Instrument Landing Aids Questions and Answers

Answer: A

Correct answer: Lateral guidance along the runway centreline

The localiser provides guidance in azimuth - it tells the pilot whether the aircraft is left or right of the extended runway centreline, and by how much. It is the horizontal half of an instrument landing system.

Vertical guidance to the touchdown zone is the glide path's function, transmitted separately in the UHF band and tuned automatically with the localiser. Distance comes from marker beacons or, increasingly, from distance measuring equipment, and height above terrain from the radio altimeter. Each element answers a different question, and an approach uses all of them together.

Answer: B

It shares the lower part of the VHF navigation band with VOR, using the odd tenth channels. The glide path lies in the UHF band around 330 megahertz and is paired automatically with the localiser. One selection tunes both.

Answer: C

Selecting the localiser frequency automatically tunes the paired glide path, so the crew makes a single selection. The pairing is fixed by international agreement, which is why no separate glide path frequency is published.

Answer: D

Two lobes are modulated at 90 and 150 hertz, and equal depth means the aircraft is on course. The receiver measures the difference in depth of modulation and drives the deviation indicator from it. Signal strength does not affect it.

Answer: A

Correct answer: To the right of the centreline as seen on approach

The localiser radiates two overlapping lobes, one modulated at 90 hertz and the other at 150 hertz. Seen from an aircraft on approach, the 150 hertz lobe predominates to the right of the centreline and the 90 hertz lobe to the left.

The receiver compares the depth of modulation of the two tones. Where they are equal the aircraft is on the centreline; where 150 hertz predominates the needle deflects to command a turn left, back towards the course. The same 90 and 150 hertz pair is used by the glide path, with 150 hertz predominating below the path.

Answer: B

Three degrees gives a descent rate that is comfortable at approach speeds and clears typical obstacles. Steeper angles are published at a few airports with terrain or noise constraints and require specific aircraft approval.

Answer: C

Correct answer: Plus and minus 2.5 degrees

Full scale localiser deflection corresponds to about \( \pm 2.5 \) degrees, giving a total course sector of roughly 5 degrees. In practice the sector width is adjusted for each installation so that it subtends about 700 feet at the threshold, which means a longer runway gets a narrower angular sector.

The glide path is far finer, at about \( \pm 0.7 \) degrees full scale. That difference matters in the cockpit: a given needle deflection represents a much smaller angular error vertically than laterally, which is why the glide path demands the more careful handling.

Answer: D

The guidance is angular, so the beam converges towards the transmitter and the same needle deflection means less and less displacement. Corrections must therefore become smaller and more precise as the runway is neared.

Answer: A

The antenna pattern produces additional lobes, the first of which lies near six degrees and gives a correct-looking indication at a much steeper angle. Cross-checking altitude against distance at the outer marker or a fix is what detects it.

Answer: B

Correct answer: 75 MHz, radiating a narrow fan vertically

All marker beacons transmit on 75 MHz, radiating a narrow fan-shaped beam vertically so that the aircraft passes through it briefly overhead.

Because they share one frequency, the receiver needs no tuning at all and the markers are told apart by their modulation instead: the outer marker gives a low 400 hertz tone in dashes with a blue light, the middle an alternating 1300 hertz tone with an amber light, and the inner a rapid 3000 hertz tone with a white light. The narrow vertical pattern is what makes the passage a sharp position fix rather than a gradual indication.