Teaching SCA as a method

[DFC]

This is not a for or against thred.

The general way that SCA is being taught is explained by BEagle in this thred;

http://www.pprune.org/flying-instruc...avigation.html

I agree that SCA is a good valid method of correction when properly used

However, very few pilots seem to use SCA correctly and unfortunately I believe and having read BEagle's explanation in the above thred even more believe that the instructors who teach this technique don't apply it correctly.

Let me explain;

Here is a quote from BEagles explanation;

Using the SCA technique is very straightforward. Let us imagine that we have been accurately flying the first leg of our navigation exercise at 90 kts on a heading of 040° when we notice that we are 4 miles left of track with some 7° of drift error as deduced from our single 10° fan line. The first correction is to turn right onto a heading of 080° and then to time for 4 minutes as we head back towards track. During this 4 minutes we can first reassess that it really was a 4 mile error and then jot down on the log that our ETA at the turning point will be 4/3 of a minute later than calculated and that there’ll be a 7° drift correction to apply when we’re back on track. When our 4 minutes are up, we turn back onto our original heading plus our drift correction, i.e. on to 047° in this example and recheck that the DI is properly aligned with the magnetic compass. With any luck and assuming that the wind doesn’t change yet again, our navigation exercise should now continue pretty well on track and we should only need to note the passing of visual fix points to revise the ETA at the turning point.

The problem is that the above is an incorrect explanation of how to use SCA and following the technique above exactly will never get the aircraft back onto track.

In fact even ignoring the effect that turning more into wind will have, following the above explanation will always result in the aircraft being still off track and also further along track than expected. The magnitude of this error caused by following the above explanation is directly proportional to the initial error.

The solution;

SCA in both theory and practice assumes that at the point where the aircraft is turned by the SCA angle it is paralleling the desired track. In other words the error in the above is that the theory measures the error miles at 90 degrees to the desired track despite the fact that the aircraft is diverging from it.

The fix is very simple;

BEagle in the above example noticed that after some time there was an error of 7 degrees and the aircraft was 4nm off track.

The first action must be to parallel track i.e. in this case turn towards the required track by 7 degrees.

Now the aircraft is paralleling the desired track and you can correctly apply the SCA and ignoring the wind effect it will work.

So actions;

1. Determine tracking error and turn towards track by the error (drift line or 1 in 60 which ever you want).

2. Turn towards the desired track by the SCA and maintain this heading for a number of minutes equal to the number of miles off track.

3. Turn in the opposite direction by the SCA - you have already corrected the drift before so no need to do it again now.

So using BEagles example;

1. Turn right by 7 degrees (parallel track) Heading = 047

2. Turn right by SCA for 4 minutes Heading = 087

3 When the 4 minutes are up turn left by SCA. Heading = 047.

So please ensure that whatever method you teach for track correction that it (in theory) will have the ability to regain track.

Saves us trying to explain (to someone who uses an SCA of 30 degrees and tries BEagles exact method when the error is 30 degrees) why they will never ever regain track because they are now paralleling it!!!!

[BEagle]

Strange, it seems to work perfectly well in practice.....

[DFC]

Strange, it seems to work perfectly well in practice

Something must be wrong then because if you sit down and draw it on paper, unless the process you describe is commenced from a position where the TMG is paralleling the desired track then simple trig shows that it can't work.

For the method to be valid it must at least in theory get the aircraft back onto track. If it can't do it in theory then the method is not valid because it fails to do what it is designed to do.

For the method to work in both theory and practice, the first step must be to parallel the desired track.

The method is sound. Just the application (as per your exact description) seems to have a minor flaw.

The steps are simple;

1. Deduce track error. and turn by this error towards the desired track.

2. Measure distance off track (this will no longer be an increasing figure).

3. Turn by SCA towards track and hold for time in minutes = distance off

4. When time up, Turn by SCA in the opposite direction to maintain desired track.


Unfortunately, your explanation and the method many are using is to do the following;

Bloggs is 3nm right of track with a 10 degree error.

Bloggs turns left by SCA and hold this for 3 minutes.

Bloggs think that they are back on track but they are not. They are 3nm to the left of the track they they had been flying.

Let's use a method that really does have the potential to get the flight back on track.

