Given:
Aircraft position 36°15'S 178°E, magnetic variation 21°W, FL 310.
UEB VOR/DME position 36°15'S 178°W, magnetic variation 21°E.
In order to read the most accurate ground speed on the DME receiver from his present position, the pilot must fly which UEB radial?
Refer to figure.
To complete this question fully, you will need knowledge from many subjects. Thankfully, however, the answers have been made very far apart, which means that we can make some approximations along the way and still get the correct answer.
This question has given us two locations with coordinates. With these types of questions, you must look for which coordinates are the same. In this case, the 036º15'S latitude is the same for both.
Looking at the coordinates again, and remembering that 178ºE is actually to the West of 178ºW (as the 180ºE/W meridian lies close in between, where the E/W longitudes reverse, and start counting down the opposite hemisphere), the aircraft is directly West of the beacon*. The question asks for the direction the aircraft must fly for the DME groundspeed readout to read correctly.
As the DME groundspeed readout uses "rate of change of DME distance" as the method for calculating groundspeed, then the only way for it to read correctly is by flying directly towards or directly away from the DME. Towards/Away from the VOR matters as well though, as the slant range becomes closer to the ground range the further we get from beacon, therefore it gets more accurate whilst flying away from the VOR.
As the aircraft is directly True West from the beacon, then the aircraft would be flying on is whatever radial is True West of the VOR, so we must simply work out what magnetic radial corresponds to True West at the beacon.
Compass | Deviation | Magnetic | Variation | True |
249º | 21ºE (+) | 270º |
We need to use the variation at the VOR as that is where the radial directions were decided, and emitted.
Therefore, the aircraft must fly along radial 249º.
*This is not strictly true, as VOR radials and DME pulses follow great circle tracks, and these include convergency due to the Earth's shape. The actual true radial outbound from the VOR would be 269º, making the final answer 248º in reality, but the answers give plenty of margin, and the question is much simpler without convergency.
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