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► 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) 15 NM
B) 14 NM
C) 16 NM
D) 17 NM
The DME distance is the slant range and not the perfect horizontal line between station and aircraft (15 NM). Picturing a triangle and using Pythagoras (Square of the hypotenuse (slant range) = sum of the squares of the remaining sides):
(Slant range)^2 = (15 NM)^2 + (36.000ft)^2
(Slant range)^2 = (15 NM)^2 + (5.9 NM)^2
(Slant range)^2 = 225 + 34.8
Slant range = SQRT(260)
Slant range = 16 NM
► The DME (distance measuring equipment) operates within the following frequencies:
A)962 to 1213 MHz
B) 108 to 118 MHz
C) 329 to 335 MHz
D) 962 to 1213 kHz
"A DME operates in the UHF band from 960MHz to 1215MHz"
► 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) 11 NM
B) 7 NM
C) 6 NM
D) 8 NM
Aircraft is directly overhead at 36.000ft
1NM = approx 6080ft
36.000 / 6080 = 5.9 NM
► 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:
Answer: Aeroplane is below the "line of sight" altitude
Formula for max theoretical range;
MTR = 1.23 x(SQRT 21000 + SQRT 0)
MTR = 1.23 x 145
MTR = 178 NM
So at the altitude of 21.000ft the maximum theoretical range at which we can receive the station is 178 NM. The Question states 220 NM meaning we are below the "line of sight altitude".
You could also convert the MRT formula (with 220NM) to find the altitude of the aircraft which would allow you to receive the station (somewhere around 30.000ft).
► Distance measuring equipment (DME) operates in the:
Answer: UHF band and is a secondary radar system
A primary radar: measures only the range and bearing of targets by detecting reflected radio signals
A secondary radar (DME for example): Relies on targets equipped with a radar transponder, that replies to each interrogation signal by transmitting a response
► On a DME, display counters rotating throughout their range indicates:
Answer: The airborne receiver is conducting a range search
► The aircraft DME receiver is able to accept replies to its own transmissions and reject replies to other aircraft interrogations because:
Answer: Pulse pairs are discreet to a particular aircraft
"The airborne interrogator unit transmits pulse pairs to the ground station with a distinct spacing on the tuned UHF frequency."
► In which situation will speed indications on an airborne distance measuring equipment (DME) most closely represent the ground speed of an aircraft flying at FL400.
Answer: When tracking directly towards the station at a range of 100 NM or more
- Ground speed is only accurate when flying TO or FROM the station
- A longer distance decreases the slant error (overhead, the GS will be least accurate)
► 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:
A) 158 NM
B) 186 NM
C) 296 NM
D) 330 NM
Method 1:
Slant range = Half the speed of light x (t-50)
Speed of light (in NM) = 162.000
t (2000 micro sec) - 50 = 1950 micro sec (= 0.00195 seconds)
Slant range = 0.5 x 162000 x 0.00195
Slant range = 158 NM
Method 2:
Remember that 1 Radar Mile = 12.36 microseconds
Time delay = 2000 - 50 = 1950 micro sec
1950 / 12.36 = 158 NM
► In the DME system:
Answer: The receive and the transmit frequency is always split by 63 MHz
"The ground station acts as a transponder and receives the pulse pairs. After a delay of 50 micro seconds, he wil transmit a response with the same pulse spacing but on a frequency different by 63MHz (to clearly distinguish retransmitted pulses from the transponder and reflected pulses from the ground)"
► The design requirements for DME stipulate that, at a range of 100 NM, the maximum systematic error should not exceed:
A) 1.5NM
B) 3NM
C) 0.25NM
D) 1.25NM
"ICAO Annex 10 requires an accuracy of 0.25 NM (+1.25% of the measured distance (Slant)) for systems installed before 1 January 1989"
0.25 + (1.25% of 100) = 1.5NM
► An aircraft at FL300 with a ground speed of 300kts, is about to pass overhead a DME station at MSL. The DME receiver is capable of determining ground speed. One minute before the overhead, the DME speed and distance indications are respectively:
A) 300 kt and 7 NM
B) less than 300 kt and 7 NM
C) less than 300 kt and 5 NM
D) 300 kt and 5 NM
► Given:
Aircraft position 34°15minN 098°E , magnetic variation 28°W , FL 280.
PTC VOR/DME position 36°12minN 098°E , magnetic variation 13°E.