As I said, draw it on paper and see that the TMG must parallel desired track for it to work. It is not a big problem and means that when correctly applied once back on track the pilot does not have to remember what the error was all they have to do is turn by the SCA again but in the opposite direction.

[DFC]

I gather from the lack of a response that you may be confused BEagle so I have managed to draw a diagram which should make it easier to understand;



In both cases the pilot departed A for Z and after some time found themselves about 3nm off track.

In the first case, the pilot turns through the SCA for 90Kt and flies that for 3 minutes (4.5nm). During this time they deduce the error and when they think they are back on track they correctly fly the correct heading.

In the second case the pilot first corrects the error and then applies the SCA.

What is common to both examples is that

a) the pilot is 3nm right of the desired track when the SCA is used; and

b) By applying the SCA the pilot moves the aircraft 3nm left of the track being followed when the turn was made

However, because in the first case the track being followed is diverging from the desired track there is an error induced and simple trig shows that SCA can never get the aircraft back onto the desired track.

Therefore to correctly apply SCA, the first action must be to get rid of the error (parallel desired track)

Hope that makes my point clearer?

[Dan Winterland]

DFC, I'm not sure you understand what BEagle is saying. He wrote "Let us imagine that we have been accurately flying the first leg of our navigation exercise at 90 kts on a heading of 040° when we notice that we are 4 miles left of track". You wrote "In the second case the pilot first corrects the error and then applies the SCA".

BEagle has told us that we have been flying accurately, therfore the error is not through flying an incorrect heading - it's down to applying the wrong drift correction, probably due to an inaccurate forecast. If this is the case, the correction needs to be applied when back on track. If the error is because we realise we have been flying the wrong heading for whatever reason, then it's quite obvious that we will have to make some sort of heading correction before applying the SCA.

You actually demonstrate this in your second diagram where you show a heading adjustment, whereas your first diagram is incorrect in that if you continued on your original heading, you will in fact still diverge from track. You do however, correctly state that the heading error itself will lead to a slight displacement. However, this will be small and in the overall scheme of things, accurate enough to get you within visual range of your next turning point.


SCA does work very well in practice. The technique was designed for military pilots operating in the low level environment where navigation is made far more difficult because you can't see your turning or check points until you are close to them and also because the workload is high. When you determine you have a track displacement error, you have to decide what caused it. Was it a cumulative error down to innacurate drift calculation, or was it a once only error due to having to avoid weather/a SAM site or as a result of evading an enemy fighter? In the former, a correction has to be made before applying the SCA , in the latter - the original heading was probably good and no correction is neccessary.

I learnt it when I trained in the RAF and then later taught it as an RAF QFI for 6 years. With lots of experience of the SCA, I would say it's a very good technique. Although it's probably of more relevance in a Tucano flying at 250' at 240knts, it is still very valid at lower speeds. Thats why I later taught it in the flying club environment. I found it superior and easier for a student to apply than other techniques such as the new track reference.

[Cows getting bigger]

.... and you guys wonder why some of us avoid SCA?

[DFC]

Dan,

I think that you have missed the whole point.

SCA is based on tracks and right angle triangles formed by tracks.

It makes little difference why the track made good (TMG) is not equal to the planned track. The important thing is that they are not the same some something has to be done.

To describe it 100% correctly, in zero wind, SCA will move the aircraft Xnm to the side of the track currently being flown if the SCA track is held for X minutes.

In the first diagram and the second diagram, the SCA moves the aircraft 3nm to the left of the track being flown at the time when it is started. However, as can been clearly seen from the first diagram, moving the aircraft 3nm to the left of a diverging TMG will never put it back on the planned track. - That is simply impossible because due to the divergence, the distance between the TMG and the planned track is increasing all the time.

Correcting the error first causes the TMG to parallel the planned track. Therefore, if the distance between the two parallel tracks is converted into time for the SCA, the SCA will get the aircraft back onto the planned track.

Again I say that this has nothing to do with headings. It is a track issue. The 18 degree track error in the above could be caused by;

Incorrectly set DI,

Correctly set DI but wrong heading flown

Wind stronger than planned

Wind not as strong as planned

Compass telling lies!!!

It makes little difference to the theory because the whole theory is based on tracks.

