INSTRUMENT LANDING SYSTEM ILS Background Components

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INSTRUMENT LANDING SYSTEM
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ILS

• Background
• Components
• Localizer
• Glide Path
• ILS dangers and problem history
• CAT II
• PRM ILS Approaches

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ILS

• Developed in the 40s
• CAT II developed in the 60s

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50 Hz
150 Hz
2920 ft
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Reliable Localizer Signal Coverage
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90 Hz
1.4 º
5º
150 Hz
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Glideslope antenna located between 750 and 1250
feet from threshold to provide 55/-5 ft TCH
offset 250-600 Beam width at OM (5nm from
runway) approximately 730ft. Beam width at DH
approximately 98ft (50 feet high or low at DH
gives full scale deflection.) Tangent 1.4degrees
x/4000ft (where 4000 is the distance from
transmitter at DH). Xtan1.44000 X98
90 Hz
1.4 º
5º
150 Hz
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• Glide path signals are bounced off the ground
• Extensive site preparation is required
• Water-soaked ground, excessive snow, or extremely
  long grass can affect the signal
• Glide slope monitors detect bad signals and
  automatically shut down the transmitter

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ILS Critical Area (US)

• Disturbances to ILS localizer and glide slope
  courses may occur when surface vehicles or
  aircraft are operated near the localizer or glide
  slope antennas. (AIM 1-1-9k)

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• ATC issues control instructions to avoid ILS
  critical areas as follows
• WX Conditions Less than 800/2
• vehicles aircraft are not authorized in the
  critical area when an arriving aircraft is
  between the ILS FAF and the airport . . .
• unless the aircraft has reported the airport in
  sight and is circling or side stepping to land on
  a rwy other than the ILS rwy
• WX Conditions At or above 800/2
• No critical area protective action is provided.
• A flight crew should advise the tower that it
  will conduct an AUTOLAND or COUPLED approach to
  ensure that the ILS critical areas are protected
  when the aircraft is inside the ILS MM.
• Note Critical areas are not protected at
  uncontrolled airports.

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ILS Critical Area(Canada)

• RAC 4.2.7 When an airport is operating under
  CAT II or III wx conditions, pilots are to
  observe CAT II or III mandatory holding position
  signs.
• When an airport is not operating under CAT II or
  III weather conditions, pilots need not abide by
  the CAT II or III taxiway holding positions and
  are expected to taxi to the CAT I taxiway holding
  position markings, unless advised otherwise by
  ATC.

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Vehicles not subject to ATC may cause momentary
deviation to ILS course or glide slope signals.
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False Glide Paths

• When the glide slope bounces off the ground,
  additional glide paths are also created.
• The 6º false signal will be a reciprocal signal
  (fly up/down commands will be reversed). The 9º
  false signal will be oriented in the same manner
  as the true glide slope.

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What to do about it

• Intercept from below
• Check your VVI
• Do a little math (300 x Dist to Rwy)

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False Glide Path vs. Erroneous Glide Path

• A false glide path is a normal by-product of the
  ILS transmission and provides a distinct, but
  incorrect, glideslope.
• An erroneous glideslope does not provide a
  defined path.
• It is a malfunction of the signal, the
  instrument, or both.
• There may be no warning flags visible.
• The autopilot is quite happy to follow an
  erroneous glidepath.

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The Main Points

• The ILS can display grossly inaccurate
  indications that look perfectly valid with no
  associated warnings to the crew
• There is no ident on the glideslope signal
• A valid ILS ident and absence of warning flags
  does not guarantee that the glideslope signal is
  correct

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Alitalia DC-9Zurich, Switzerland14 Nov 1990
A
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• AC Pull, pull, pull, pull! (Autopilot
  disconnect)
• FO Go around
• AC No, no, no, no . . .catch the glide,
• Pitch from -2 to 5, power up, VVI from -1100
  to -190
• AC (9 sec later) Can you hold it?
• FO Yes.
• (1 sec later) Radio altimeter warning (200
  AGL)
• (2 sec later) Impact

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Accident Investigation Notes

• Glideslope indication 1 was unreliable.
• Autopilot was probably following a frozen
  glideslope signal.
• A go around is seen as a failure, a lack of
  professional competence and even loss of
  prestige.
• There are newer, more advanced navigation
  receivers that are not able to detect a signal
  break or open circuit between the receiver and
  the cockpit instruments.
• Alitalia pilots in general were not aware that
  the instrument indications could be incorrect
  with no off flags showing.

