DGCA Air Navigation: Dme Questions
239 exam-style Dme 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
A DME station is located 1000 feet above MSL. An aircraft flying at FL 370, 15 NM away from the DME station, will have a DME reading of:
- A 16 NM
- B 14 NM
- C 15 NM
- D 17 NM
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Correct answer: A. 16 NM
DME measures slant range, so with the aircraft 36,000 ft (about 6 NM) above the station and 15 NM horizontally, the slant distance is the square root of 15 squared plus 6 squared, about 16 NM.
- Q2
Which of the following will give the most accurate calculation of aircraft ground speed?
- A A DME station sited on the flight route
- BAn ADF sited on the flight route
- C A VOR station sited on the flight route
- DA DME station sited across the flight route
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Correct answer: A. A DME station sited on the flight route
A DME sited on the flight route measures range directly along track, so its rate of change gives groundspeed most accurately. A DME offset across the track or a bearing-only aid gives a poorer result.
- Q3
In which situation will speed indications on an airborne Distance Measuring Equipment (DME) most closely represent the groundspeed of an aircraft flying at FL400?
- A When tracking directly towards the station at a range of 100 NM or more
- BWhen passing abeam the station and within 5 NM of it
- CWhen overhead the station, with no change of heading at transit
- DWhen tracking directly away from the station at a range of 10 NM
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Correct answer: A. When tracking directly towards the station at a range of 100 NM or more
DME gives slant range, so its rate output equals groundspeed only when the slant angle is small. Tracking directly toward (or away from) the station at long range and high altitude keeps the aircraft on an almost horizontal line to the beacon, so the range rate closely matches the true groundspeed; abeam or overhead the reading falls to near zero.
- Q4
The time taken for the transmission of an interrogation pulse by a Distance Measuring Equipment (DME) to travel to the ground transponder and return to the airborne receiver was 2000 micro-second. The slant range from the ground transponder was:
- A165 NM
- B186 NM
- C 296 NM
- D 330 NM
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Correct answer: A. 165 NM
Radio energy covers about 12.36 microseconds per nautical mile for the round trip. A 2000 microsecond total (allowing for the fixed transponder reply delay) corresponds to a slant range of roughly 165 NM.
- Q5
An aircraft DME receiver does not lock on to its own transmissions reflected from the ground because:
- Athey are not on the receiver frequency
- B DME transmits twin pulses
- C the pulse recurrence rates are varied
- D DME uses the UHF band
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Correct answer: A. they are not on the receiver frequency
The airborne interrogator transmits on one frequency and the ground transponder replies on a paired frequency 63 MHz different, so any of its own pulses reflected from the ground are not on the receiver's tuned reply frequency and are ignored.
- Q6
The DME (Distance Measuring Equipment) operates within the following frequencies:
- A962 to 1213 MHz
- B108 to 118 MHz
- C329 to 335 MHz
- D 962 to 1213 kHz.
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Correct answer: A. 962 to 1213 MHz
DME operates in the UHF band between 962 and 1213 MHz, sharing this band with TACAN.
- Q7
A DME is located at MSL.An aircraft passing vertically above the station at flight level FL 360 will obtain a DME range of approximately:
- A 6 NM
- B7 NM
- C 11 NM
- D 8 NM
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Correct answer: A. 6 NM
Directly overhead a station at MSL, the DME slant range equals the aircraft height. FL360 is 36 000 ft, about 6 NM, so the DME reads roughly 6 NM.
- Q8
During a flight at FL 210, a pilot does not receive any DME distance indication from a DME station located approximately 220 NM away.The reason for this is that the:
- A aeroplane is below the 'line of sight' altitude
- Baeroplane is circling around the station
- C altitude is too high
- Drange of a DME system is always less than 200 NM
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Correct answer: A. aeroplane is below the 'line of sight' altitude
At FL210 the line-of-sight range to a ground DME is only about 178 NM, so a station 220 NM away lies below the radio horizon and gives no indication; the aircraft is below the line-of-sight altitude for that range.
- Q9
A typical frequency employed in Distance Measuring Equipment (DME) is:
- A 1000 MHz
- B 10 MHz
- C 100 MHz
- D 100 GHz
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Correct answer: A. 1000 MHz
DME operates around 1000 MHz, in the 962 to 1213 MHz UHF band, so 1000 MHz is a typical operating frequency.
- Q10
Distance Measuring Equipment (DME) operates in the:
- AUHF band and is a secondary radar system
- B VHF band and uses the principle of phase comparison
- C UHF band and is a primary radar system
- DSHF band and uses frequency modulation techniques
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Correct answer: A. UHF band and is a secondary radar system
DME works in the UHF band and is a secondary radar system, since the aircraft interrogates and the ground beacon actively transponds rather than reflecting the signal.
- Q11
For a conventional DME facility 'Beacon Saturation' will occur whenever the number of simultaneous interrogations exceeds:
- A 100
- B200
- C60
- D80
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Correct answer: A. 100
A conventional DME transponder is designed to handle a fixed maximum interrogation load, and once more than about 100 aircraft interrogate it simultaneously the receiver reaches saturation. Beyond this the transponder replies only to the strongest signals, so weaker or more distant aircraft are progressively locked out.
- Q12
On a DME, display counters rotating throughout their range indicates:
- A the airborne receiver is conducting a range search
- Bairborne equipment failure
- C ground equipment failure
- D the airborne equipment is conducting a frequency search
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Correct answer: A. the airborne receiver is conducting a range search
Counters spinning continuously through their full range indicate the airborne DME is in search mode, sweeping to acquire the paired replies from the ground station. Once matching replies are found the counters settle and lock onto the measured range.
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