The procedure will not be 100% accurate with winds unless one adjusts the headings and timings for wind efffects - again I say that it is a track procedure. However, over short times in relatively light winds, it will not be that far out - provided that the procedure is correctly used.

Finally, Replace the above track error with 40 degrees!!

In example 1, the SCA track would initially parallel the planned track and after the time has elapsed the pilot having noticed the 40 degree error would revise their heading by 40 degrees and fly a track paralleling the planned track - still 3nm to the right.

In example 2, the pilot would first deduce the error and turn by 40 degrees to parallel track. Then they would apply the SCA which would move the aircraft 3nm left of the current track - which happens to be where the planned track is.

Try it both ways on a piece of paper - unless the TMG is paralleling the planned track it is simply a mathematical imposibility to get back onto the planned track.......if for no other reson than there is no right angle triangle!!!

The procedure is good and sound if operated correctly.

[jb5000]

Basically what I think you're saying is...

Fly the SCA back towards your track from the 'new' drift-corrected heading rather than the 'old' one that got you off track in the first place?

Why I doubt I ever really noticed the difference:

1) In your example 18 degrees is a huge track error! This translates to only a 1 mile difference.

2) Are you able to judge the 3nm off track so accurately? If it's further away than that your method actually makes things worse.

3) Small instrument errors, slight heading / speed inaccuracies, wind changes etc. all add up as well.

4) 90knots is a fairly sedentary TAS, anything faster and the effect becomes less and less pronounced.

[DFC]

Basically what I think you're saying is...

Fly the SCA back towards your track from the 'new' drift-corrected heading rather than the 'old' one that got you off track in the first place?

Yes.

True that if the errors are small initially then the difference will also be small and that many errors in the system also come into play.

However, in order to be a valid method of "regaining planned track" then it must at least in theory acheive the aim. This is impossible unless the initial error is corrected (the planned track is paralleled) first.

Here are some examples that while extreme, demonstrate the point;



The first example is where the error is 40 degrees - the resultant of applying SCA and then track error correction is that the aircraft parallels the planned track throughout.

The second example (even more extreme) shows that the SCA track takes the aircraft further away from the planned track.

However:



The above example shows that even taken to the absurd (but not unheard of!!) error of 120 degrees, correcting the track error and then applying the SCA will always bring the aircraft back onto the planned track.

So fix the error and then SCA back to planned track is a valid use of SCA to regain planned track while apply SCA to the track being flown in error followed by correcting the track error will never do better than a small error and could make the error worse.

I am not suggesting a major change - just a simple change to how SCA is being used which makes it 100% work. The only thing that needs to be changed is the sequence of actions - which make it foolproof (in theory)!

[what next]

Hello!

Are you able to judge the 3nm off track so accurately?

I always wondered (same with the rule of sixty): If people are able to determine their track error with such precision, why did they get off track in the first place?

To me, all these methods seem purely academic anyway, especially in the year 2009 (soon 2010!). I rather train my students during their all-too-short and outrageously expensive (at least in my part of the world) flying lessons in state of the art navigation methods that will enable them to stay on their intended track all the time.

Greetings,
Max

[DFC]

what next,

I see that I am overhead X when I should be overhead Y. The map tells me that;

a) From the pre-drawn drift line the track error is ... and

b) the distance from X to Y is ........

More importantly the place to teach navigation is on the ground. The place to practice well understood and used techniques is in the air.

However, as I said at the start this is not a debate about methods. This is about using SCA in a way that will work rather than having to rely on the error being within a very limited rage in order to get almost back onto the planned track.

Please feel free to start another thread telling us about your methods that kep flights on track. You can include the method you use to (once clear of the airspace) return to planned track when ATC clear you to leave the zone on a track that is 120 degrees different from what you expected - refer to the above diagram for the SCA method. I would be interested in your methods that don't inlcude GPS as a sole source of the information.

[Dan Winterland]

No DFC, I think it's you who is missing the point. The whole point of the SCA is that it's quick and easy to use. You seem to be transforming it into your next thesis. Navigation can be made simple and pilots like me like simple. The way the RAF teach navigation is that you should be on track as much as possible and if you are off track, you should either fly to a feature on track you can see, of if you can't see one - use the SCA.

