on your own on a feature-less leg..

[Pilot16]

So...your on a solo trip (you can add a passenger if you like :p)...turn onto your next leg... a very long and featureless leg...which will make it very difficult to judge any track error hence a closing angle (at the half way point)...However you know that even without a heading correction, you can easily spot your next waypoint as it draws nearer because its quite a nice large land feature...

I was wondering if this technique would work in such an occasion...say it was a 40nm leg...and there wasnt any suitable landmark to confirm half way position to update an ETA for destination, but THERE IS a town about 8nm down the track which can easily be identified...
Can this be used for an updated ETA over destination? even though there is another 31nm to run?

I figure this can be done very easily by setting the time taken on the inner-scale of whizwheel against 9 on the outer and then by looking under 31 on the outer-scale for the time to run.
And this updated ETA can be given to the ATSU.

Im still learning so just need a confirmation on my ideas, allthough im prettty sure, it will work...thanks

[Keef]

If you can hold heading and speed *very* precisely, and if you know exactly where you started, the 8nm checkpoint will give you some idea of drift and windspeed/airspeed.

Me, I'm lazy. I try to find a suitable VOR/DME, or if it's nice VFR, I use the GPS. I've done long legs (including over water) by dead reckoning, but prefer the easy way these days.

[reynoldsno1]

I don't see any real problem with what you suggest - of course, you could always re-plan the trip and adjust your legs so that you do get check features to assist in your navigation....

[Evo]

The problem with checking at 8nm is that it's quite sensitive - if you decide that you need a 5 degree heading change then you are going to hold that over the next 30nm. Small errors in the correction will lead to large errors in where you end up. It's ok if you get it right, but be careful

<soapbox>
As for the whizz-wheel, leave it on the ground. :)
</soapbox>

[PhilD]

Assuming your plane has some kind of radio navaid or GPS why on earth would you nav by dead-reckoning? Long legs over featureless terrain (or water) is what GPS was invented for.

[BEagle]

Well, that's the problem with the outdated 'half way' method of navigation! Throw it out and use SCA and proportional timing instead - it's sooooo much simpler!

SCA I've described many a time; proportional timing uses estimated times at obvious visual features at approximate fractions along track - e.g 1/4 or 1/3 or whatever. So 1/5 would be plausible, but surely there must be something more to look at unless you're over a desert?

You pre-plan your times at visual fix points by putting 60 against your cruising GS on the whizz wheel. Then measure the distance of each fix along track; if you're doing 90kts GS and a feature is 8 miles along track, fairly obviously you should pass it at 5:20 and be at your 40 mile destination at 26:40. Now, if you set off on the correct heading and fly precisely the correct planned IAS,if you find yourself 10 sec early at your 'town' 1/5 of the way along track, then you'll be 50 sec early at destination due only to a different wind compared to the one you used for planning. However, if you can't be certain that you've been flying accurately, or have been air-trafficked about, then just carry the 10 sec error to destination. But 1/5 of the way along track is a bit close to the start to be a huge amount of use....on a 40 mile leg you shouldn't need more than a couple of good, obvious visual fix points.

Often I've seen - or rather used to see until I stopped people teaching it - someone mark a chart in the middle of nowhere with this stupid 'half-way' point; whereas there were obvious visual fixes at roughly 1/3 and 2/3 of the way along track which could easily have been used instead.

Pilot navigation shouldn't be difficult - professional Flight Naviagtors used to need their chart tables, astrolabes, lodestones and astrological manuals to conduct the black art of their craft; however, you have to be able to fly and navigate, so the Keep It Simple, Stupid approach is needed!

[FlyingForFun]

I'm with BEagle in not liking the "1/2 way point" method. The one I prefer is the "6 minute mark" method.

Calculate your groundspeed (which you do anyway, before the flight). Divide it by 10. That's the number of miles you should cover in 6 minutes. Make a mark along your track every 6 minutes, and mark them: 6, 12, 18 etc.

Now, your town that's 8nm along your leg might be, let's say, just past the first 6-minute mark. So you might reckon it's going to take 7 minutes to get there. You find yourself overhead the town after just 6 minutes. So you're ahead of schedule - but by how much? And at what time will you arrive at other landmarks along the leg? The answer is simple - your 6-minute marks are no longer 6-minut marks. They've now become 5-minute marks - and everything else should fall into place.

Also, to pick up on a point in your original question, you really shouldn't be using the flight computer in-flight. If you do decide to use a 1/2 way point, and you come across this type of problem, just estimate the time and heading errors - much safer than having your head inside looking at the flight computer.

FFF
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[BEagle]

F4F - I go along with the 6 minute marks, but would still advocate precise times at pre-planned obvious visual fix points (about 15 min apart is sufficient).

And yes - don't use the whizz-wheel in flight, use mental calculations instead and look out of the window!