In order to read the most accurate ground speed given by the DME receiver from his present position, the pilot must fly on which PTC radial?
A) 167°
B) 332°
C) Aircraft will not receive DME information from PTC due to the line of sight rule
D) 193°
2 Things to consider:
- GS accuracy reduces as the aircraft nears the DME
- Radials are magnetic
The station is to the NORTH of the aircraft (360°) so to get the most accurate GS reading we need to fly the opposite (180°) SOUTH
Radials are magnetic so add VARIATION (for a VOR, we take variation at station and for an NDB we take the variation at the aircraft)
180° - 13E = 167°
► Given:
Aircraft position 52°09minS 024°E , magnetic variation 14°W , FL 310.
BIT VOR/DME position 54°42minS 024°E , magnetic variation 14°E.
In order to read the most accurate ground speed given by the DME receiver from his present position, the pilot must fly on which BIT radial?
A) 194°
B) 014°
C) 166°
D) 346°
2 Things to consider:
- GS accuracy reduces as the aircraft nears the DME
- Radials are magnetic
The station is to the SOUTH of the aircraft (180°) so to get the most accurate GS reading we need to fly the opposite (360°) NORTH
Radials are magnetic so add VARIATION (for a VOR, we take variation at station and for an NDB we take the variation at the aircraft)
360° - 14E = 346°
► Given:
Aircraft position 36°15minS 178°E , magnetic variation 21°W , FL 310.
UEB VOR/DME position 36°12minS 178°W , magnetic variation 21°E.
In order to read the most accurate ground speed given by the DME receiver from his present position, the pilot must fly on which UEB radial?
A) 111°
B) 069°
C) 291°
D) 249°
2 Things to consider:
- GS accuracy reduces as the aircraft nears the DME
- Radials are magnetic
The station is to the EAST of the aircraft (90°) so to get the most accurate GS reading we need to fly the opposite (270°) WEST.
We are at the back side of the earth, aircraft is at 178°E and flying towards the anti-meridian 180° in an EAST direction towards the station at 178°W
Radials are magnetic so add VARIATION (for a VOR, we take variation at station and for an NDB we take the variation at the aircraft)
270° - 21E = 249°
A) 15 NM
B) 14 NM
C) 16 NM
D) 17 NM
The DME distance is the slant range and not the perfect horizontal line between station and aircraft (15 NM). Picturing a triangle and using Pythagoras (Square of the hypotenuse (slant range) = sum of the squares of the remaining sides):
(Slant range)^2 = (15 NM)^2 + (36.000ft)^2
(Slant range)^2 = (15 NM)^2 + (5.9 NM)^2
(Slant range)^2 = 225 + 34.8
Slant range = SQRT(260)
Slant range = 16 NM
► The DME (distance measuring equipment) operates within the following frequencies:
A)962 to 1213 MHz
B) 108 to 118 MHz
C) 329 to 335 MHz
D) 962 to 1213 kHz
"A DME operates in the UHF band from 960MHz to 1215MHz"
► 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) 11 NM
B) 7 NM
C) 6 NM
D) 8 NM
Aircraft is directly overhead at 36.000ft
1NM = approx 6080ft
36.000 / 6080 = 5.9 NM
► 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:
Answer: Aeroplane is below the "line of sight" altitude
Formula for max theoretical range;
MTR = 1.23 x(SQRT 21000 + SQRT 0)
MTR = 1.23 x 145
MTR = 178 NM
So at the altitude of 21.000ft the maximum theoretical range at which we can receive the station is 178 NM. The Question states 220 NM meaning we are below the "line of sight altitude".
You could also convert the MRT formula (with 220NM) to find the altitude of the aircraft which would allow you to receive the station (somewhere around 30.000ft).
► Distance measuring equipment (DME) operates in the:
Answer: UHF band and is a secondary radar system
A primary radar: measures only the range and bearing of targets by detecting reflected radio signals
A secondary radar (DME for example): Relies on targets equipped with a radar transponder, that replies to each interrogation signal by transmitting a response
► On a DME, display counters rotating throughout their range indicates:
Answer: The airborne receiver is conducting a range search
► The aircraft DME receiver is able to accept replies to its own transmissions and reject replies to other aircraft interrogations because:
Answer: Pulse pairs are discreet to a particular aircraft
"The airborne interrogator unit transmits pulse pairs to the ground station with a distinct spacing on the tuned UHF frequency."