But to try and explain how the SCA should really work, look at the following diagrams, for which I didn't use a computer as I'm not that clever - I used a Tucano nav plotter which is set for a speed of 240knots. These diagrams are drawn as accurately as I could - which is probably more accurate than I can fly!



In the left hand diagram, the pilot sets off on a track of 360. He has chosen a good check point at 4 minutes, a mast with good vertical profile which he thinks he will be able to see and he can fly to if he's off track. However, he's made the mistake of believing the met man when he said there was no wind, when there's actually a wind of 270 at 60. Getting clost to his mast, he can't see it because it's into sun, so he can't use the ''fly to a feature'' technique. At 4 minutes he sees it to the left at what he estimates is a range of 4 miles. His brain goes into action and he realises that

a) he must get back on track and....

b) he's got to do something about maintaining track when back on.

He activates the SCA part of his brain which has already been programmed that flying at 240 knots, 240 divided by 60 equals 4 miles a minute, 60 divided by 4 is 15 and therefore 15 degrees is his SCA. (He doesn't have to do the maths in the air, he already knows the SCA for his speed). So, he decides to change his heading 15 degrees for 4 minutes, (one minute on the heading for as many miles you are off track) and he will fly this heading of 345 until the stopwatch gets to 8 minutes.

Now he's turned, he wonders what sent him off track. Was it a cumulative error or was it a once only error? He noticed he was drifting quite a lot, so he decided it's cumulative caused by believing the met man. He does a quick calculation and work out that after 16 miles he was 4 miles off track, and that equates to about 15 degrees of error. So he makes a decision that he will change his heading to 330 until 8 minutes and at 8 minutes he will fly heading 345. He flys this accurately and as you can see, he is almost on track. Not exactly on track, but will certainly be able to see his waypoint at 10 minutes.



In the right hand diagram, the pilot encounters weather at 2 minutes. He's flown to the right of track, but he doesnt know how far until he sees the mast to his left at what he estimates to be about four miles. The SCA training kicks in and he applies a heading of 345 which he will hold until 8 minutes. He then decides whether it's a cumulative or a once only error. he knows he got off track avoiding weather, therefore it's a once only error, so there is no additional correction to add.



It's not perfect, but it's the best there is in the circumstances. Errors include the fact that the trigonmetry of the SCA based on the 1 in 60 rule isn't exact, the groundspeed may not exactly be divisble by 60, the distance off track can only be estimated and that there is usually a short delay while mental gymnastics take place - for example, the pilot in case A above will have spent a bit of time on the heading of 345 before deciding he should really be on 330. But, the point is that the pilot will end up close enough to track to identify and fly to his next turning point.

SCA sounds complex at first, but the beauty is that it's quick and easy to use once you have had a bit of practice at it - and it doesn't require a visual feature on track to fly to.

As I said, I've used it all my career while flying visual navigation and it works well in practice.

[DFC]

Dan,

First you have not followed the sequence used by BEagle and many others when teaching SCA. Therefore, you lie somewhere between. However, as can be seen from the following diagram where I have accurately drawn your example a simple change in how you think of what your are doing will

1. Improve the accuracy; and

2. Show that the method has at least the potential to get you exactly back on track - provided the correct actions are taken.



On the left is your example drawn accurately;

A is the start point - B is the point where you apply the SCA.
C is the point 1 minute later where you decide the track error and apply that also.
D is where you are when you think that you are back on track.

Note that in your example [B]the SCA has not taken the aircraft back towards planned track (since B to C is paralleling the planned track). It is only through application of the wind correction that the aircraft starts closing with the planned track. How confusing is that to a student ( the standard closing angle does not close with anything)?

To the right is the exact same scenario using the sequence I am talking about;

A is the strat point - B is the point where the track error is corrected.
C is the point 1 minute later where the SCA is applied for 4 minutes
D is where you are back on track - and had you done as I say you would be exacly back on track!!

SCA does dork very well if it is done properly.

Both examples would suffer equally because the leg back towards track is more into wind thus - your example would be more off track that shown and my method would be slightly off track.

The circles approximate the minimum visibility possible for VFR in class G. (they are 0.9nm radius which is a little generous!!)

I am not seeking to reinvent SCA or do something that you are not already doing.

You applied SCA then MDR (drift correction).

I am saying that MDR (drift correction) followed by SCA is more accurate, involved the same actions by the pilot (but in different order) and can in theory actually work whereas SCA first can never.

