Figure 20-8: Tracking
In Figure 20-8 the line A-B passes through Point C where the heading change is made, and is parallel to the required track. At Point C, which is 4 degrees to left of track, a 4-degree right heading change would parallel the track. Angle X equals Angle Y (the closing angle) and using the 10 degrees drift lines we can determine that Angle Y equals 7 degrees. Therefore at Point C, an 11-degree right heading change (4 degrees + 7 degrees = 11 degrees) should permit the aircraft to track directly to destination.
Now let’s look at a practical application. In Figure 20-9 the calculated heading to fly from Leech Airport to Carnegie Airport was found to be 105 degrees compass.
Figure 20-9: Tracking-a practical application
Set heading time was 15:00 Z. No reliable pinpoint was available until 45 minutes later, close to where the north-south railway crosses Highway 10, 82 miles along the track. Using the 10-degree drift lines we find the opening angle to be 4 degrees and the closing angle to be 6 degrees. Therefore, in order to fly direct to Carnegie Airport, a 10-degree right heading change (4 degrees + 6 degrees = 10 degrees) leads onto a heading of 115 degrees (105 + 10 degrees). As the ground speed is found to be 109 knots and there are 49 miles left to go, the revised ETA would be 16:12 Z. It should be understood at this time that the opening and closing method can be employed at any distance along the required track, and is not limited to use after passing the halfway point. Alternatively, a visual correction could have been made to the point where the original required track meets the curve in the railway line at Blackburn Mines, where a heading of 109 degrees (105 degrees + 4 degrees opening angle) taken at the time would have kept the helicopter on the track to Carnegie.
The Drift Compensation Method
Experienced pilots often establish heading by constantly compensating for drift as the flight progresses. When the heading to maintain a desired track is established by this method, drift angle can then be easily computed if you need it. Visibility must be such that enough checkpoints are visible to maintain an accurate track.
Select at least two prominent check-points a sufficient distance apart ( 5 to 10 miles or more, depending on the terrain), on the desired track ahead of the helicopter. Maintain a heading which keeps the nearer checkpoint aligned with the farther one. When the helicopter and the check-points remain on the same line, the heading indicated on the compass will be the heading which, provided there is no wind change, should keep the helicopter on the track required for the remainder of the flight. To continually compensate for wind effect, select another checkpoint on track in the distance before the nearer checkpoint is reached; then repeat the alignment and drift compensation procedure. Calculate ground speed by time and distance between checkpoints to keep ETA accurate.
THE FLIGHT
Setting Heading
After takeoff, establish the helicopter on the desired compass heading for the first leg, either over the heliport or over a convenient prominent checkpoint. Note the time and enter it in the flight log and calculate the ETA for the destination and the first checkpoint. Remember, to ensure accuracy in determining the actual amount of drift, it is necessary to begin the navigation exercise directly over the planned set-heading point. Immediately after you are established on the first heading, check the chart and pinpoints on the ground to confirm that the aircraft is in fact heading in the correct direction.
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