ADF / NDBRadio Navigation — DGCA CPL practice questions
Question 1 of 19
The phenomenon of coastal refraction which affects the accuracy of ADF bearings:
All 19 questions — ADF / NDB
Radio Navigation · DGCA CPL. The correct option is marked on each.
Q1. The phenomenon of coastal refraction which affects the accuracy of ADF bearings:
- A.is most marked at night
- B.can be minimized by using beacons situated well inland
- C.can be minimized by taking bearings where the signal crosses the coastline at right angles✓
- D.is most marked one hour before to one hour after sunrise and sunset
Why: When the signal crosses the coast at 90°, there is no speed differential component along the wavefront — so no bending occurs. This is the primary mitigation. Option (b) is wrong because inland beacons mean the signal crosses MORE coast/water boundary. Option (a) and (d) describe night effect, not coastal refraction.
Q2. An aircraft is intending to track from NDB 'A' to NDB 'B' on a track of 050°(T), heading 060°(T). If the RBI shows the relative bearing of 'A' to be 180° and the relative bearing of 'B' to be 330° then the aircraft is:
- A.port of track and nearer 'A'
- B.port of track and nearer 'B'
- C.starboard of track and nearer 'A'
- D.starboard of track and nearer 'B'✓
Why: Working: Heading = 060°T. RBI B = 330° → True bearing of B = 060° + 330° − 360° = 030°T On the correct 050° track, B should bear 050°T from the aircraft (dead ahead of track). B actually bears 030°T — 20° to the LEFT (port) of where expected. This means the aircraft is displaced to the RIGHT (starboard) of the track. A is at 180° (dead astern, just passed) — B is at 330° rel (30° port of ahead). B being only 30° off dead ahead while A is dead astern confirms the aircraft is closer to B. The "starboard or port" decision: if the destination beacon is further to the port side than its nominal…
Q3. ADF quadrantal error is caused by:
- A.static build up on the airframe and St. Elmo's Fire
- B.the aircraft's major electrical axis, the fuselage, reflecting and re-radiating the incoming NDB transmissions✓
- C.station interference and/or night effect
- D.NDB signals speeding up and bending as they cross from a land to water propagation path
Why: Quadrantal error is caused by the fuselage (major fore/aft electrical axis) distorting the ADF's polar diagram by reflecting and re-radiating incoming signals. Error is maximum at 045°, 135°, 225°, 315° relative bearings (in the quadrants). Option (d) describes coastal refraction.
Q4. The overall accuracy of ADF bearings by day within the promulgated range (DOC) is:
- A.± 3°
- B.± 5°✓
- C.± 6°
- D.± 10°
Why: ADF accuracy = ±5° within DOC by day. Note this is the bearing accuracy only and does not include any additional compass error.
Q5. In order to Tune, Identify and Monitor N0NA1A NDB emissions the BFO should be used as follows:
- A.Tune ON / Identify ON / Monitor OFF
- B.Tune ON / Identify ON / Monitor ON✓
- C.Tune ON / Identify OFF / Monitor OFF
- D.Tune OFF / Identify OFF / Monitor OFF
Why: N0NA1A = interrupted unmodulated carrier. An unmodulated carrier cannot be heard without a BFO. The BFO creates an audible tone by mixing the carrier with an offset frequency. Therefore BFO must remain ON during all phases: tuning, identification, and monitoring. Option (a) is for N0NA2A (where monitoring can be done with BFO off).
Q6. The magnitude of the error in position lines derived from ADF bearings that are affected by coastal refraction may be reduced by:
- A.selecting beacons situated well inland
- B.only using beacons within the designated operational coverage
- C.choosing N0NA2A beacons
- D.choosing beacons on or near the coast✓
Why: Beacons on or near the coast minimise the over-water propagation path — so the signal spends less distance over the sea before crossing the coast, reducing refraction. Option (a) — inland beacons increase the refraction. Option (b) — DOC applies to signal quality, not coastal geometry. Option (c) — emission type has no effect on refraction.
Q7. The BFO facility on ADF equipment should be used as follows when an NDB having N0NA1A type emission is to be used:
- A.BFO on for tuning and identification but may be turned off for monitoring
- B.BFO on for tuning but can be turned off for monitoring and identification
- C.BFO off during tuning, identification and monitoring because this type of emission is not modulated
- D.BFO should be switched on for tuning, ident and monitoring✓
Why: N0NA1A contains an interrupted unmodulated carrier (A1A) which requires the BFO to produce any audible signal. BFO must remain ON throughout — tuning, identification, and continuous monitoring. This is identical to Q5 but phrased differently.
Q8. The protection ratio of 3:1 that is provided within the promulgated range/DOC of an NDB by day cannot be guaranteed at night because of:
- A.long range sky wave interference from other transmitters✓
- B.sky wave signals from the NDB to which you are tuned
- C.the increased skip distance that occurs at night
- D.the possibility of sporadic E returns occurring at night
Why: At night, the D-region disappears — distant NDBs on similar frequencies can propagate via sky wave over thousands of miles and cause interference even within another NDB's DOC. The "wanted" NDB's signal/noise ratio drops below 3:1 because of these long-range sky wave intruders. Option (b) is incorrect — it's other stations, not the tuned NDB, that cause night interference.
Q9. Each NDB has a range promulgated in the COMM section of the AIP. Within this range interference from other NDBs should not cause bearing errors in excess of:
- A.day ±5°✓
- B.night ±10°
- C.day ±6°
- D.night ±5°
Why: Within the promulgated DOC, the 3:1 protection ratio is guaranteed by day, ensuring bearing errors stay within ±5°. The protection is not guaranteed at night due to sky wave interference from distant stations.