Looking at your righthand diagram. The reason why SCA works perfectly in that example is because the TMG is paralleling the planned track when SCA is applied - exactly what I am talking about.

I don't know why your friend flying in Australia does not know how far off track they are having avoided the weather. Had the used SCA to move the required number of nm right of track to clear the wx then (all being well), they would apply the same SCA to get back to track.

SCA sounds complex at first, but the beauty is that it's quick and easy to use once you have had a bit of practice at it - and it doesn't require a visual feature on track to fly to.

SCA is not complex. How easy is it to check the drift line, kill the drift and then SCA for the required number of minutes back to track. - Very.

Your "fly to a feature on track" system is indeed valid - provided that it is not repeated again and again. In other words - look out, see the mast, fly to the mast and when there fly the correct heading - the one that would have taken you to the mast with no error. To do that the pilot still has to be able to determine the drift and MDR the correct heading. Otherwise it is track crawling.

In sumary, I am not adding or removing anything from what you describe. I am merely changing the order of actions to make the resuly more accurate and the method valid as a "get you back on track" method.

Get the idea?

[DFC]

Dan,

To clarify a little further I have changed your scenario slightly.

In this case, the Met man has told the pilot that the wind is 090 at 60 and the pilot has planned based on that. It turns out that the wind is calm. Let's see what your pilot does;



The left example follows your use of the SCA.

Pilot Departs A. At B they discover that they are off track and apply the SCA. The think about it and decide that the Metman was wrong and there is no wind so the heading to fly the planned track is 000. So they do not change the SCA heading further. Unfortunately, after the SCA time is up they are no closer to planned track than they were at B!!

Following what I say (right diagram) - pilot determines that metman was wrong and there is no wind so they fly 360 becuase that is the heading they should have flown since A and it kills the drift. They then apply SCA for 4 minutes because they are 4nm off track and hey presto they are exactly back on track.

So SCA and then drift correction does not work in this case.

Can we not use a SCA procedure that will work and not just in certain cases?

Ask yourself why the right hand diagram is the exact same in both cases while the diagram of your system (the left one) changes?

BEagle is being very quiet. Will be stride in and put us both right?

[Dan Winterland]

Yes, I can see what you are getting at. However, although I didn't allow any time to re calculate the drift after applying the SCA and therfore that would have put me off track, I see you have allowed one whole minute, which puts you over a mile off track at the conclusion of 4 minutes. The drift calcualtion in real life should take less than that - I would reckon more than 15 seconds is excessive so it would happen in less than a mile. This would put you about a quarter of a mile off track which in the scheme of things is nothing when you are trying to find your next turning point. And my example of an extra 60 knots drift is excessive - I used that figure to keep the maths easy and to illustrate the point. If the wind is more than 15 knots more than planned, I would say that was unusual. This would make the error 1/16th of a mile at most.

I was telling you how it's taught at the RAF Flying Training Schools. I'm not too concerned how to refine the technique for my own use as I don't do much of this sort of navigation these days. Most of my navigation is now done by a triple IRS with GPS updates and with my feet on the footrests while reading the newspaper and drinking coffee! And I doubt the RAF are going to change their technique to include a second change of heading to make the track correction extremely accurate in the case of a cumulative error. That would make the tecnique too complex for the environment it was designed for.

When your at low level in turbulence so that you can't read the map properly and you're contour flying and looing out for other aircraft which may be trying to bounce you, the KISS (Keep It Simple Stupid) principle is a good one.

You have to remember that ther are a lot of variables in navigation, especailly at low level. Are you flying the exact speed? Have you recalculated the drift correctly? have you assessed your track error correctly? When you take all these into account, you will see there is a heavy element of the TLAR (That Looks About Right) technique involved.

The level of accuracy you advocate just doesn't happen in real life.

[Dan Winterland]

DFC - replying to your post at 11.56.

Your case A hasn't applied a correction. He should have realised that the error is a cumulative one and a correction to get back on track as well as the heading correction is applicable. He should fly 345 for 4 minutes then correct to 360.

Just like your case B has!

In all cases the SCA will get you back on track, but if it's a cululative error such as an incorrectly applied drift, that has error has to be corrected as well.







