Plotting — Page 486, Lesson 482

Right, let's get into the plotting chapter. We're dealing with radio bearings, and the very first thing I want you to understand is the difference between the two ways an aircraft measures a bearing to a ground transmitter. Since the bearings are measured relative to the fore-and-aft axis of the aircraft, it is necessary to convert them into true directions by use of the aircraft's compass. The compass is affected by the local variation at the aircraft. Therefore, AIRCRAFT variation is used. So, when we convert, we use the variation at the aircraft's position, not at the transmitter. Now, let's look at the two types of bearing presentation. First, the Relative Bearing. This is a bearing measured not from the direction of true or magnetic north, but from the direction in which the aircraft is pointing. For example, if an aircraft is heading in the direction of 030°(T) and there is an NDB due east of the aircraft, the bearing of the NDB from the aircraft is 060° relative. This bearing would appear on a relative bearing indication, or RBI, in the aircraft. It can be seen from Figure 28.2 that adding together the relative bearing and the true heading will give the true bearing. Second, the Magnetic Bearing. This is a bearing measured from the direction of magnetic north, and such a bearing would appear on a radio magnetic indicator, or RMI, in the aircraft. To change this into a true bearing, all that is necessary is the application of variation at the aircraft's position, where the measuring is being done. Now, let's bring in the AWR. An AWR bearing is a relative bearing from the aircraft. This is combined with true heading of the aircraft to give a true bearing TO the response, as above. This is then reciprocated to give the true bearing to plot FROM the response. So, let's formalise the conversion of a relative bearing into a true bearing. AWR bearings are always in the form of relative bearings. ADF bearings will also be relative if the presentation is on an RBI. It is necessary to convert these into true bearings which can then be plotted from the radar response or the NDB, as appropriate. Here's the sequence. First, apply AIRCRAFT variation to the magnetic heading to get true heading. Second, add relative bearing to true heading in order to get true bearing TO the NDB or radar response. If the answer comes to more than 360°(T), subtract 360. Third, take the reciprocal in order to get the True Bearing to plot FROM the NDB or radar response. Let's work through the example together. We have an RBI reading of 207. The aircraft heading is 315°(M). The NDB variation is 7°W, and the aircraft variation is 9°W. The question is: what is the true bearing to plot from the NDB? We start with the magnetic heading, 315°(M). We apply the aircraft variation, which is 9°W. Since it's west, we subtract to get the true heading. 315 minus 9 gives us 306°(T). Now we add the relative bearing, 207°(R). 306 plus 207 gives us 513°(T). That's the true bearing TO the NDB. Since it's more than 360, we subtract 360, giving us 153°(T) TO the NDB. Finally, we take the reciprocal by adding 180, which gives us 333°(T). That's the true bearing to plot FROM the NDB. Notice we used the aircraft variation of 9°W, not the NDB variation of 7°W, because the measurement is being done at the aircraft. The NDB variation is given, but it's not used in this conversion. That's the key distinction.