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Aircraft position 34°15'N 098°E, magnetic variation 28°W, FL 280.
PTC VOR/DME position 36°12'N 098°E, magnetic variation 13°E.

In order to read the most accurate ground speed given by the DME receiver from his present position, the pilot must fly on which PTC radial?

  • A
  • B
    Aircraft will not receive DME information from PTC due to the line of sight rule.
  • C
  • D

This question has given us two locations with coordinates, and these can be drawn out if necessary. With these types of questions, you must look for which coordinates are the same. In this case, the 98ºE longitude is the same for both, meaning that the aircraft and the VOR/DME are on the same meridian.

Looking at the coordinates again, the aircraft is directly south (True South) 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. Technically, going directly away slightly reduces the errors due to slant distance vs ground distance, as the aircraft is getting further from the DME, which is more accurate. This does not affect this question though, as the radial is the same one no matter which direction we fly along it.

As the aircraft is directly True South from the beacon, then the radial that the aircraft would be flying on is whatever is True South of the VOR, so we must simply work out what magnetic radial corresponds to True South at the beacon.

Compass Deviation Magnetic Variation True
167º 13ºE (+) 180

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 167º

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This question has appeared on the real examination, you can find the related countries below.