► In which situation will speed indications on an airborne distance measuring equipment (DME) most closely represent the ground speed of an aircraft flying at FL400.
Answer: When tracking directly towards the station at a range of 100 NM or more
- Ground speed is only accurate when flying TO or FROM the station
- A longer distance decreases the slant error (overhead, the GS will be least accurate)
► 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:
A) 158 NM
B) 186 NM
C) 296 NM
D) 330 NM
Method 1:
Slant range = Half the speed of light x (t-50)
Speed of light (in NM) = 162.000
t (2000 micro sec) - 50 = 1950 micro sec (= 0.00195 seconds)
Slant range = 0.5 x 162000 x 0.00195
Slant range = 158 NM
Method 2:
Remember that 1 Radar Mile = 12.36 microseconds
Time delay = 2000 - 50 = 1950 micro sec
1950 / 12.36 = 158 NM
► In the DME system:
Answer: The receive and the transmit frequency is always split by 63 MHz
"The ground station acts as a transponder and receives the pulse pairs. After a delay of 50 micro seconds, he wil transmit a response with the same pulse spacing but on a frequency different by 63MHz (to clearly distinguish retransmitted pulses from the transponder and reflected pulses from the ground)"
► The design requirements for DME stipulate that, at a range of 100 NM, the maximum systematic error should not exceed:
A) 1.5NM
B) 3NM
C) 0.25NM
D) 1.25NM
"ICAO Annex 10 requires an accuracy of 0.25 NM (+1.25% of the measured distance (Slant)) for systems installed before 1 January 1989"
0.25 + (1.25% of 100) = 1.5NM
► An aircraft at FL300 with a ground speed of 300kts, is about to pass overhead a DME station at MSL. The DME receiver is capable of determining ground speed. One minute before the overhead, the DME speed and distance indications are respectively:
A) 300 kt and 7 NM
B) less than 300 kt and 7 NM
C) less than 300 kt and 5 NM
D) 300 kt and 5 NM
► Given:
Aircraft position 34°15minN 098°E , magnetic variation 28°W , FL 280.
PTC VOR/DME position 36°12minN 098°E , magnetic variation 13°E.
In order to read the most accurate ground speed given by the DME receiver from his present position, the pilot must fly on which PTC radial?
A) 167°
B) 332°
C) Aircraft will not receive DME information from PTC due to the line of sight rule
D) 193°
2 Things to consider:
- GS accuracy reduces as the aircraft nears the DME
- Radials are magnetic
The station is to the NORTH of the aircraft (360°) so to get the most accurate GS reading we need to fly the opposite (180°) SOUTH
Radials are magnetic so add VARIATION (for a VOR, we take variation at station and for an NDB we take the variation at the aircraft)
180° - 13E = 167°
► Given:
Aircraft position 52°09minS 024°E , magnetic variation 14°W , FL 310.
BIT VOR/DME position 54°42minS 024°E , magnetic variation 14°E.
In order to read the most accurate ground speed given by the DME receiver from his present position, the pilot must fly on which BIT radial?
A) 194°
B) 014°
C) 166°
D) 346°
2 Things to consider:
- GS accuracy reduces as the aircraft nears the DME
- Radials are magnetic
The station is to the SOUTH of the aircraft (180°) so to get the most accurate GS reading we need to fly the opposite (360°) NORTH
Radials are magnetic so add VARIATION (for a VOR, we take variation at station and for an NDB we take the variation at the aircraft)
360° - 14E = 346°
► Given:
Aircraft position 36°15minS 178°E , magnetic variation 21°W , FL 310.
UEB VOR/DME position 36°12minS 178°W , magnetic variation 21°E.
In order to read the most accurate ground speed given by the DME receiver from his present position, the pilot must fly on which UEB radial?
A) 111°
B) 069°
C) 291°
D) 249°
2 Things to consider:
- GS accuracy reduces as the aircraft nears the DME
- Radials are magnetic
The station is to the EAST of the aircraft (90°) so to get the most accurate GS reading we need to fly the opposite (270°) WEST.
We are at the back side of the earth, aircraft is at 178°E and flying towards the anti-meridian 180° in an EAST direction towards the station at 178°W
Radials are magnetic so add VARIATION (for a VOR, we take variation at station and for an NDB we take the variation at the aircraft)
270° - 21E = 249°
