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!!!!
