Learn QDM and QDR bearings, convert ADF indications, track to or from an NDB, apply reciprocals and avoid wind-correction errors.
In general flight simulation, QDM is the magnetic bearing from the aircraft to an NDB—the no-wind heading that would take you to it. QDR is the reciprocal magnetic bearing from the NDB to the aircraft. Pilots use QDM inbound and QDR outbound, correcting the flown heading for wind.
QDM versus QDR at a glance
| Term | Direction | Typical use | RMI indication |
|---|---|---|---|
| QDM | Aircraft to station | Inbound bearing | Needle head |
| QDR | Station to aircraft | Outbound bearing | Needle tail |
Both are expressed as three-figure magnetic bearings. For normal short-range NDB work, they differ by 180°. Add or subtract 180°, then keep the result between 000° and 359°.
For example, if the QDM is 090°, the NDB lies east of the aircraft. The corresponding QDR is 270°, meaning the aircraft lies west of the beacon.
What does the ADF needle show?
An ADF needle points towards the NDB, but the number displayed depends on the instrument fitted. It does not always present QDM directly.
- Fixed-card ADF or RBI: the needle shows relative bearing from the aircraft's nose. Add that relative bearing to magnetic heading to calculate QDM.
- RMI: the needle head indicates QDM directly against the magnetic compass card; the tail indicates QDR.
- Manually rotated ADF card: once the card is set to the aircraft's magnetic heading, it behaves much like an RMI. It must be reset after every heading change.
Suppose the aircraft is heading 070° magnetic and a fixed-card ADF shows the beacon 40° to the right. The QDM is 070° + 040° = 110°. Its reciprocal QDR is 290°. If a total exceeds 359°, subtract 360°.
Instrument presentation is a frequent source of reversed bearings. Our explanation of fixed-card ADF, RBI and RMI indications shows how the same bearing appears on each display.
How are QDM and QDR used with an NDB?
Use QDM to define an inbound line to the beacon and QDR to define an outbound line from it.
- Tune and identify the NDB. Confirm its identifier rather than trusting needle movement alone; an untuned or incorrectly tuned ADF can still produce misleading indications in some simulator aircraft.
- Find the present bearing. Read the RMI head and tail directly, or calculate QDM from magnetic heading plus relative bearing.
- Choose the required line. An instruction to track QDM 090° means approach the NDB along the line west of it. Tracking QDR 270° means depart westwards from the beacon along that same line.
- Intercept the line. Use a suitable intercept heading, commonly around 30° to 45° from the required course when there is enough room. Monitor the RMI head for an inbound QDM or its tail for an outbound QDR.
- Turn onto the track. As the required bearing is reached, reduce the intercept and establish an initial wind-corrected heading close to the QDM inbound or QDR outbound.
- Correct drift. If the required bearing starts changing, adjust the wind correction rather than repeatedly pointing the aircraft directly at the needle.
Near station passage, the ADF needle may swing rapidly or become unstable because a tiny position change produces a large bearing change. Do not chase it with abrupt turns. Continue through the passage, then establish the outbound QDR once the indication settles.
Why can the flown heading differ from QDM or QDR?
QDM represents the heading to the station only in still air; QDR defines the aircraft's bearing from it. With a crosswind, the aircraft must crab into wind, so its magnetic heading differs from the desired track.
A mistake we see constantly is keeping the ADF needle on the nose. That is homing, not wind-corrected tracking, and it produces a curved path downwind. We cover the practical correction method in our guide to ADF homing, tracking and wind correction.
Which QDM and QDR mistakes cause reversed tracks?
- Reading the wrong end of an RMI needle: the head is QDM; the tail is QDR.
- Adding 180° twice: once QDR has been derived from QDM, do not reverse it again when selecting the outbound course.
- Mixing magnetic and true values: QDM and QDR are magnetic. A simulator map or GPS may display a true track, depending on its settings.
- Forgetting to update a rotatable ADF card: an old heading setting makes every displayed bearing wrong.
- Confusing bearing with heading: the required bearing defines the line over the ground; heading is adjusted to hold that line in wind.
Does an NDB provide distance as well as bearing?
An NDB and ADF provide direction, not distance. Distance requires another source such as DME, GPS or a timed navigation calculation. A nearby DME is not necessarily co-located with the NDB, so use the published position rather than assuming both signals originate from one point.
Where the procedure combines both aids, our guide to using an NDB bearing with DME distance explains how the two indications fix the aircraft's position.