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Provided the helicopter has not passed the halfway point, the double track error method or the visual alteration method can be used. If the helicopter is beyond the halfway point, you can still use the visual alteration method, or alternatively, the opening and closing angle method. Each method will be discussed in detail later in this text but first some basic geometry may help.
The key to understanding how these various methods work lies in the fact that if the helicopter heading is altered in the direction of the required track by a number of degrees equal to the track error or opening angle, the resulting heading will produce a track parallel to the required track. The desirable thing now is to get us back over track so that we can fly that heading. Some orderly method must be followed to take the aircraft from its present position off track to a point on the required track. Once this is accomplished we can then take up and fly the new corrected heading. Alternatively, we have the chance of flying direct from the position off track to the destination.
Using the 10-degree drift lines in Figure 20-4, it can be seen that the track made good is widening from the required track by an angle of 7 degrees. By altering the heading 7 degrees to the right, we now have the heading that produces a track parallel to the required track and is approximately the heading to steer when the required track is regained. (This is not strictly true, because in either method mentioned below the new heading calculated to keep the helicopter over track may not be exactly correct, due to the fact that the direction of the wind relative to the new heading is altered and drift may remain what it was before. Yet for most practical purposes the change of drift caused by such a small alteration in heading can be neglected.)
If you decide to return to the required track, the primary method is the double track error method. Simply double the amount of track error or opening angle, and apply this to the original heading in the direction of the required track: the helicopter will regain track in approximately the same period of time as it took to drift off track, and the required track will be intercepted again at a distance along track equal to twice the distance from the set heading point to the point when the heading change was made (Point A in Fig 20-3). On regaining track it is necessary to subtract half the correction applied to the original heading to obtain the heading to keep the aircraft on the required track.
If a positively identified landmark on the required track can be seen, the visual alteration method can be used. Once again the key fact is the heading change that would produce a track parallel to the required track. After determining the opening angle and applying it to the heading that produced the error, we now have a heading that we can fly once we are back on the required track. By simply flying visually from the present position (A in Fig 20-4) to the positively identified landmark, the new heading can now be flown and the helicopter should remain on the required track.
Figure 20-5: Methods of making track corrections
Figure 20-6: Opening and Closing angles/ visual alteration
Now let’s look at a practical example of these two methods in Figure 20-7.
Figure 20-7: Tracking by double track error and visual alteration methods
We take off from Patterson Airport and set heading at 16:00 Z over the point where the railway line crosses the Isaacs River, at the town of Patterson. Our calculations have indicated that we must steer a compass heading of 065 degrees on our trip to Twin Cities Airport, 148 miles distant. After 14 minutes (16:14 Z) we pinpoint ourselves at the north end of Arbuckle Lake, and using the 10 degree drift lines determine that we are 7 degrees to right of track. A heading change of 7 degrees (058 degrees) left would result in a heading whereby we would parallel the required track. But as we wish to regain track using the double track error method, we alter heading to 051 degrees (065 degrees - 14 degrees = 051 degrees). This new heading is held for an additional 14 minutes and we regain track just south of the bend in the Carey River at 16:28 Z. At this time we alter heading 7 degrees right and steer 058 degrees. Any physical features near or along the track line, which will confirm that the track has been regained, will be helpful, but if none are available, the heading should be altered at the calculated time. Revised ETA can be made while flying towards the required track. If we had decided to use the visual alteration method, we could have flown visually to Danford, which is right on track and easily identifiable, and then steered 058 degrees (065 - 7 = 058) which should keep the helicopter on track.
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