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• Operators of aircraft equipped with the
  following navigation receivers must include
  procedures for detecting malfunctions that result
  in the display of disparate information
• Collins 51RV-1
• Collins 51RV-4
• Wilcox 806
• King KNR 6030
• Bendix RNA 26C

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B
Air New Zealand 30 July 2000
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Air New Zealand

• Cleared for (autocoupled) ILS Rwy 08 to Faleolo
  Intl, W. Samoa
• Autopilot captured the localizer
• APP was armed and glideslope was captured
• On glideslope throughout the approach
• PF noted an anomaly in DME vs altitude
• PNF noticed that visual cues did not match
• A go-around was commanded at 400 AGL, 6 mi. from
  runway.
• A second approach was flown using LOC only
  procedures
• Flt instruments showed on glideslope during
  second approach

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• Go-around initiated On glide path for approach
  go-around until abeam the rwy
• No OFF FLAGS
• Good ident
• GP monitor was in a test mode, which inhibited
  the automatic shutdown of a faulty transmitter
  that was not radiating all of the GP signal.
• The crew had read the NOTAMs, which showed the
  VOR, ILS/GP, ILS/DME were unmonitored, and the
  ILS/GP had no standby transmitter.

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Missed Approach

• PF lost SA--went into survival mode
• From inquiry to action in less than 10 sec
• Crew supported the pilots decision to go-around
• Most conservative/safest response is always best.

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Fredericton Incident
C
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Recommendations

• The glideslope check is imperative
• If the check at the Outer Marker reveals any
  discrepancies, then a go-around should be
  initiated and the problem solved before starting
  another approach.
• Crosscheck altitude and distance periodically
  (DME or FMS)
• Ø Crosscheck radio altitude and barometric
  altitude
• Ø Crosscheck ground speed and rate of descent
• Question ATC when indications do not appear to be
  correct

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Glideslope Check Altitude

• Is it an altimeter check or a glideslope check?

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Localizer False Course Capture

• Caused by incompatibility of ground and air
  equipment
• Even if both meet requirements
• Report it to ATC
• Most likely to occur when the aircraft is 8º to
  12º from the published localizer course

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ILS False Localizer CaptureVictoria, June 2001
D
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Procedural Defences AIM COM 3.13.1

• To reduce the risk of false captures
•  approach mode should not be selected until the
  aircraft is within 18 nm of the threshold and
  positioned within 8º of the inbound ILS course
•  pilots should use raw data sources to ensure
  that the aircraft is on the correct localizer
  course before initiating a coupled approach

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LDALocalizer Type Directional Aid

• Localizer transmitter but not aligned with runway
• Has same accuracy as localizer but course will
  not be within 3 degrees of centerline.
• Straight-in mins if alignment lt 30 degrees
• May have glideslope

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SDFSimplified Directional Facility

• Similar to localizer
• Course beam is wider (6 or 12 degrees)
• Antenna may be offset from centerline
• Normally within 3

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CAT II ILS
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Cat II ILS

• Only the Airbus, Challenger and Dash 8 are
  certified for CAT II ops
• Special equipment including
• Dual everything except single FD
• Dual Autopilot,
• Dual Rad alts,
• Dual VHF, etc.
• Special procedures published
• Crews need certification

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PARALLELILS APPROACHES

• Dependent Parallel
• Simultaneous Parallel
• Simultaneous Close Parallel (PRM)

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SIMULTANEOUS CLOSE PARALLEL ILS PRM APPROACHES

• Centerlines lt 4300 apart but gt3400
• Precision Runway Monitor (PRM)
• Special Procedures

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Simultaneous Offset Instrument Approach (SOIA)
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Questions on PRM ?