1
$\begingroup$

If the magnetic variation at the aircraft and at the NDB station is different, then magnetic bearings would also change, so QDR = QDM + 180 would not be true. Can someone please help me understand this?

For example:

  • The True bearing to NDB is 090 degrees, so the true bearing to the aircraft will be 270 degrees
  • Variation at NDB is 10 degrees west
  • Variation at aircraft is 20 degrees west
  • So magnetic bearing to NDB will be 110 degrees and magnetic bearing to aircraft will be 280 degrees

Clearly these are not reciprocal of each other.

$\endgroup$
3
  • $\begingroup$ I’m tempted to wade in here, but you seem to have a high level of knowledge and IQ, so I am puzzled at your confusion. Because the short answer is that they aren’t necessarily the reciprocal of each other, and the reason is MagVar. As you point out in your well thought out question! Is there something still unclear? (BTW, true bearings aren’t alway the reciprocal of each other either, but for different reasons…) $\endgroup$ Dec 4, 2021 at 17:18
  • $\begingroup$ @MichaelHall in the books it's mentioned they are +-180 reciprocal of each other. which isn't true as you can see $\endgroup$ Dec 4, 2021 at 19:47
  • $\begingroup$ Can you cite a reference? I think context is important, because from a practical standpoint they might as well be reciprocal. $\endgroup$ Dec 4, 2021 at 20:30

2 Answers 2

5
$\begingroup$

QDM and QDR are abbreviations from Q code. Originally:

  • QDM: The magnetic heading for you to steer to reach me (or ...) with no wind is/was ... degrees (at ... hours).

  • QDR: Your magnetic bearing from me (or from ...) is/was ... degrees (at ... hours).

enter image description here

Actual determination of QDR and QDM according to Q Code

From these definitions it is clear the location used when determining heading for QDM and bearing for QDR is not the same, so if the variation is different the angles won't be reciprocal.

However this is not this convention which is used when talking about QDR/QDM with navaids.

NDB

A NDB station broadcasts no bearing information. So the aircraft ADF actually determines the QDM by sensing the transmitter bearing relative to its local magnetic north.

If a QDR is needed in this context it will not be computed using the actual variation at the NDB location, it will be the reciprocal angle QDR = QDM + 180°.

If the variation changes on the route to the NDB, the ADF will show an updated QDM, and the heading will be adjusted. At the end the route will reach the NDB, though the heading will have changed on the route.

enter image description here

ADF: Actual QDM sensed by ADF, QDR reciprocal of QDM

VOR

The VOR signals carry the QDR information. The VOR indicator displays this information. The broadcast angle is based on the VOR own magnetic north.

The QDM which the VOR indicates is just the reciprocal of the QDR. Therefore if the variation at the aircraft location is different, this QDM is wrong. This is the reason why it's clearer with VOR navigation to talk about radial to/from rather than QDM/QDR.

enter image description here

VOR: Actual QDR broadcast by the VOR, QDM is the reciprocal of QDR

$\endgroup$
12
  • $\begingroup$ "VOR airborne equipment receives phases of two different signals (reference and variable) and derives QDR by taking the difference between the two. Its reciprocal is QDM. An RMI accepts relative bearings and not QDMs. Therefore the VOR phase difference giving QDM is first converted to give relative bearing. This is achieved by use of a differential synchro in the VOR navigation unit which subtracts the aircraft's magnetic heading to give relative bearing before the information is fed to the RMI. The RMI subsequently adds the aircraft's heading to give the QDM." $\endgroup$ Dec 3, 2021 at 21:18
  • $\begingroup$ So it's the relative bearing that gets wrong, correct? $\endgroup$ Dec 3, 2021 at 21:19
  • $\begingroup$ I'm going through more references, Incase of an "NDB", it turns out that the QDM actually changes as the variation changes. While it is not the case for VOR. since the measurement is done at the Aircraft for NDB, and for the VOR - i've explained the process how it goes $\endgroup$ Dec 3, 2021 at 21:25
  • 1
    $\begingroup$ You guys are probably way smarter than me, but I thought the ADF was just a dumb radio receiver, and the needle simply points towards the non-directional signal. (kinda like a mag compass) Words like "determines" "approximated" "recompute" and "adjusted" imply some sort of programmable logic capability that I'm pretty sure they don't have. I think the answer is misleading because of the use of these terms. $\endgroup$ Dec 4, 2021 at 2:16
  • $\begingroup$ @MichaelHall Your comments are valid. $\endgroup$
    – mins
    Dec 4, 2021 at 11:20
1
$\begingroup$

A statement that bearing from a station to the aircraft is the EXACT reciprocal of the aircraft’s bearing to the station is almost never true. For either magnetic, (for the exact reason you describe…) or true bearing. However, from a practical standpoint and as a general rule of thumb, it just doesn't matter. The statement is accurate enough to consider it true for the purpose of navigation.

Considering your example, look at an isogonic chart and figure out how far you would have to fly to go from an area of 10 degrees magvar to 20 degrees. If you were flying an aircraft with the range to make this flight you would likely have navigation equipment more sophisticated than just an ADF receiver.

Most GA aircraft that would need or want to home on an NDB are only going to cover an area where magvar varies by a few degrees. Given the effect of wind, instrument accuracy, and pilot ability, trying to resolve for and accommodate the miniscule effects of the changing magvar just isn’t worth worrying about.

If you are a pilot, use 180 degrees if you need to figure the reciprocal for any reason, and don’t spend any time overthinking how you might want to adjust in flight.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .