DGCA Air Navigation: Adf & Ndb Questions
149 exam-style Adf & Ndb questions for the DGCA Air Navigationpaper (CPL & ATPL). The first 12 are free below with answers and explanations — sign up free to practise all of them with progress tracking.
- Q1
Which of the following is the ICAO allocated frequency band for ADF receivers?
- A 200 - 1750 kHz
- B 255 - 455 kHz
- C300 - 3000 kHz
- D 200 - 2000 kHz
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Correct answer: A. 200 - 1750 kHz
The ICAO band allocated for ADF and NDB operation is approximately 200 to 1750 kHz, lying in the LF and MF ranges.
- Q2
In order to obtain an ADF bearing the:
- A signal must be received by both the sense and loop aerials
- B sense aerial must be tuned separately
- Cmode selector should be switched to 'loop'
- D BFO switch must be selected to 'ON'
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Correct answer: A. signal must be received by both the sense and loop aerials
An ADF bearing requires the signal from both the loop aerial, which gives the figure-of-eight pattern, and the sense aerial, which resolves the 180 degree ambiguity into a single bearing.
- Q3
Factors liable to affect most NDB/ADF system performance and reliability include:
- Astatic interference - night effect - absence of failure warning system
- B static interference - station interference - latitude error
- C height error - station interference - mountain effect
- D coastal refraction - lane slip - mountain effect
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Correct answer: A. static interference - night effect - absence of failure warning system
NDB and ADF accuracy is degraded by static interference, notably from thunderstorms, by night effect from sky-wave contamination, and by the absence of any failure warning in the equipment.
- Q4
Which one of the following disturbances is most likely to cause the greatest inaccuracy in ADF bearings?
- A Local thunderstorm activity
- BCoastal effect
- C Quadrantal error
- D Precipitation interference
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Correct answer: A. Local thunderstorm activity
Local thunderstorm activity produces the greatest ADF bearing errors, as the needle is drawn towards the powerful electrical discharges rather than the beacon.
- Q5
The BFO selector on an ADF receiver is used to:
- A hear the IDENT of some NDB stations radiating a continuous wave signal
- B stop loop rotation
- Chear the IDENT and must always be switched ON
- D find the loop 'null' position
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Correct answer: A. hear the IDENT of some NDB stations radiating a continuous wave signal
The Beat Frequency Oscillator lets the receiver produce an audible tone from a continuous-wave (unmodulated) NDB carrier so its Morse identification can be heard; without it such a carrier would be silent.
- Q6
An NDB transmits a signal pattern in the horizontal plane which is :
- Aomnidirectional
- Bbi-lobal circular
- C a cardioid balanced at 30 Hz
- D a beam rotating at 30 Hz
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Correct answer: A. omnidirectional
An NDB radiates equally in all directions in the horizontal plane, giving an omnidirectional (non-directional) signal from which the ADF derives relative bearing.
- Q7
An aircraft is ""homing"" to a radio beacon whilst maintaining a relative bearing of zero. If the magnetic heading decreases, the aircraft is experiencing :
- Aright drift
- Bleft drift
- Czero drift
- Da wind from the west
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Correct answer: A. right drift
When homing with a relative bearing of zero, a decreasing heading means the aircraft must keep turning left to hold the beacon ahead, which indicates it is experiencing left drift.
- Q8
'Night Effect' which causes loss of signal and fading, resulting in bearing errors from NDB transmissions, is due to:
- Askywave distortion of the null position and is maximum at dawn and dusk
- B interference from other transmissions and is maximum at dusk when east of the NDB
- C static activity increasing at night particularly in the lower frequency band
- Dthe effect of the Aurora Borealis
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Correct answer: A. skywave distortion of the null position and is maximum at dawn and dusk
Night effect on NDB bearings is caused by sky waves returning from the ionosphere and distorting the loop null position, and it is worst around dawn and dusk when the ionospheric layers are changing. This produces fading and bearing errors, particularly at longer ranges.
- Q9
Quadrantal errors associated with aircraft Automatic Direction Finding (ADF) equipment are caused by:
- Asignal bending by the aircraft metallic surfaces
- Bsignal bending caused by electrical interference from aircraft wiring
- C misalignment of the loop aerial
- D skywave/groundwave contamination
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Correct answer: A. signal bending by the aircraft metallic surfaces
Quadrantal error in ADF arises because the incoming signal is bent by the aircraft's metallic structure before reaching the loop, biasing the indicated bearing. The error is greatest on quadrantal relative bearings, hence the name, and is compensated during calibration.
- Q10
Errors caused by the effect of coastal refraction on bearings at lower altitudes are maximum when the NDB is:
- A inland and the bearing crosses the coast at an acute angle
- B near the coast and the bearing crosses the coast at right angles
- C inland and the bearing crosses the coast at right angles
- Dnear the coast and the bearing crosses the coast at an acute angle
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Correct answer: A. inland and the bearing crosses the coast at an acute angle
Coastal refraction bends a radio wave as it crosses from land to sea, and the resulting bearing error is greatest when the NDB is inland and the bearing crosses the coastline at an acute (shallow) angle. A bearing crossing the coast at right angles suffers little or no refraction error.
- Q11
There are two NDBs, one 20 NM inland, and the other 50 NM inland from the coast. Assuming that the error caused by coastal refraction is the same for both propagations, the extent of the error in a position line plotted by an aircraft that is over water will be:
- A greater from the beacon that is 50 NM inland
- Bthe same from both beacons when the aircraft is on a relative bearing of 180° and 360°
- C greater from the beacon that is 20 NM inland
- D the same from both beacons when the aircraft is on a relative bearing of 090° and 270°
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Correct answer: A. greater from the beacon that is 50 NM inland
For the same refraction error, the angular bearing error translates into a larger position-line displacement the further the beacon lies from the aircraft. The beacon 50 NM inland is more distant, so its position line is displaced the most.
- Q12
Which of the following is likely to have the greatest effect on ADF accuracy?
- AInterference from other NDBs, particularly at night
- BFrequency drift at the ground station
- C Interference from other NDBs, particularly during the day
- DMutual interference between aircraft aerials
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Correct answer: A. Interference from other NDBs, particularly at night
The dominant limit on ADF accuracy in practice is interference from other NDBs, which becomes far worse at night when sky-wave propagation lets distant beacons on the same or nearby frequencies be received. By day ground-wave-only reception keeps such interference low.
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