Q10. The range promulgated in the AIP and flight guides for all NDBs in the UK is the range:
- A.within which a protection ratio of 3:1 is guaranteed by day and night
- B.up to which bearings can be obtained on 95% of occasions
- C.within which bearings obtained by day should be accurate to within 5°✓
- D.within which protection from sky wave protection is guaranteed
Why: The DOC defines the range within which (by day) the 3:1 protection ratio ensures ±5° accuracy. Options (a) and (d) are wrong — protection is NOT guaranteed at night. Option (b) is an invented distractor not matching the DOC definition.
Q11. In order to resolve the 180° directional ambiguity of a directional LOOP aerial its polar diagram is combined with that of a SENSE aerial _____ to produce a _____ whose single null ensures the ADF needle moves the shortest distance to indicate the correct _____:
- A.at the aircraft / cardioid / radial
- B.at the transmitter / limacon / bearing
- C.at the aircraft / limacon / bearing
- D.at the aircraft / cardioid / bearing✓
Why: The sense aerial is combined with the loop aerial at the aircraft (within the goniometer). The combined diagram is a CARDIOID. The cardioid's single null ensures the needle takes the shortest path to indicate the correct BEARING. Option (a) is wrong — the output is a bearing, not a radial (radials are from a ground station). Options (b) and (c) are wrong — the combined diagram is a cardioid, not a limacon.
Q12. The protection ratio afforded to NDBs in the UK within the promulgated range (DOC) applies:
- A.by day only✓
- B.by night only
- C.both day and night
- D.at dawn and dusk
Why: The 3:1 protection ratio is based on daytime conditions when the D-region absorbs sky wave from distant stations. At night, the D-region disappears — sky wave propagation from distant co-channel stations renders the protection ratio meaningless within the DOC.
Q13. The phenomenon of coastal refraction affecting ADF bearings is caused by the signal _____ when it reaches the coastline and bending _____ the normal to the coast:
- A.accelerating / towards
- B.decelerating / towards
- C.accelerating / away from✓
- D.decelerating / away from
Why: Radio waves travel faster over water (lower attenuation). When a signal crosses the coast from land to sea, it accelerates . By Snell's law, acceleration causes the wavefront to bend away from the normal (perpendicular) to the coast. This is the opposite of light entering a denser medium. Options (b) and (d) would describe deceleration (entering a denser medium).
Q14. In an ADF system, night effect is most pronounced:
- A.during long winter nights
- B.when the aircraft is at low altitude
- C.when the aircraft is at high altitude
- D.at dusk and dawn✓
Why: At dusk and dawn the D-region is in transition — partially present. The mixture of surface wave and sky wave at this time creates maximum phase interference, producing the worst bearing errors. The term "night effect" is somewhat misleading — it peaks at dusk/dawn, not during the middle of the night.
Q15. When the induced signals from the loop and the sense antenna are combined in an ADF receiver, the resultant polar diagram is:
- A.a limacon
- B.a cardioid✓
- C.figure of eight shaped
- D.circular
Why: Loop (figure-8) + sense aerial (circle) = CARDIOID when the sense aerial signal has equal amplitude to the loop signal. A cardioid has a single null, resolving the 180° ambiguity. Option (c) is the loop alone; option (d) is the sense aerial alone.
Q16. When flying over the sea and using an inland NDB to fix position with a series of position lines, the plotted position in relation to the aircraft's actual position will be:
- A.further from the coast
- B.closer to the coast✓
- C.co-incident
- D.inaccurate due to the transmitted wave front decelerating
Why: The signal from the inland NDB crosses the coast and travels over the sea toward the aircraft. Over the sea it speeds up and bends away from the coast's normal — making the signal appear to come from a direction slightly further inland than actual. The aircraft, applying this bearing to fix position, will plot itself closer to the coast than its actual position. Option (d) is wrong — the wave accelerates, it does not decelerate.
Q17. An aircraft on a heading of 235°(M) shows an RMI reading of 090° with respect to an NDB. Any quadrantal error which is affecting the accuracy of this bearing is likely to be:
- A.a maximum value✓
- B.a very small value
- C.zero, since quadrantal error affects only the RBI
- D.zero, since quadrantal error affects only the VOR
Why: Working: NDB magnetic bearing = 090°M (from RMI). Aircraft heading = 235°M. Relative bearing = 090° − 235° + 360° = 215° Quadrantal error is maximum at relative bearings of 045°, 135°, 225°, 315°. 215° is close to 225° — near a maximum quadrantal error position. So the error is near maximum. Options (c) and (d) are completely wrong — quadrantal error affects ALL ADF systems using a loop aerial, including both RBI and RMI.
Q18. The principal propagation path employed in an NDB/ADF system is:
- A.sky wave
- B.surface wave✓
- C.direct wave
- D.ducted wave
Why: NDBs operate in LF/MF bands where surface wave propagation is the primary mode. Surface waves follow the earth's curvature and provide reliable, predictable bearings. Sky wave exists at night but causes errors (night effect) — it is an undesired mode, not the intended propagation path.
Q19. The ADF of an aircraft on a heading of 189°(T) will experience the greatest effect due to quadrantal error if the NDB bears:
- A.234°(T)✓
- B.279°(T)
- C.225°(T)
- D.145°(T)
Why: Working: Quadrantal error is maximum at relative bearings 045°, 135°, 225°, 315°. Aircraft heading 189°T. For each maximum error relative bearing: — 189° + 045° = 234°T (matches option a) — 189° + 135° = 324°T — 189° + 225° = 054°T (414° − 360°) — 189° + 315° = 144°T Only 234°T appears in the options → answer is (a). Note: option (c) 225°T gives relative bearing = 225° − 189° = 036° — not a maximum error position.