Crosswind navigation - how do YOU handle drift?
 

You are flying across country, you don't know the area, and your GPS has packed up. You don't have radio nav.

However, you checked the met and there is a 25 knot wind at right angles to your intended track. You have worked out your max drift and have a magnetic heading written down on your kneepad.

Vis is good, you can see surface detail 15 miles ahead, so you pick feature on track and fly towards it.

If you continue to fly towards the feature, get there, and then select another, without reference to the compass/DI in between, won't you be gradually getting off track in an upwind direction? because when you select the feature (assuming you're flying on magnetic, drift-compensated heading) the feature will be upwind of your intended, true track.

Of course, in reality you will keep checking the compass/DI as well.

So what's the best way to resolve the two?

The flippant answer is that you reach in your bag and pull out the other GPS.

However, your question appears to be based on the premise that someone using a GPS or VOR tracking has not done any planning.

In reality one should always have a plog. This can be worked out manually (in the traditional PPL way) or can be printed out from say Navbox (much quicker and less error prone). This will show the leg distances, timings, and the wind corrected headings to fly.

If you are flying with any track-based device (VOR or GPS) then you will be constantly aware of the wind correction, even if you never look at the plog, because you can see the heading you are having to fly in order to maintain the track.

If you do this really properly, you will also be timing each leg.

Then if you lose radio nav, you just fly the plog in the traditional way.

To be honest, few people that fly with radio nav are also doing leg timing, and if you get a total loss of electrics you will pay dearly for that. There is no decent solution to this (especially if one might be in IMC) which is why I carry a spare GPS and a handheld radio.

Hi IO540

I'm not suggesting use of GPS implies lack of proper planning... my question is a bit more subtle than that, but if you're flying to GPS or VOR the issue doesn't arise, so I eliminated those options in my scenario. As you say, if you are flying to a GPS and/or VOR, you will be aware of the drift.

It only arises if you're navigating by dead reckoning, which some of us like to do from time to time, even if we also own a GPS :)

The questions is, IF you are navigating by DR, and fixing a visual reference point on the horizon, how do you reconcile the magnetic heading with the track you want, given a decent amount of sideways drift?

I'm a little confused! Forgive me! If I am flying by DR then the way I was taught I would fly by accurate calculated heading and time. IF I could get a good fix en route before my waypoint then I could assess actual track and calculate a correcting heading to get me to my waypoint, as wind is obviously a little different than forecast. If you fly a heading and time you wont go wrong by far, by picking ground features to fly to you can quickly convince yourself in to getting lost.

Er... you can't, can you? Surely you've either got to rely on the DI and your DR calculations, or you've got to rely on distant visual reference points and let the DI go hang.

If I want (say) a 70 degree track, and I've worked out from the wind that I need to fly (say) a 110 degree heading, then I can't aim visually for some specific reference point in the distance. I have to fly the calculated heading for the calculated time, and hope and trust that my planned reference feature will swing into view as it approaches, no? At the start of the leg the reference point will be too far off to port to see it.

Alteratively I can pick a distant reference that is at 70 degrees from my current position, and fly towards it, but I won't be able to rely on the DI until I get there, because in order to fly the proper track I will end up pointing the nose into wind, and I'll arrive at the reference point flying sideways.

I don't see how it is possible to reconcile these incompatible navigational approaches.

Best wishes
Bear

i am lost here, you are either DR or not? If you are DR then you are correcting as you go.

Please explain more clearly.

What you are describing is not dead reckoning. It's known as pilotage or "track crawling" and it does suffer from the problems you describe, so isn't generally taught as a navigation technique.

Dead reckoning, involves calculating a heading to fly for a given time, and flying that. It is NOT pointing the aircraft at a distant object that you want to get to.

On your map you should have a line drawn between your starting point, and the end point of that leg. Then along route you can try to fix your position on the map. If you find that you are left or right of your line, then you know that the wind is blowing you off course, more (or less) than you took into your calculations. You may also find it's taking you longer or shorter to reach points along your route. You must then correct your heading (or time estimates) to A) stop the error increasing, and B) to correct the error that has already happened.

There are different techniques for this. The one I learnt was called "fan lines". Another common one is the "1 in 60" rule.

You can find this described in the Trevor Thom Navigation book which you are likely to be using for your training.

The important thing to remember is NOT to point your aircraft at the distant object that you "think" is where you need to be, but instead fly the planned headings and timings, making corrections as necessary when you have a positive fix on your location enroute.

dp

humaround

You wrote "your GPS has packed up" so I assumed that was your intended scenario.

If you are flying 100% by reference to the ground only, then you need to point the plane so that the chosen feature on the horizon remains in a constant position relative to your field of view.

In other words, you have to set yourself up to be on a collision course with it. (if another plane is on a true collision course with yours, it will be stationary in your field of view - same principle)

Yet another way of looking at this: it is exactly the same as tracking towards an NDB. You adjust the heading so that the ADF needle remains on a constant heading.

It's easy enough, using the above, to fly a direct track to a point on the ground. They do this in the FAA PPL, incidentally. What you cannot tell (without a GPS, etc) is what that track actually is...

I am also not sure I understand correctly the scenario you describe.

What I believe you are saying is, you know you are flying the correct heading for the leg b/c by steering it you have just arrived overhead a chosen landmark, then you have just picked your next landmark on the horizon and all of a sudden all of your nav equipment packs up, including for the sake of argument your DI, your magnetic compass, and your watch. And you are asking, in this case, how do I know I will still be steering the correct heading to arrive at said next landmark?

If this indeed is your question, then the answer is pretty simple: wherever your chosen landmark is on the windscreen, draw a (hopefully imaginary) vertical line through it and make sure it stays on that line. That tells you you are tracking in the right direction.

