Components of the ILS
Think of the ILS as a very accurate VOR
- has 4 separate parts
1. Localizer
2. Glide slope
3. Marker beacons
4. Visual elements/approach lights
ILS (instrument landing system)
The ILS is a precision approach. This means we not only get information on course, but also altitude.
Localizer (AIM 1-1-9)
- works on 40 reserved frequencies between 108.10 and 111.95 MHz
- morse code begins with letter “I” plus 3 letters
- 2 overlapping lobes overlap energy is equal, providing a path in line with extended center line
- blue 150 Hz
- yellow 90 Hz
- course width is 700’ at threshold (3 degrees to 6 degrees average 5 degrees)
- roughly 4 times more sensitive than the VOR
- Range of locator
- 35 degrees of center line up to 10 NM
- 10 degree of centerline up to 18 NM
Localizer back course; the reciprocal of the localizer
- many localizers transmit a back
- can be used for tracking, missed approach or taking off
- no glide slope on a back course
- some are available for non-precision approaches
- reverse sensing- can get this when flying the front or back course with an HSI. Always tune the OBS to the localizer front course
Glide slope
Provides vertical guidance toward runway touchdown point
- usually at a slope of approximate 3 degrees
Same concept as localizer, but on its side
- width of glide slope usually 1.4 degrees from top to bottom or .35 degrees per dot
- glide slope is 12 times more sensitive then the VOR
- uses same frequency as localizer, just different modulations of that frequency
- glide slope has range of about 10 NM
False glide slope possible at 12.5 degrees
- ground reflection of the transmission causes more than one overlapping lobe
- there will be reverse sensing with a false glide slope which will cause needle to oscillate
- intercept the glide slope from below
- check your rate of decent V.S. Ground speed on IAP and altitude
Marker beacons
- provide accurate range fixes along the approach path (for situational awareness)
- aircraft equipment is 3 lights and a tone generator
3 types of beacons:
1. OM (outer marker) - typically at 4-7 miles out on approach
- usually about 1400’ AGL when passing OM
- blue light, low tone, dash-dash-dash-dash (2 per second)
- LOW pitch
- “hey you should be descending now”
2. MM (middle marker) - usually 3500 feet from runway
- signal consists of medium pitch, Amber light, dot-dash (6 per second)
- normally at the DH (normally 200’ AGL)
- hey, look outside
3. IM (inner marker) - typically only on category 2 & 3 approaches
- all the way down to the runway
- white light, high pitch, all dots
- HIGH pitch
- “hey, here’s the ground
4. Back course - identifies a back course fix
- white light
Approach light system (AIM 2-1-1)
- Lights help to transition from IFR flight to VFR flight (guide pilot to runway)
- normally extended 2400 to 3000 feet from threshold
- RAIL (runway alignment indicator lights) sequenced flashing lights extended center line “the rabbit”
- blinks twice per second
REIL (runway end identifier lights
- identify runway end surrounded by many other lights (strobe)
- identify a runway end which lacks contrast with the surrounding terrain
- identify runway end in poor visibility
VASI (visual approach slope indicator)
- 2 bar VASI- for smaller aircraft
- 3 bar VASI- for larger aircraft
- all white, out of sight
- all red, your dead
- red and white, your all right
PAPI (precision approach path indicator)
- similar to a VASI, but all 4 lights are in a row
Tri color VASI
- Amber, green and red
Pulsating VASI
- fast pulsing white; well above the glide path
- pulsing white; above glide path
- steady white; on glide path
- pulsing red; below glide path
- fast pulsing red; well below the glide path
Precision instrument runway markings (AIM 2-3-3)
Threshold marker: - bars show the width of the runway - also show start of runway Touchdown zone marker: - 500 foot increments show runway length Aiming point markers: - placed 1000 feet from end of runway
Flying the ILS
- transition from VOR to ILS
Cyclic movements for glide slope - to estimate glide slope, use rate of descent equal to 5 times ground speed
- 80 Knot groundspeed times 5= 400 fpm rate of descent
Inoperative components
- inoperative localizer- do not fly approach
- inoperative glide slope- fly like a VOR approach
1. Consists of step-down fixes along the way
2. Called a localizer only approach
3. Non precision
4. Use MDA from chart - outer markers can be replaced by
1. VOR- flying over or cross radial
2. DME fix
3. Compass locator
4. PAR (precision approach radar)
5. ASR (airport surveillance radar) - middle marker can be replaced by
1. PAR
2. Compass locator - approach lights out
1. Look for adjusted minimums on approach
2. Visibility will be higher
Wind shear on approach
Undershoot effect:
- airspeed drops, you must increase power to keep from descending
Overshoot effect:
- airspeed increases, you must decrease power to slow airspeed and keep from climbing
Simultaneous approaches
- minimum 4300’ between 2 runways
Parallel approach
- at least 2500’ laterally between runways
- must also have a 2 mile stagger between aircraft
Converging approach
- MAP must be 3 miles away from each other and going in different directions
Sidestep
- use normal maneuvers
- runways must be closer than 1200’ apart
- must have both runways visible
Other ILS type approaches 3 types
LDA
SDF
MLS
LDA (localizer-type directional aid)
Not ILS because
- may not have all the components of an ILS
- not aligned with runway
- not always a glide slope
- no approach lights
Localizer approach usually to the side of a runway
If approach is off more than 30 degrees, circle to land minimums only
- also assigned approach letter rather than runway #
If approach is less than 30 degrees both straight in and circling minimums exist
SDF (simplified directional facility)
- 6 or 12 degree course width
- no glide slope
- may or may not be offset to runway centerline
- same rules for circling and straight in minimums
MLS (microwave landing system)
- can have a curved flight path
- more accurate
- must have special airborne equipment
$ very expensive