[ozplane]

Just a detail point on the 6 minute practice. As the runway heading always seems to be 180 degrees away from the direction of the flight you have to allow for this or start timing when you are overhead or abeam the airfield having completed that part of the circuit. This can take a couple of minutes in a slow aircraft and would affect the first 6-minute check point.

[Keef]

Yes, the "start point" is important - I reckon a good plog always includes a "set heading" point - probably very soon after takeoff if going in the right direction, or abeam the field on the correct onward heading if the other way. It's nominally "overhead the field after takeoff".

It might take 1 minute to reach it going one way, and 5 minutes the other (depending on other traffic).

[Whirlybird]

I can't think of ANY area of the UK, except over water, with a featureless 40 nm. There is always something. There may not be towns; but what about roads crossing railway lines, unusual shaped bits of woodland, hills, power stations, wind farms, isolated bits of higher ground, lakes. All of these will do, and the more apparently featureless the terrain, the better they work, as you won't confuse them with something similar in the same area. For instance, today as a turning point I used a small village, because it was a convenient place to turn. Small...but it happened to have a river going through it, and a motorway just to one side, so fairly easy to positively ID.

As for half and quarter way marks versus six minutes, I've always used the half way point etc, and I still do. But I tend to make sure I always have a pretty good idea of where I am anyway. I guess that just takes practice, but since I virtually always use visual nav, with radio aids and GPS just in marginal vis or for backup, I get the practice.

So I don't think it matters exactly how you do it; just practise doing it. The UK is not a desert; it's FULL of ground features everywhere. Recognising them from the air just takes time.

[ozplane]

I partly agree with Whirlybird but then I learnt in Africa and it was dead easy out of Nairobi. You just picked one of the 4 extinct volcanos on the cardinal points i.e. Mt Kenya, Kilimanjairo, Ol Donyo Sapuk and Longonot and headed off for your game park or whatever. This doesn't work in East Anglia!! The Charles Wells advert might claim it's not flat but it blooming well is. Ely Cathedral is about the only thing that's more than 3 feet above sea level. (For the pedants out there I am exaggerating)

[compressor stall]

Assuming your plane has some kind of radio navaid or GPS why on earth would you nav by dead-reckoning? Long legs over featureless terrain (or water) is what GPS was invented for.


Because GPS are not infalliable. Batteries fail, contacts corrode, satellites drop out (particulalry hand helds).

You can also be out of range of a navaid (or not have any fitted to the aircraft).

And there ARE points on the planet (admittedly not in the UK) that have featureless terrain for a 100 miles....welcome to my world. :O

[Gertrude the Wombat]

This doesn't work in East Anglia!!Sure doesn't.(For the pedants out there I am exaggerating) Well, yes, but not by very much.

[Whirlybird]

But East Anglia has quite a lot of towns, and big roads, and lots and lots of interesting canal like things and other stretches of water...it's not that difficult.

[Gertrude the Wombat]

But East Anglia has quite a lot of towns, and big roads, and lots and lots of interesting canal like things and other stretches of water...it's not that difficult. Some bits are easier than others. It doesn't help that only some of the drains are on the chart, so spotting a drain on the ground and trying to find the blue line on the map often doesn't work, and it doesn't help that there are hundreds of undistinguished little villages connected by identical little wriggly roads. It's easy enough on a day with 20 miles viz, but when all you can see is a few boring looking villages merging into haze ...

I have on occasion taken the 1:50 000 map with me as well as the half-mil chart, when wanting to find a particular building from the air for my passenger to look at, and that makes things a bit easier because the actual shapes of the villages are shown.

[Whirlybird]

Gertrude,

I must admit that I think I've only flown in East Anglia in very nice weather, with at least 10nm vis. But I would still think that for "normal" cross countries, there are enough features - a featureless 5nm or so is really not an issue since if you hold your heading you're highly unlikely to go too far off track in that time.

But looking for specific features like buildings - that's a different matter altogether. Yes, the 1:50,000 OS maps are very useful for that, once you get used to following them at high speed!! But I find that on the whole the quarter mil charts are excellent for this usually - they have a lot more detail than the half mil, and usually enough even to find quite isolated features. I used the quarter mil when I was looking for a specific field on a specific farm in North Wales (which my passenger could recognise if we found it); I had the OS map, but didn't need it.

[FlyingForFun]

The ease of using any particular type of feature is related to whether or not you're used to it.

When I first flew in Arizona, I looked at the chart and saw huge areas with no towns or roads for mile after mile. The local instructors told me to navigate using the mountains, but I didn't understand how that was possible - to me, one mountain looked exactly the same as the next.

Two month, and about 100 flying hours, later, I returned to the UK, and I headed off to my local area - and I was shocked. I thought I knew the area, but suddently, I found that I couldn't navigate, because there were no mountains nearby!!!

It's just a case of what you're used to.

FFF
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