Note: your landmark should move progressively lower along your reference line. If it stays exactly on the same spot on your windscreen, well, either you will be crashing into it or your landmark is moving away from you in the exact same direction and slope you are tracking :E

Or have I got your question completely wrong?

Bah, I see IO5-something is on a faster connection than mine. We are saying the same thing though.

So you are left with your magnetic compass, watch and map? Keep flying the heading for the required track and dig out the whizz wheel to calculate the next Heading/Track to fly at the T/P! Failing that, get into the nearest airfield and get your plane fixed!

(Sorry IO not digging at you about GPS again)

This is my point exactly about reliance on GPS these days. The point is with your PPL trainin gyou should be able to navigate and cope with drift calculations on the fly (excuse the pun).

Mag compass, watch/timer and Map are all you need (maybe a whizz wheel or E6B).

You should not need a GPS, VOR, DME or NDB's to navigate.

.... and then we have another thread about why new pilots are not being attracted into this wonderful olde WW2 game.

I challenge you to operate the slide rule usefully while airborne, on a nice summer day, maintaining VFR of course, underneath little white fluffy clouds...

I am not taking the bait anymore though :)

Back to technical things: there are two different methods involved here.

(1) Flying a direct track to some feature, which is easy, since a) you can see that feature all along and b) you don't care what the actual track is and c) you don't need to know the wind. No stopwatch or calculations of any sort needed. I think this is called "feature crawling" and is fine if you know the area and the viz is good.

(2) This is called dead reckoning, and is taught in the UK PPL, and involves flying for X minutes on wind-corrected heading YYY and when your stopwatch indicates X (or shortly before) you need to verify the waypoint visually. They key thing here is that you will very possibly not be visual with the waypoint until you are nearly there.

I think the original poster was referring to method (1). You are referring to method (2).

Forgive me for stirring things up here, but I don't think Humaround's question has anything to do with any method of navigation.

I think what he is referring to is simply flying straight+level.

You were (hopefully) taught by your instructor that, when flying straight+level, you should pick a distant feature and fly towards it. What Humaround has realised is that this won't work in high crosswinds.

The solution is:

- Pick a feature as far in the distance as possible. Generally, high winds come with good visibility, so this shouldn't be a problem.

- As soon as a further, more distant object comes into view, fly to this instead of your previous point.

- Cross-check against the DI more frequently than you would do in other circumstances.

I hope that answers the question.

FFF
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Haven't read all the posts so pardon me if I repeat something here but the easiest thing to do is

1. pick a good landmark feature on your chart that's just to the side of your line (so you can see it as it you pass over it. If it's right under the line, you lose sight of it under the nose a few miles out).
2. Find said point on the ground
3. Fly to it as your line relates to it on the chart, correcting your flight path as required by what you're seeing out the window.
4. Reach said point
5. Repeat steps 1-4 until the chosen point is your destination, then land
6. Replace batteries in GPS

Hope this helps! :)
Pitts2112

Yes that will happen.

Same thing happens when using an NDB or a VOR pointer or the pointer feature on a GPS by simply keeping the pointer straight ahead.

If you do not watch the heading you are flying then you will follow a line of persuit towards the position, (feature, beacon, VOR, GPS coordinate). At that place, you will be pointing more into the wind than you were at the start and if you simply select a feature 12 O'clock, you will eventually end up tracking into the wind and from then on your track will be constant. Well actually it will not be constant because you will follow the isobars! But over short distances it will be!

It makes little difference what system of navigation one uses, this can happen!

In order to maintain a constant track in a crosswind, you need two points on the earth and a straight line passing through them which you will use as a reference.

These two points could be two ground features ahead of the aircraft which you keep aligned, it could be your initial position and the next position when using NDB, VOR, GPS or Visual Navigation using a map and a line.

You are describing the situation for a single feature ahead such as a Visual feature, an NDB, a VOR or a GPS position.

The moment you start, your initial position is fixed and so too is the track to the feature, NDB, VOR or GPS position.

Since the navigation is the same with them all, I will describe one simple method of maintaining track such as you describe to a feature;

1. Point nose at feature.

2. Read track on DI and maintain track as heading.

3. In crosswind, the relative bearing of the feature will change i.e. it will move left or right of the nose.

4. Turn back towards desired track by twice the error until the feature is on the other side of the nose by twice the error.

5. Pick a heading that will keep the feature on the same relative bearing. i.e. if the nose is pointing 20deg left of the desired track, make sure that the feature is 20deg right of the nose.

If you already have the wind and the drift then simply ensure that if you point the nose 20deg right to maintain track that the feature remains 20deg left of the nose.

Forget the idea that GPS etc is different. No they are not, it is simply a case of the information being provided in a different way and no matter how good the GPS it will never turn an idiot into a navigator.

Regards,

DFC

1. Draw line on map
2. Point a/c at feature along said line
3. Assess drift and set crab angle to remain on track -- Note that feature will not be at center of windscreen unless zero x-wind.
4. Pick further features along track as they become visible
5. Repeat until reaching destination or turnpoint

This exercise is exactly the same as taking out the drift on downwind in the circuit except that you spend more time at it;)

A few years ago I bought a Book at the PFA rally entitled 'Diversion Planning' (using only a stopwatch and a pencil) this gives lots of Handy Tips. It's based on RAF methods and as the author Martin Smith says if it's good enough at 400Kts it's good enough for GA. Other diversion planning books might be available. ;)

WHY is this post even here? This is BASIC Navigation and should have been learnt in the PPL. Start with your maximum drift which should be calculated before you go, either by whizz wheel or by formulae (MD= windspeed/TAS x60) then apply it by clock code or any other means that you should have been taught and can be found by searching these forums.
- Sorry if this is a bit of WW11 teaching for some.:p

Foxmoth, couldn't have said it better myself :D