Herewith my idiots' guide to the SCA method:
Most PPL text books give students a number of suggestions concerning methods of visual navigation. All these are based on the well-proven ‘1 in 60’ method which is a technique for calculating correction angles using estimates of off-track distance errors. Equally, it is also possible to make track corrections based upon estimates of angular track errors, a technique which is still quite popular.
However, most of these methods suffer from the drawbacks of either requiring relatively difficult mental arithmetic or correcting not back to the planned track with its associated pre-planned visual fixes, but direct to the next planned turning point. Recent advice from senior ex-CAA Examiners suggests that a rigorous navigation technique is required which instead does allow pilots to correct back onto their pre-planned track. Traditional techniques have not provided pilots with a simple method for achieving this; however a method originated in the RAF is available which makes track correction from observed off-track distances extremely straightforward.
Those of you whose eyes glaze over at the thought of trigonometry can skip thes paragraph as far as the bold portion if you like, but for the rest of you it works like this: If you realise that you are a miles off track and wish to fly b miles back on to track, then you need to turn through an angle φ whose sine is equal to a/b. Now the 1 in 60 rule tells us that φ is more or less equal to (a/b)x60 and if you fly your distance b at v miles per minute for t minutes, then φ = (60/v)x(a/t). If a and t are made numerically the same, that is you fly for the same number of minutes as your number of miles off track, then a = t and a Standard Closing Angle φ of (60/v) can be used where v is expressed in miles per minute. Hence the SCA at 360 kts is 10°, at 120 kts it is 30° and at 90 kts the SCA is 40°.
This method is really only completely accurate when TAS equals GS; it was originally used for navigation in fast aircraft at low level where the difference between these two values is not significant. The error will be greater at lower speeds, but is quite acceptable as the SCA technique assists pilots in reducing track error to a point from which readily identifiable pre-planned visual fixes can be observed and overflown. Similarly, timing errors will be introduced with a large SCA as the aircraft’s along track velocity (more trigonometry, sorry!) is v cos φ rather than v. This can be overcome either by reducing the SCA and increasing the correction time correspondingly, or by making an appropriate timing correction. In practice it is better to return to track as soon as possible, but only if a simple method for correcting the timing error can also be achieved.
Considering the PA28 with a 90 kt cruising speed, things now become quite simple. The SCA is 40° and cos 40° is 0.766 which is as near as makes no odds 3/4, so what should have taken 3 minutes on track will now take 4 minutes on a 40° SCA, i.e. 1/3 longer. These values will later be used in summarising the SCA method for use by PPL students cruising at 90 kts. (In a Warrior at 105 kts, theoretically the SCA is 34° and the corresponding ETA delay is 1/4 the track correction time, but for all intents and purposes it’s easier just to stick to the same 40° and 1/3 as for the Cherokee).
It is also necessary to examine why the aircraft was off-track in the first place. Assuming that pre-flight planning was correctly completed, several factors could have caused the aircraft to be off-track. For example, was the DI correctly set against the compass and was the slip ball properly centred? Did the pilot fly the aircraft accurately on the planned heading? If the answer to all those questions is yes, then the only possible cause of the error (barring ATC or divine intervention) must be that the wind velocity was other than the forecast value – a not unknown phenomenon! Having regained track, due correction can also be made for the change in drift which can readily be deduced by reference to a drift line drawn on the map. Because, if the pilot flew the aircraft accurately and yet discovered a track angle error of ψ°, then when back on track and with the DI re-aligned, the heading may be altered by the same angle ψ to correct for drift. In the correct direction, of course!
To assist in making these estimates, consider now the subject of map preparation. The start point and turning points should be marked with a circle and the track between drawn in. Timing marks every 6 minutes may be added as must the exact elapsed time at readily identifiable visual fixes roughly corresponding to easy fractions of the way along the leg (to make proportional timing correction reasonably straightforward) and at the turning point. A single 10° fan line from the start point for each leg should be drawn, to allow assessment and correction of drift error as described above. Finally the heading (not track) for each leg should be written on the map and a note made of the W/V at the level being flown together with the associated max drift value, as well as the safety altitude. Estimating distance from the CAA ½ million chart is straightforward enough by reference to the known dimensions of ATZs, MATZs and, of course, the latitude marks.
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.
Although SCA has its sceptics, it is a very simple and easy way for pilots to correct navigation errors and to regain their pre-planned track and it’s the method I require to be taught to all new students. But none of this is going to be much use if a pilot hasn’t planned accurately in the first place, flown accurately or thought ahead!
BEags