I doubt BEagle will say much different to me when he comes back on. He learned the SCA at the same place a me, we were on the same Squadron and later he was the CFI of the flying club I instructed at.

[BEagle]

Agreed, Dan.

1. You find yourself off-track, despite having flown your calculated heading and speed as accurately as you thought possible.
2. Turn towards track by the SCA - do not re-synch the DI at this stage.
3. Whilst puttering back towards track, re-assess the original distance off - was it 3 or 4 miles? Make a note of the drift angle.
4. When time is up, turn onto original heading.
5. THEN re-synch DI, double check rudder trim or ball deflexion.
6. If DI and ball were spot on, error must have been wind. So apply whatever the drift angle was.
7. Amend ETA for any SCA of more than about 30 deg.
8. K.I.S.S

It might not get you back within .035 miles of track, but it will enable you to identify your visual fix points more easily.

AND IT WORKS JUST FINE!!

[DFC]

3. Whilst puttering back towards track,.......

Problem is (as can be seen from several of the above diagrams) - you may not be "puttering back towards track"!!!!!

To use the most recent example;

1. You find yourself off-track, despite having flown your calculated heading and speed as accurately as you thought possible.

No problem so far
2. Turn towards track by the SCA - do not re-synch the DI at this stage.

a left 15 degrrees turn in the above examples

3. Whilst puttering back towards track, re-assess the original distance off - was it 3 or 4 miles? Make a note of the drift angle.

Problem - you are not "puttering back towards track - you are paralleling the planed track

4. When time is up, turn onto original heading.

And start diverging in the same way as before having got no closer to planned track

5. THEN re-synch DI, double check rudder trim or ball deflexion.

OK - DI out by 15 degrees. Turn onto correct heading and now parallel track - further off then when the arror was first noticed.

6. If DI and ball were spot on, error must have been wind. So apply whatever the drift angle was.

[B] OK so the DI is spot on and we now turn 15 degrees into wind. We parallel the planned track still further away than when we discovered the problem!!![B]

7. Amend ETA for any SCA of more than about 30 deg.

You are never going to get to the fixc so there is no ETA!!!

8. K.I.S.S

Is that KISS?

Here is a picture of what you describe as KISS. I have copied your numbered instructions to the appropriate points so that there is no confusion as to what happens when we do exactly as you say;




I again ask the question is your KISS method doing what it is expected to do? The picture above says no!

Why not simply;

1. check the track error (drift line)

2. Turn by track error

3. Assess miles off.

3. Turn by SCA and hold for required minutes

4 Turn by SCA in the opposite direction.

Now that is KISS

[DFC]

I think we have to go back to basics;

SCA works on the approximate application of SIN / COS in a right angle triangle.



In the left diagram, A - B is parallel to C - D. The triangle formed by a line crossing the two lines will be a right angle triangle.

In the right diagram, the lines are not parallel. Just like in navigation where we are diverging from the planned track.

The triangle formed will not be a right angle triangle.

Thus SCA (which is based on a right angled triangle) will never work when the lines are diverging. It is a mathematical imposibility.

So are we asking the student to (as well as everything else) decide if the error is big or small?, going to leave us with a big or small (or even bigger than we started with) error after using SCA.

That is not KISS.

[groundfloor]

Whew! Learnt the academic 1 in 60 rule which was used for basic flying training - worked like a charm!

When arriving at a checkpoint selected at distance gone that divided into 60 reasonably easily - 15 20 30 - mapread to see the distance off track left or right. So 1Nm off to the right at 30Nm tracking 2deg right. 1 Nm right at 15Nm tracking 4deg right.

Then a corrective turn (double drift) to regain track in the same amount of time to the checkpoint from your set heading point, followed by a turn back onto track with a heading adjusted for the drift experienced to the first checkpoint.

Went something like this (20 Nm Point with 10 min elapsed). " I pinpoint myself on the river between Newcastle and the railway bridge, I should have been 3 miles left of the bridge, I am 5 miles left of the bridge, therefore i am 2 miles left of track which means I have drifted 6 deg left. My heading was 070 deg therefore I will steer 082 deg for 10min and then 076 deg.

Its independant of groundspeed and works

For Nav test purposes this had to be achieved before halfway on every leg.

In the real world - turn onto heading - pick a point on the horizon and fly to it.