ILS Flashcards
ILS provides what
Vertical guidance down glide path + Horizontal guidance along centreline (localiser)
Localiser
Provides guidance left and right to runway
VHF transmitter with 40 dedicated frequencies between 108/111.95MHz
Glide Path
Up and down guidance on approach
Transmits in the UHF/paired with VHF localiser frequency
ILS Frequency Band
VHF - 108MHz to 112MHz
Odd numbers less than 112 in the VHF band
Components of ILS
Localiser
Glide slope
DME range to indicate distance
Market beacons
ILS is identified by
3 letter Morse code
Principles of localiser
Difference of depth of modulation
Blue lobe - 150Hz
Yellow lobe - 90Hz
Modulation increases across lobe to max furthest from centreline
Ac receiver detects and measures depth of modulation in each lobe and calculates the difference.
More 90Hz than 150Hz = left of centreline
More 150Hz than 90Hz = right of the centreline
Errors associated with false localiser beams
Reversed sense side lobes cause incorrect indication of the detected modulated frequency (fly right command when already right of centre line)
Localiser coverage range
Protected ranges +- 10 degrees of runway centreline out to 25nm
+- 35 degrees out to 17nm
Localiser back course
Non precision approach on the opposite side on runway - azimuth guidance
Glide path - principles of operation
Vertical guidance by overlapping beams of UHF carrier waves
Lower (blue) - 150Hz
Upper (yellow) - 90Hz
More 150Hz signal than 90Hz = below the GP (fly up command)
More 90Hz signal than 150Hz = above the GP (flow down command)
Equal amount of each signal = DDM zero on glide path
Glide path Coverage
+- 8 degrees either side of runway centreline - 10nm
False glide path
Lower lobe is blocked by the ground/occurs above true glide path 2x
Standard 3 degree glide path = first false glide path occurs at 3 x 2 = 6 degrees
Outer Marker
Indicated the final approach fix
4-7nm from runway threshold
Lowest frequency of the three (400hz)
Blue flashing lights
2 dashes per second - - - - - - - -
Beam Bends
Slight curves which are caused by reflections of the radio wave due to obstructions. Can be followed by the navigation system of aircraft
Middle Marker
1nm from threshold
Amber
Medium frequency (1300Hz)
Dots and dashes - 3 dashes per second - . - . - . - . - . -
FM broadcast just below 108MHz can cause
Interference with localiser signal which may lead to deviation
ILS beam bends are
Slight curves caused by reflection of radio waves due to permanent obstructions. Can be followed by nav systems.
Checked by flight tests
ILS warning flag indicates one of the following
Absence of carrier frequency
Absence of modulation
% of modulation reduced to zero
Different type of precision approaches in ILS
CAT I - Not below 200ft (DH) - RVR 550m
CAT II - DH 100ft - RVR 300M
CAT IIIA - DH below 100ft - RVR 200m
CATIIIB - DH below 100ft or no DH - 75m
CATIIIC - No limits - guides along runway
Scalloping and ILS
Causes rapid indicator changes from side to side of the intended approach path which cannot be followed by the aircraft
To be fully established on the localiser before descending on the glide slop the ac must be within
1 dot half scale deflection on either side of the localiser centreline.
Paired VOR and ILS on approach with half scale deflection of VOR =
Full scale ILS (2.5 max deviation)
FM immunity filters
Used to protect against interference from VHF broadcast transmissions on the localiser and VORs
Not ILS glide slope as this is UHF
Warning flag on ILS will trigger when
No carrier frequency
No modulation
% modulation reduced to zero
Localiser coverage
25Nm within 10 degrees of approach bearing
17N, within 35 degrees of approach bearing
ILS back course on OBI and CDI
OBI will always display reverse sensing
HSI can be turned to face the front course to reverse the indication to give correct sensing of localiser
Full scale deflection of a localiser is
2.5 degrees (Each dot is 0.5 degrees)
Identification of ILS is by
3 letter Morse code
Transmitted every 10 seconds on audible channel of the ILS
Via the audio control panel in NAV
ILS/DME - co located meaning
DME will read 0nm at the threshold
Automatically tuned when localiser frequency is used
How does ILS produce guidance
A radiation pattern which is amplitude modulated by a 90Hz and 150Hz signal
Full deflection on a glide slope indicator indicates the aircraft is
0.7 degrees above or below the correct glide path
ILS lobes - frequencies
The same carrier frequency - VHF - 75MHz
Both Amplitude modulated
Different amplitude modulation
Outer Marker is modulated at
400hz
- - - - - - -
Rate of 2 per second
4-7nm for threshold
Middle Marker is modulated at
1300hz
.-.-.-.
2 dashes and 6 dots per second
0.4nm
Inner Marker is modulated at
3000 hertz (highest frequency)
6 dots per second
. . . . . . . .
Audio frequency of markers
Inner - 3000hz
Middle - 1300hz
Outer - 400hz
ILS facility performance categories (ILS equipment capability\0
Not the same as the normal operation figures. Lowest minimums capability of the facility.
CAT I - 100ft or less
CAT II - 50ft or less
CAT III - Down to the runway
Glide path Coverage formula
(Glide slope in degrees) x 0.45 (lower)
(Glide slope in degrees) x 1.75 (upper)
3 x 0.45 = 1.35 degrees above the horizontal
3 x 1.75 = 5.25 degrees above the horizontal
Tuned to a ILS - rotating the OBS will
Only rotate the compass card and not have any effect on the deviation display
Warning flag appears for ILS when
Absence of the modulation
Percentage modulation signal reduced to 0
Absence of the carrier frequency
Working out height along glideslope
3 degree glide slope = 300ft per mile
4 degree glide slope = 400ft per mile
+100ft per1 degree
ILS components - Distances of localiser/glide path/middle market/outer marker
Localiser - 300 meters behind end of runway
Glide path - 300 meter behind threshold
Middle marker - 1000 meter from threshold
Outer marker - 4nm from threshold
Type A vs Type B
Type A - Above 250 DH
Type B - Below 250 DH
What is the reversal procedure
Starts at fix
Straight leg with track guidance/timed
80 degree turn
260 turn in opposite direction to intercept inbound track
Multi path Errors are to be expected in the
ILS coverage area
Reflect the ILS signals
