Wednesday, 27 August 2008

PPL Ex 18 (iv): NAVIGATION: (Measuring the Track Page 1)

This exercise follows on from the ‘briefing’ for “The Dead Reckoning Computer or Wizz Wheel”.
Remember: Click on an image to enlarge!

In this ‘briefing’ it should suffice to remind you that with the air navigation charts that we use the angles are correctly represented. The most useful chart to use is AERONAUTICAL CHART ICAO 1:500,000. It is possible to use an ordinary protractor for measuring angles (i.e. tracks) and a chart with these properties is called ORTHOMORPHIC. A desirable property of the chart is that it should be constant scale so that you can use an ordinary scale rule to measure distance. The chart presupposes a scale model of the Earth called the reduced earth on which is drawn the required details of the Earth – coastlines, railways, rivers, roads etc, and also a graticule of latitude and longitude. The graticule is made up of a series of circular lines on the sphere, but, of course, parallel lines on the chart; the lines of longitude are called MERIDIANS and join the geographic poles. The PRIME MERIDIAN joins the North Pole to Greenwich and to the South Pole. All meridians are GREAT CIRCLES and are labelled East or West. The equator is at right angles to the axis of the Earth, the lines of latitude are circles on the Earth whose plane is parallel to the equator; they are therefore small circles and are named North or South.
Assuming the Earth to be a true sphere, a nautical mile is defined as the length of arc of a great circle which subtends an angle of one minute at the centre of the Earth. This is the reason that the Airspeed Indicator (ASI) is graduated in nautical miles per hour i.e. Knots. (Sorry; I know that YOU know that!)
OK, so we have resolved the little matter of representing a spherical Earth on a flat chart. Now it is essential to understand how to measure a True Track [TRK(T)] on an Aeronautical Chart using a Douglas (square) Protractor. Accurately place the centre crosshairs of the protractor somewhere on the track that you have drawn, ensuring that the North (360˚) mark is at the top (where North is, of course) and in such a position that the track line extends beyond the side of the protractor. Now adjust the protractor so that the vertical lines of the protractor align exactly with the North/South graticule lines keeping the centre of the protractor exactly on the measured track line. It is now possible to read the True Track [TRK(T)] in the direction that you intend to fly. Note: The direction is measured clockwise around the edge of the protractor from 000˚N through 090˚E, 180˚S, 270˚W and 360˚ (000˚) N. In the picture above: 250-degrees True.

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