

For completeness, I made a spreadsheet to calculate the theoretical scale variation across the chart, and the theoretical distance was 60nm +/- 0.05 nm, which is not measurable.

So the moral? The most accurate way to measure distances on a chart is indeed to compare a distance on the chart against a scale on the chart (either against the main scale or against minutes on a nearby line of meridian), but not for the "official" reason.

For this chart at least, the reason has nothing to do with the theoretical scale variation across the chart, it is because of shrinkage!

Another wasted morning...

For my CPL GNAV the only way to get adequate accuracy was to use dividers then reference against the chart itself.

However, out here in the real world, where I'm trying to determine the best VFR route from A to B, with a minimum of dog-legs around danger areas and controlled airspace, then a long straight edge is definitely the optimal paper-based tool (there are clearly some great software tools available as well, and as it happens I've just ordered the latest version of Navbox Proplan as well). And running at the world at somewhere between 90-110 knots, an accuracy of about a nautical mile is perfectly good for all VFR navigation.

Of course, I could just use a 1m school blackboard ruler or a bit of wood from B&Q, but it's much more useful to have something with nautical miles at 1:500,000 on it - to save ALSO having to use the dividers, bit of string, etc.

Aircraft Spruce seem to have dispatched my tape measure, so I look forward to having a play with it.

G

My Jeppessen VFR/GPS chart (EB/EH) says "1:500.000" in bold letters. And in normal font: "True Scale at N51º - 1:515.000"

So it might not be that your chart has shrunk, but the difference is somehow designed in.

It's possible, but I don't think so.

My LF-2 chart for France also has 1 : 514 000, but near the scale it clearly says to use a 1.03 factor for distances.

The UK chart just has a scale with the Lambert projection parameters above, and that's it. I would assume their printing target is indeed 1 : 500 000 at the standard parallels, and guess that the variation is down to the variable water content of the paper, especially after printing.

If it's the same as mine (current issue EG-5), it actually says 1:501,500 which is an error of 0.3%, or 1 mile in 333 - or approximately the width of the chart.

I'd love to be able to say that I can fly to an accuracy that comes close to that mattering. But I can't.

G

You could always try buying the MemoryMap digital charts. You can play around adjusting your route as much as you like & the computer will happily update the bearing & distance for each leg. Either print your route out or transfer to a real chart once you're happy.

Folks might find this useful - I certainly do. On the wall I have one of those all-Europe charts which shows airfields. Also, I have a home-made ruler-like piece of wood which is marked in still-air hours (at the map scale) for my aircraft. So, at the pre-planning stage it's very quick to use this 'ruler' and to obtain ball-park flight time figures. It helps greatly when guess-timating leg times etc.

To do a diversion in Canada, the drill is to:

1. mark where you are and where you want to go on the chart
2. Measure the distance with a pencil
3. hold pencil against longitude line to read off nm
4. simulate parallel rules by placing pencil along course and sliding to VOR rose.

On desk at home, string and sheets of paper are even more accurate than pencil between thumb and index finger:8

Standard flight school equipment is a map of the wide area (think 1/2 of Europe) of the school with a sturdy pin or nail exactly through the home airfield. Attached to said pin is a piece of wire. On the map itself (in a convenient location - usually over the sea) a scale is added in nm and flight time, whose origin is the same home base. Works wonders for visitors: "Can you really fly to London in just two hours?"