DGCA Air Navigation: Ils Questions
249 exam-style Ils 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
What is the approximate angular coverage of reliable navigation information for a 3° ILS glide path out to a distance of 10 NM?
- A 1.35° above the horizontal to 5.25° above the horizontal and 8° each side of the localiser centreline
- B 0.45° above the horizontal to 1.75° above the glide path and 8° each side of the localiser centreline
- C 0.7° above and below the glide path and 2.5° each side of the localiser centreline
- D 3° above and below the glide path and 10° each side of the localiser centreline
Show answer
Correct answer: A. 1.35° above the horizontal to 5.25° above the horizontal and 8° each side of the localiser centreline
For a 3 degree glidepath the reliable coverage runs from 0.45 to 1.75 times the glidepath angle in elevation, that is 1.35 to 5.25 degrees above the horizontal, and to about 8 degrees either side of the localiser centreline.
- Q2
An aircraft is at 1.9 DME showing 2 dots below glideslope. What is the height of the aircraft above the glideslope?
- AApproximately 380 ft below glideslope
- BApproximately 190 ft below glideslope
- CApproximately 760 ft below glideslope
Show answer
Correct answer: A. Approximately 380 ft below glideslope
At 1.9 DME on a 3° glideslope, the ideal height is 1.9 × 300 ft/DME = 570 ft above threshold. Two dots below (full deflection on a standard ILS) is about 0.7° low, or 1.9 × 0.7 × 6076 ft/NM ÷ 60 ≈ 134 ft. So, the aircraft is 570 - 134 ≈ 436 ft above threshold, or roughly 380 ft below the glideslope.
- Q3
ILS is subject to false glide paths resulting from:
- Amultiple lobes of radiation patterns in the vertical plane
- Bspurious signals reflected by nearby obstacles
- C back-scattering of antennas
- D ground returns ahead of the antennas
Show answer
Correct answer: A. multiple lobes of radiation patterns in the vertical plane
False glidepaths occur because the glidepath aerial produces several lobes in the vertical plane, creating additional balance points at multiples of the true glide angle.
- Q4
What is the colour sequence when passing over an Outer, Middle and Inner Marker beacon?
- A blue - amber - white
- Bamber - white - green
- C white - amber - blue
- D blue - green - white
Show answer
Correct answer: A. blue - amber - white
The marker beacons are identified by lights in the sequence blue for the outer marker, amber for the middle marker and white for the inner marker.
- Q5
An aircraft carrying out an ILS approach is receiving more 90 Hz than 150 Hz modulation notes from both the localiser and glidepath transmitters. The ILS indication will show:
- AFly right and fly down
- B Fly left and fly down
- C Fly right and fly up
- DFly left and fly up
Show answer
Correct answer: A. Fly right and fly down
Excess 90 Hz on the localiser means the aircraft is left of the centreline and excess 90 Hz on the glidepath means it is above the slope, so the indications command fly right and fly down.
- Q6
An aircraft carrying out a 3° glidepath ILS approach experiences a reduction in groundspeed from 150 kt at the outer marker to 120 kt over the threshold. The effect of this change in groundspeed on the aircraft's rate of descent will be a decrease of approximately:
- A 150 FT/MIN
- B 250 FT/MIN
- C50 FT/MIN
- D 100 FT/MIN
Show answer
Correct answer: A. 150 FT/MIN
On a 3° glidepath the rate of descent is about five times the groundspeed in knots, so slowing from 150 kt (roughly 750 ft/min) to 120 kt (roughly 600 ft/min) reduces the rate of descent by approximately 150 ft/min.
- Q7
The principle of operation of an ILS localiser transmitter is based on two overlapping lobes that are transmitted on (i).......... frequencies and carry different (ii)..........
- A (i) the same (ii) modulation frequencies
- B (i) the same (ii) phases
- C (i) different (ii) modulation frequencies
- D (i) different (ii) phases
Show answer
Correct answer: A. (i) the same (ii) modulation frequencies
The localiser radiates two overlapping lobes on the same carrier frequency, distinguished by different modulation tones of 90 Hz and 150 Hz whose depth-of-modulation difference defines the centreline.
- Q8
In which frequency band does an ILS glide slope transmit?
- AUHF
- B VHF
- C SHF
- DEHF
Show answer
Correct answer: A. UHF
The ILS glide path transmits in the UHF band, around 329–335 MHz, paired automatically with the VHF localiser frequency.
- Q9
Assuming a five dot display, what does each of the dots on either side of the ILS localizer cockpit display represent :
- A0.5 degrees
- B 1.5 degrees
- C2.5 degrees
- D 2.0 degrees
Show answer
Correct answer: A. 0.5 degrees
On a five-dot localiser display full-scale deflection corresponds to about 2.5° each side, so each dot represents approximately 0.5° of angular displacement.
- Q10
Outer marker transmits on 75 MHz and has an aural frequency of:
- A400 Hz
- B 1300 Hz
- C2000 Hz
- D 3000 Hz
Show answer
Correct answer: A. 400 Hz
All ILS marker beacons transmit on 75 MHz; the outer marker is keyed with a 400 Hz tone (dashes), distinct from the middle and inner marker tones.
- Q11
Every 10 kt decrease in groundspeed, on a 3° ILS glidepath, will require an approximate:
- Adecrease in the aircraft's rate of descent of 50 FT/MIN
- B increase in the aircraft's rate of descent of 50 FT/MIN
- Cdecrease in the aircraft's rate of descent of 100 FT/MIN
- D increase in the aircraft's rate of descent of 100 FT/MIN
Show answer
Correct answer: A. decrease in the aircraft's rate of descent of 50 FT/MIN
On a 3° glidepath the required rate of descent is about five times the groundspeed in knots, so each 10 kt change in groundspeed alters the rate of descent by roughly 50 ft/min. A reduction in groundspeed therefore needs a decrease in rate of descent of about 50 ft/min to stay on the glidepath.
- Q12
Instrument Landing Systems (ILS) Glide Paths provide azimuth coverage (i) .......... ° each side of the localiser centre-line to a distance of (ii) .......... NM from the threshold.
- A (i) 8 (ii) 10
- B (i) 25 (ii) 17
- C (i) 35 (ii) 25
- D(i) 5 (ii) 8
Show answer
Correct answer: A. (i) 8 (ii) 10
The ILS glide path provides azimuth coverage of 8° either side of the localiser centre-line out to a range of 10 NM from the threshold. Beyond this sector reliable glidepath guidance is not guaranteed.
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